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Title: The Evolution of Man

Author: Ernst Haeckel

Release date: August 1, 2005 [eBook #8700]
Most recently updated: April 5, 2020

Language: English

Other information and formats: www.gutenberg.org/ebooks/8700

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*** START OF THE PROJECT GUTENBERG EBOOK THE
EVOLUTION OF MAN ***

The Evolution of Man
A POPULAR SCIENTIFIC STUDY

by Ernst Haeckel

Translated from the Fifth (enlarged) Edition by Joseph McCabe

[Issued for the Rationalist Press Association, Limited]

WATTS & CO.
17 Johnson’s Court, Fleet Street, London, E.C.
1912

Contents

LIST OF ILLUSTRATIONS
GLOSSARY
TRANSLATOR’S PREFACE
TABLE: CLASSIFICATION OF THE ANIMAL WORLD

Chapter I. THE FUNDAMENTAL LAW OF ORGANIC EVOLUTION
Chapter II. THE OLDER EMBRYOLOGY
Chapter III. MODERN EMBRYOLOGY
Chapter IV. THE OLDER PHYLOGENY
Chapter V. THE MODERN SCIENCE OF EVOLUTION
Chapter VI. THE OVUM–THE AMŒBA
Chapter VII. CONCEPTION
Chapter VIII. THE GASTRÆA THEORY
Chapter IX. THE GASTRULATION OF THE VERTEBRATE
Chapter X. THE CŒLOM THEORY
Chapter XI. THE VERTEBRATE CHARACTER OF MAN
Chapter XII. THE EMBRYONIC SHIELD–GERMINATIVE AREA
Chapter XIII. DORSAL BODY–VENTRAL BODY
Chapter XIV. THE ARTICULATION OF THE BODY
Chapter XV. FŒTAL MEMBRANES AND CIRCULATION
Chapter XVI. STRUCTURE OF THE LANCELET AND THE SEA-
SQUIRT
Chapter XVII. EMBRYOLOGY OF THE LANCELET AND THE SEA-
SQUIRT
Chapter XVIII. DURATION OF THE HISTORY OF OUR STEM
Chapter XIX. OUR PROTIST ANCESTORS

Chapter XX. OUR WORM-LIKE ANCESTORS
Chapter XXI. OUR FISH-LIKE ANCESTORS
Chapter XXII. OUR FIVE-TOED ANCESTORS
Chapter XXIII. OUR APE ANCESTORS
Chapter XXIV. EVOLUTION OF THE NERVOUS SYSTEM
Chapter XXV. EVOLUTION OF THE SENSE-ORGANS
Chapter XXVI. EVOLUTION OF THE ORGANS OF MOVEMENT
Chapter XXVII. EVOLUTION OF THE ALIMENTARY SYSTEM
Chapter XXVIII. EVOLUTION OF THE VASCULAR SYSTEM
Chapter XXIX. EVOLUTION OF THE SEXUAL ORGANS
Chapter XXX. RESULTS OF ANTHROPOGENY

INDEX

LIST OF ILLUSTRATIONS

Fig. 1. The human ovum
Fig. 2. Stem-cell of an echinoderm
Fig. 3. Three epithelial cells
Fig. 4. Five spiny or grooved cells
Fig. 5. Ten liver-cells
Fig. 6. Nine star-shaped bone-cells
Fig. 7. Eleven star-shaped cells
Fig. 8. Unfertilised ovum of an echinoderm
Fig. 9. A large branching nerve-cell
Fig. 10. Blood-cells
Fig. 11. Indirect or mitotic cell-division
Fig. 12. Mobile cells
Fig. 13. Ova of various animals
Fig. 14. The human ovum
Fig. 15. Fertilised ovum of hen
Fig. 16. A creeping amœba
Fig. 17. Division of an amœba
Fig. 18. Ovum of a sponge
Fig. 19. Blood-cells, or phagocytes
Fig. 20. Spermia or spermatozoa
Fig. 21. Spermatozoa of various animals
Fig. 22. A single human spermatozoon
Fig. 23. Fertilisation of the ovum
Fig. 24. Impregnated echinoderm ovum
Fig. 25. Impregnation of the star-fish ovum
Figs. 26–27. Impregnation of sea-urchin ovum
Fig. 28. Stem-cell of a rabbit
Fig. 29. Gastrulation of a coral
Fig. 30. Gastrula of a gastræad

Fig. 31. Gastrula of a worm
Fig. 32. Gastrula of an echinoderm
Fig. 33. Gastrula of an arthropod
Fig. 34. Gastrula of a mollusc
Fig. 35. Gastrula of a vertebrate
Fig. 36. Gastrula of a lower sponge
Fig. 37. Cells from the primary germinal layers
Fig. 38. Gastrulation of the amphioxus
Fig. 39. Gastrula of the amphioxus
Fig. 40. Cleavage of the frog’s ovum
Figs. 41–44. Sections of fertilised toad ovum
Figs. 45–48. Gastrulation of the salamander
Fig. 49. Segmentation of the lamprey
Fig. 50. Gastrulation of the lamprey
Fig. 51. Gastrulation of ceratodus
Fig. 52. Ovum of a deep-sea bony fish
Fig. 53. Segmentation of a bony fish
Fig. 54. Discoid gastrula of a bony fish
Figs. 55–56. Sections of blastula of shark
Fig. 57. Discoid segmentation of bird’s ovum
Figs. 58–61. Gastrulation of the bird
Fig. 62. Germinal disk of the lizard
Figs. 63–64. Gastrulation of the opossum
Figs. 65–67. Gastrulation of the opossum
Figs. 68–71. Gastrulation of the rabbit
Fig. 72. Gastrula of the placental mammal
Fig. 73. Gastrula of the rabbit
Figs. 74–75. Diagram of the four secondary germinal layers
Figs. 76–77. Cœlomula of sagitta
Fig. 78. Section of young sagitta
Figs. 79–80. Section of amphioxus-larvæ
Figs. 81–82. Section of amphioxus-larvæ
Figs. 83–84. Chordula of the amphioxus
Figs. 85–86. Chordula of the amphibia
Figs. 87–88. Section of cœlomula-embryos of vertebrates
Figs. 89–90. Section of cœlomula-embryo of triton
Fig. 91. Dorsal part of three triton-embryos

Fig. 92. Chordula-embryo of a bird
Fig. 93. Vertebrate-embryo of a bird
Figs. 94–95. Section of the primitive streak of a chick
Fig. 96. Section of the primitive groove of a rabbit
Fig. 97. Section of primitive mouth of a human embryo
Figs. 98–102. The ideal primitive vertebrate
Fig. 103. Redundant mammary glands
Fig. 104. A Greek gynecomast
Fig. 105. Severance of the discoid mammal embryo
Figs. 106–107. The visceral embryonic vesicle
Fig. 108. Four entodermic cells
Fig. 109. Two entodermic cells
Figs. 110–114. Ovum of a rabbit
Figs. 115–118. Embryonic vesicle of a rabbit
Fig. 119. Section of the gastrula of four vertebrates
Figs 120. Embryonic shield of a rabbit
Figs. 121–123. Dorsal shield and embryonic shield of a rabbit.
Fig. 124. Cœlomula of the amphioxus
Fig. 125. Chordula of a frog
Fig. 126. Section of frog-embryo
Figs. 127–128. Dorsal shield of a chick
Fig. 129. Section of hind end of a chick
Fig. 130. Germinal area of the rabbit
Fig. 131. Embryo of the opossum
Fig. 132. Embryonic shield of the rabbit
Fig. 133. Human embryo at the sandal-stage
Fig. 134. Embryonic shield of rabbit
Fig. 135. Embryonic shield of opossum
Fig. 136. Embryonic disk of a chick
Fig. 137. Embryonic disk of a higher vertebrate
Figs. 138–142. Sections of maturing mammal embryo
Figs. 143–146. Sections of embryonic chicks
Fig. 147. Section of embryonic chick
Fig. 148. Section of fore-half of chick-embryo
Figs. 149–150. Sections of human embryos
Fig. 151. Section of a shark-embryo
Fig. 152. Section of a duck-embryo

Figs. 153–155. Sole-shaped embryonic disk of chick
Figs. 156–157. Embryo of the amphioxus
Figs. 158–160. Embryo of the amphioxus
Figs. 161–162. Sections of shark-embryos
Fig. 163. Section of a Triton-embryo
Figs. 164–166. Vertebræ
Fig. 167. Head of a shark-embryo
Figs. 168–169. Head of a chick-embryo
Fig. 170. Head of a dog-embryo
Fig. 171. Human embryo of the fourth week
Fig. 172. Section of shoulder of chick-embryo
Fig. 173. Section of pelvic region of chick-embryo
Fig. 174. Development of the lizard’s legs
Fig. 175. Human-embryo five weeks old
Figs. 176–178. Embryos of the bat
Fig. 179. Human embryos
Fig. 180. Human embryo of the fourth week
Fig. 181. Human embryo of the fifth week
Fig. 182. Section of tail of human embryo
Figs. 183–184. Human embryo dissected
Fig. 185. Miss Julia Pastrana
Figs. 186–190. Human embryos
Fig. 191. Human embryos of sixteen to eighteen ays
Figs. 192–193. Human embryo of fourth week
Fig. 194. Human embryo with its membranes
Fig. 195. Diagram of the embryonic organs
Fig. 196. Section of the pregnant womb
Fig. 197. Embryo of siamang-gibbon
Fig. 198. Section of pregnant womb
Figs. 199–200. Human fœtus–placenta
Fig. 201. Vitelline vessels in germinative area
Fig. 202. Boat-shaped embryo of the dog
Fig. 203. Lar or white-handed gibbon
Fig. 204. Young orang
Fig. 205. Wild orang
Fig. 206. Bald-headed chimpanzee
Fig. 207. Female chimpanzee

Fig. 208. Female gorilla
Fig. 209. Male giant-gorilla
Fig. 210. The lancelet
Fig. 211. Section of the head of the lancelet
Fig. 212. Section of an amphioxus-larva
Fig. 213. Diagram of preceding
Fig. 214. Section of a young amphioxus
Fig. 215. Diagram of a young amphioxus
Fig. 216. Transverse section of lancelet
Fig. 217. Section through the middle of the lancelet
Fig. 218. Section of a primitive-fish embryo
Fig. 219. Section of the head of the lancelet
Figs. 220. Organisation of an ascidia
Figs. 221. Organisation of an ascidia
Figs. 222–224. Sections of young amphioxus-larvæ
Fig. 225. An appendicaria
Fig. 226. Chroococcus minor
Fig. 227. Aphanocapsa primordialis
Fig. 228. Protamœba
Fig. 229. Original ovum-cleavage
Fig. 230. Morula
Figs. 231–232. Magosphæra planula
Fig. 233. Modern gastræads
Figs. 234–235. Prophysema primordiale
Figs. 236–237. Ascula of gastrophysema
Fig. 238. Olynthus
Fig. 239. Aphanostomum langii
Figs. 240–241. A turbellarian
Figs. 242–243. Chætonotus
Fig. 244. A nemertine worm
Fig. 245. An enteropneust
Fig. 246. Section of the branchial gut
Fig. 247. The marine lamprey
Fig. 248. Fossil primitive fish
Fig. 249. Embryo of a shark
Fig. 250. Man-eating shark
Fig. 251. Fossil angel-shark

Fig. 252. Tooth of a gigantic shark
Figs. 253–255. Crossopterygii
Fig. 256. Fossil dipneust
Fig. 257. The Australian dipneust
Figs. 258–259. Young ceratodus
Fig. 260. Fossil amphibian
Fig. 261. Larva of the spotted salamander
Fig. 262. Larva of common frog
Fig. 263. Fossil mailed amphibian
Fig. 264. The new zealand lizard
Fig. 265. Homœosaurus pulchellus
Fig. 266. Skull of a permian lizard
Fig. 267. Skull of a theromorphum
Fig. 268. Lower jaw of a primitive mammal
Figs. 269–270. The ornithorhyncus
Fig. 271. Lower jaw of a promammal
Fig. 272. The crab-eating opossum
Fig. 273. Fœtal membranes of the human embryo
Fig. 274. Skull of a fossil lemur
Fig. 275. The slender lori
Fig. 276. The white-nosed ape
Fig. 277. The drill-baboon
Figs. 278–282. Skeletons of man and the anthropoid apes
Fig. 283. Skull of the java ape-man
Fig. 284. Section of the human skin
Fig. 285. Epidermic cells
Fig. 286. Rudimentary lachrymal glands
Fig. 287. The female breast
Fig. 288. Mammary gland of a new-born infant
Fig. 289. Embryo of a bear
Fig. 290. Human embryo
Fig. 291. Central marrow of a human embryo
Figs. 292–293. The human brain
Figs. 294–296. Central marrow of human embryo
Fig. 297. Head of a chick embryo
Fig. 298. Brain of three craniote embryos
Fig. 299. Brain of a shark

Fig. 300. Brain and spinal cord of a frog
Fig. 301. Brain of an ox-embryo
Fig. 302. Brain of a human embryo
Fig. 303. Brain of a human embryo
Fig. 304. Brain of the rabbit
Fig. 305. Bead of a shark
Figs. 306–310. Heads of chick-embryos
Fig. 311. Section of mouth of human embryo
Fig. 312. Diagram of mouth-nose cavity
Figs. 313–314. Heads of human embryo
Figs. 315–316. Face of human embryo
Fig. 317. The human eye
Fig. 318. Eye of the chick embryo
Fig. 319. Section of eye of a human embryo
Fig. 320. The human ear
Fig. 321. The bony labyrinth
Fig. 322. Development of the labyrinth
Fig. 323. Primitive skull of human embryo
Fig. 324. Rudimentary muscles of the ear
Figs. 325–326. The human skeleton
Fig. 327. The human vertebral column
Fig. 328. Piece of the dorsal cord
Figs. 329–330. Dorsal vertebræ
Fig. 331. Intervertebral disk
Fig. 332. Human skull
Fig. 333. Skull of new-born child
Fig. 334. Head-skeleton of a primitive fish
Fig. 335. Skulls of nine primates
Figs. 336–338. Evolution of the fin
Fig. 339. Skeleton of the fore-leg of an amphibian
Fig. 340. Skeleton of gorilla’s hand
Fig. 341. Skeleton of human hand
Fig. 342. Skeleton of hand of six mammals
Figs. 343–345. Arm and hand of three anthropoids
Fig. 346. Section of fish’s tail
Fig. 347. Human skeleton
Fig. 348. Skeleton of the giant gorilla

Fig. 349. The human stomach
Fig. 350. Section of the head of a rabbit-embryo
Fig. 351. Shark’s teeth
Fig. 352. Gut of a human embryo
Figs. 353–354. Gut of a dog embryo
Figs. 355–356. Sections of head of lamprey
Fig. 357. Viscera of a human embryo
Fig. 358. Red blood-cells
Fig. 359. Vascular tissue
Fig. 360. Section of trunk of a chick-embryo
Fig. 361. Merocytes
Fig. 362. Vascular system of an annelid
Fig. 363. Head of a fish-embryo
Figs. 364–366. The five arterial arches
Figs. 367–370. The five arterial arches
Figs. 371–372. Heart of a rabbit-embryo
Figs. 373–374. Heart of a dog-embryo
Figs. 375–377. Heart of a human embryo
Fig. 378. Heart of adult man
Fig. 379. Section of head of a chick-embryo
Fig. 380. Section of a human embryo
Figs. 381–382. Sections of a chick-embryo
Fig. 383. Embryos of sagitta
Fig. 384. Kidneys of bdellostoma
Fig. 385. Section of embryonic shield
Figs. 386–387. Primitive kidneys
Fig. 388. Pig-embryo
Fig. 389. Human embryo
Figs. 390–392. Rudimentary kidneys and sexual organs
Figs. 393–394. Urinary and sexual organs of salamander
Fig. 395. Primitive kidneys of human embryo
Figs. 396–398. Urinary organs of ox-embryos
Fig. 399. Sexual organs of water-mole
Figs. 400–401. Original position of sexual glands
Fig. 402. Urogenital system of human embryo
Fig. 403. Section of ovary
Figs. 404–406. Graafian follicles

Fig. 407. A ripe graafian follicle
Fig. 408. The human ovum

GLOSSARY

Acrania: animals without skull (cranium).
Anthropogeny: the evolution (genesis) of man (anthropos).
Anthropology: the science of man.
Archi-: (in compounds) the first or typical—as, archi-cytula, archi-gastrula,
etc.
Biogeny: the science of the genesis of life (bios).
Blast-: (in compounds) pertaining to the early embryo (blastos = a bud);
hence:—
Blastoderm: skin (derma) or enclosing layer of the embryo.
Blastosphere: the embryo in the hollow sphere stage.
Blastula: same as preceding.
Epiblast: the outer layer of the embryo (ectoderm).
Hypoblast: the inner layer of the embryo (endoderm).
Branchial: pertaining to the gills (branchia).
Caryo-: (in compounds) pertaining to the nucleus (caryon); hence:—
Caryokineses: the movement of the nucleus.
Caryolysis: dissolution of the nucleus.
Caryoplasm: the matter of the nucleus.
Centrolecithal: see under Lecith-.
Chordaria and Chordonia: animals with a dorsal chord or back-bone.
Cœlom or Cœloma: the body-cavity in the embryo; hence:—
Cœlenterata: animals without a body-cavity.
Cœlomaria: animals with a body-cavity.
Cœlomation: formation of the body-cavity.
Cyto-: (in compounds) pertaining to the cell (cytos); hence:—
Cytoblast: the nucleus of the cell.
Cytodes: cell-like bodies, imperfect cells.
Cytoplasm: the matter of the body of the cell.
Cytosoma: the body (soma) of the cell.

Cryptorchism: abnormal retention of the testicles in the body.
Deutoplasm: see Plasm.
Dualism: the belief in the existence of two entirely distinct principles (such
as matter and spirit).
Dysteleology: the science of those features in organisms which refute the
“design-argument”.
Ectoderm: the outer (ekto) layer of the embryo.
Entoderm: the inner (ento) layer of the embryo.
Epiderm: the outer layer of the skin.
Epigenesis: the theory of gradual development of organs in the embryo.
Epiphysis: the third or central eye in the early vertebrates.
Episoma: see Soma.
Epithelia: tissues covering the surface of parts of the body (such as the
mouth, etc.)
Gonads: the sexual glands.
Gonochorism: separation of the male and female sexes.
Gonotomes: sections of the sexual glands.
Gynecomast: a male with the breasts (masta) of a woman (gyne).
Hepatic: pertaining to the liver (hepar).
Holoblastic: embryos in which the animal and vegetal cells divide equally
(holon = whole).
Hypermastism: the possession of more than the normal breasts (masta).
Hypobranchial: underneath (hypo) the gills.
Hypophysis: sensitive-offshoot from the brain in the vertebrate.
Hyposoma: see Soma.
Lecith-: pertaining to the yelk (lecithus); hence:—
Centrolecithal: eggs with the yelk in the centre.
Lecithoma: the yelk-sac.
Telolecithal: eggs with the yelk at one end.
Meroblastic: cleaving in part (meron) only.
Meta-: (in compounds) the “after” or secondary stage; hence:—
Metagaster: the secondary or permanent gut (gaster).
Metaplasm: secondary or differentiated plasm.
Metastoma: the secondary or permanent mouth (stoma).
Metazoa: the higher or later animals, made up of many cells.
Metovum: the mature or advanced ovum.
Metamera: the segments into which the embryo breaks up.

Metamerism: the segmentation of the embryo.
Monera: the most primitive of the unicellular organisms.
Monism: belief in the fundamental unity of all things.
Morphology: the science of organic forms (generally equivalent to
anatomy).
Myotomes: segments into which the muscles break up.
Nephra: the kidneys; hence:—
Nephridia: the rudimentary kidney-organs.
Nephrotomes: the segments of the developing kidneys.
Ontogeny: the science of the development of the individual (generally
equivalent to embryology).
Perigenesis: the genesis of the movements in the vital particles.
Phagocytes: cells that absorb food (phagein = to eat).
Phylogeny: the science of the evolution of species (phyla).
Planocytes: cells that move about (planein).
Plasm: the colloid or jelly-like matter of which organisms are composed;
hence:—
Caryoplasm: the matter of the nucleus (caryon).
Cytoplasm: the matter of the body of the cell.
Deutoplasm: secondary or differentiated plasm.
Metaplasm: secondary or differentiated plasm.
Protoplasm: primitive or undifferentiated plasm.
Plasson: the simplest form of plasm.
Plastidules: small particles of plasm.
Polyspermism: the penetration of more than one sperm-cell into the ovum.
Pro- or Prot: (in compounds) the earlier form (opposed to Meta); hence:—
Prochorion: the first form of the chorion.
Progaster: the first or primitive stomach.
Pronephridia: the earlier form of the kidneys.
Prorenal: the earlier form of the kidneys.
Prostoma: the first or primitive mouth.
Protists: the earliest or unicellular organisms.
Provertebræ: the earliest phase of the vertebræ.
Protophyta: the primitive or unicellular plants.
Protoplasm: undifferentiated plasm.
Protozoa: the primitive or unicellular animals.
Renal: pertaining to the kidneys (renes).

Scatulation: packing or boxing-up (scatula = a box).
Sclerotomes: segments into which the primitive skeleton falls.
Soma: the body; hence:—
Cytosoma: the body of the cell (cytos).
Episoma: the upper or back-half of the embryonic body.
Somites: segments of the embryonic body.
Hyposoma: the under or belly-half of the embryonic body.
Teleology: the belief in design and purpose (telos) in nature.
Telolecithal: see Lecith-.
Umbilical: pertaining to the navel (umbilicus).
Vitelline: pertaining to the yelk (vitellus).

PREFACE

[BY JOSEPH MCCABE]

The work which we now place within the reach of every reader of the
English tongue is one of the finest productions of its distinguished author.
The first edition appeared in 1874. At that time the conviction of man’s
natural evolution was even less advanced in Germany than in England, and
the work raised a storm of controversy. Theologians—forgetting the
commonest facts of our individual development—spoke with the most
profound disdain of the theory that a Luther or a Goethe could be the
outcome of development from a tiny speck of protoplasm. The work, one of
the most distinguished of them said, was “a fleck of shame on the
escutcheon of Germany.” To-day its conclusion is accepted by influential
clerics, such as the Dean of Westminster, and by almost every biologist and
anthropologist of distinction in Europe. Evolution is not a laboriously
reached conclusion, but a guiding truth, in biological literature to-day.
There was ample evidence to substantiate the conclusion even in the first
edition of the book. But fresh facts have come to light in each decade,
always enforcing the general truth of man’s evolution, and at times making
clearer the line of development. Professor Haeckel embodied these in
successive editions of his work. In the fifth edition, of which this is a
translation, reference will be found to the very latest facts bearing on the
evolution of man, such as the discovery of the remarkable effect of mixing
human blood with that of the anthropoid ape. Moreover, the ample series of
illustrations has been considerably improved and enlarged; there is no
scientific work published, at a price remotely approaching that of the
present edition, with so abundant and excellent a supply of illustrations.
When it was issued in Germany, a few years ago, a distinguished biologist
wrote in the Frankfurter Zeitung that it would secure immortality for its
author, the most notable critic of the idea of immortality. And the Daily

Telegraph reviewer described the English version as a “handsome edition of
Haeckel’s monumental work,” and “an issue worthy of the subject and the
author.”
The influence of such a work, one of the most constructive that Haeckel
has ever written, should extend to more than the few hundred readers who
are able to purchase the expensive volumes of the original issue. Few pages
in the story of science are more arresting and generally instructive than this
great picture of “mankind in the making.” The horizon of the mind is
healthily expanded as we follow the search-light of science down the vast
avenues of past time, and gaze on the uncouth forms that enter into, or
illustrate, the line of our ancestry. And if the imagination recoils from the
strange and remote figures that are lit up by our search-light, and hesitates
to accept them as ancestral forms, science draws aside another veil and
reveals another picture to us. It shows us that each of us passes, in our
embryonic development, through a series of forms hardly less uncouth and
unfamiliar. Nay, it traces a parallel between the two series of forms. It
shows us man beginning his existence, in the ovary of the female infant, as
a minute and simple speck of jelly-like plasm. It shows us (from analogy)
the fertilised ovum breaking into a cluster of cohering cells, and folding and
curving, until the limb-less, head-less, long-tailed fœtus looks like a worm-
shaped body. It then points out how gill-slits and corresponding blood-
vessels appear, as in a lowly fish, and the fin-like extremities bud out and
grow into limbs, and so on; until, after a very clear ape-stage, the definite
human form emerges from the series of transformations.
It is with this embryological evidence for our evolution that the present
volume is concerned. There are illustrations in the work that will make the
point clear at a glance. Possibly too clear; for the simplicity of the idea and
the eagerness to apply it at every point have carried many, who borrow
hastily from Haeckel, out of their scientific depth. Haeckel has never shared
their errors, nor encouraged their superficiality. He insists from the outset
that a complete parallel could not possibly be expected. Embryonic life
itself is subject to evolution. Though there is a general and substantial law
—as most of our English and American authorities admit—that the
embryonic series of forms recalls the ancestral series of forms, the parallel
is blurred throughout and often distorted. It is not the obvious resemblance
of the embryos of different animals, and their general similarity to our
extinct ancestors in this or that organ, on which we must rest our case. A

careful study must be made of the various stages through which all embryos
pass, and an effort made to prove their real identity and therefore
genealogical relation.
This is a task of great subtlety and delicacy. Many scientists have worked
at it together with Professor Haeckel—I need only name our own Professor
Balfour and Professor Ray Lankester—and the scheme is fairly complete.
But the general reader must not expect that even so clear a writer as
Haeckel can describe these intricate processes without demanding his very
careful attention. Most of the chapters in the present volume (and the
second volume will be less difficult) are easily intelligible to all; but there
are points at which the line of argument is necessarily subtle and complex.
In the hope that most readers will be induced to master even these more
difficult chapters, I will give an outline of the characteristic argument of the
work. Haeckel’s distinctive services in regard to man’s evolution have been:
(1) The construction of a complete ancestral tree, though, of course, some
of the stages in it are purely conjectural, and not final; (2) The tracing of the
remarkable reproduction of ancestral forms in the embryonic development
of the individual. Naturally, he has not worked alone in either department.
The second volume of this work will embody the first of these two
achievements; the present one is mainly concerned with the latter. It will be
useful for the reader to have a synopsis of the argument and an explanation
of some of the chief terms invented or employed by the author.
The main theme of the work is that, in the course of their embryonic
development, all animals, including man, pass roughly and rapidly through
a series of forms which represents the succession of their ancestors in the
past. After a severe and extensive study of embryonic phenomena, Haeckel
has drawn up a “law” (in the ordinary scientific sense) to this effect, and has
called it “the biogenetic law,” or the chief law relating to the evolution
(genesis) of life (bios). This law is widely and increasingly accepted by
embryologists and zoologists. It is enough to quote a recent declaration of
the great American zoologist, President D. Starr Jordan: “It is, of course,
true that the life-history of the individual is an epitome of the life-history of
the race”; while a distinguished German zoologist (Sarasin) has described it
as being of the same use to the biologist as spectrum analysis is to the
astronomer.

But the reproduction of ancestral forms in the course of the embryonic
development is by no means always clear, or even always present. Many of
the embryonic phases do not recall ancestral stages at all. They may have
done so originally, but we must remember that the embryonic life itself has
been subject to adaptive changes for millions of years. All this is clearly
explained by Professor Haeckel. For the moment, I would impress on the
reader the vital importance of fixing the distinction from the start. He must
thoroughly familiarise himself with the meaning of five terms. Biogeny is
the development of life in general (both in the individual and the species),
or the sciences describing it. Ontogeny is the development (embryonic and
post-embryonic) of the individual (on), or the science describing it.
Phylogeny is the development of the race or stem (phulon), or the science
describing it. Roughly, ontogeny may be taken to mean embryology, and
phylogeny what we generally call evolution. Further, the embryonic
phenomena sometimes reproduce ancestral forms, and they are then called
palingenetic (from palin = again): sometimes they do not recall ancestral
forms, but are later modifications due to adaptation, and they are then called
cenogenetic (from kenos = new or foreign). These terms are now widely
used, but the reader of Haeckel must understand them thoroughly.
The first five chapters are an easy account of the history of embryology
and evolution. The sixth and seventh give an equally clear account of the
sexual elements and the process of conception. But some of the succeeding
chapters must deal with embryonic processes so unfamiliar, and pursue
them through so wide a range of animals in a brief space, that, in spite of
the 200 illustrations, they will offer difficulty to many a reader. As our aim
is to secure, not a superficial acquiescence in conclusions, but a fair
comprehension of the truths of science, we have retained these chapters.
However, I will give a brief and clear outline of the argument, so that the
reader with little leisure may realise their value.
When the animal ovum (egg-cell) has been fertilised, it divides and
subdivides until we have a cluster of cohering cells, externally not unlike a
raspberry or mulberry. This is the morula (= mulberry) stage. The cluster
becomes hollow, or filled with fluid in the centre, all the cells rising to the
surface. This is the blastula (hollow ball) stage. One half of the cluster then
bends or folds in upon the other, as one might do with a thin indiarubber
ball, and we get a vase-shaped body with hollow interior (the first stomach,
or “primitive gut”), an open mouth (the first or “primitive mouth”), and a

wall composed of two layers of cells (two “germinal layers”). This is the
gastrula (stomach) stage, and the process of its formation is called
gastrulation. A glance at the illustration (Fig. 29) will make this perfectly
clear.
So much for the embryonic process in itself. The application to evolution
has been a long and laborious task. Briefly, it was necessary to show that all
the multicellular animals passed through these three stages, so that our
biogenetic law would enable us to recognise them as reminiscences of
ancestral forms. This is the work of Chapters VIII and IX. The difficulty
can be realised in this way: As we reach the higher animals the ovum has to
take up a large quantity of yelk, on which it may feed in developing. Think
of the bird’s “egg.” The effect of this was to flatten the germ (the morula
and blastula) from the first, and so give, at first sight, a totally different
complexion to what it has in the lowest animals. When we pass the reptile
and bird stage, the large yelk almost disappears (the germ now being
supplied with blood by the mother), but the germ has been permanently
altered in shape, and there are now a number of new embryonic processes
(membranes, blood-vessel connections, etc.). Thus it was no light task to
trace the identity of this process of gastrulation in all the animals. It has
been done, however; and with this introduction the reader will be able to
follow the proof. The conclusion is important. If all animals pass through
the curious gastrula stage, it must be because they all had a common
ancestor of that nature. To this conjectural ancestor (it lived before the
period of fossilisation begins) Haeckel gives the name of the Gastræa, and
in the second volume we shall see a number of living animals of this type
(“gastræads”).
The line of argument is the same in the next chapter. After laborious and
careful research (though this stage is not generally admitted in the same
sense as the previous one), a fourth common stage was discovered, and
given the name of the Cœlomula. The blastula had one layer of cells, the
blastoderm (derma = skin): the gastrula two layers, the ectoderm (“outer
skin”) and entoderm (“inner skin”). Now a third layer (mesoderm = middle
skin) is formed, by the growth inwards of two pouches or folds of the skin.
The pouches blend together, and form a single cavity (the body cavity, or
cœlom), and its two walls are two fresh “germinal layers.” Again, the
identity of the process has to be proved in all the higher classes of animals,
and when this is done we have another ancestral stage, the Cœlomæa.

The remaining task is to build up the complex frame of the higher
animals—always showing the identity of the process (on which the
evolutionary argument depends) in enormously different conditions of
embryonic life—out of the four “germinal layers.” Chapter IX prepares us
for the work by giving us a very clear account of the essential structure of
the back-boned (vertebrate) animal, and the probable common ancestor of
all the vertebrates (a small fish of the lancelet type). Chapters XI–XIV then
carry out the construction step by step. The work is now simpler, in the
sense that we leave all the invertebrate animals out of account; but there are
so many organs to be fashioned out of the four simple layers that the reader
must proceed carefully. In the second volume each of these organs will be
dealt with separately, and the parallel will be worked out between its
embryonic and its phylogenetic (evolutionary) development. The general
reader may wait for this for a full understanding. But in the meantime the
wonderful story of the construction of all our organs in the course of a few
weeks (the human frame is perfectly formed, though less than two inches in
length, by the twelfth week) from so simple a material is full of interest. It
would be useless to attempt to summarise the process. The four chapters are
themselves but a summary of it, and the eighty fine illustrations of the
process will make it sufficiently clear. The last chapter carries the story on
to the point where man at last parts company with the anthropoid ape, and
gives a full account of the membranes or wrappers that enfold him in the
womb, and the connection with the mother.
In conclusion, I would urge the reader to consult, at his free library
perhaps, the complete edition of this work, when he has read the present
abbreviated edition. Much of the text has had to be condensed in order to
bring out the work at our popular price, and the beautiful plates of the
complete edition have had to be omitted. The reader will find it an immense
assistance if he can consult the library edition.

JOSEPH MCCABE

Cricklewood, March, 1906.

HAECKEL’S CLASSIFICATION OF THE
ANIMAL WORLD

Unicellular animals (Protozoa)
Bacteria
1. Unnucleated Monera
Protamæbæ
Amœbina
a. Rhizopoda
Radiolaria
2. Nucleated
Flagellata
b. Infusoria
Ciliata
Catallacta
3. Cell-colonies
Blastæada

Unicellular animals (Protozoa)
Gastremaria
a. Gastræads
Cyemaria
Protospongiæ
I b. Sponges
Metaspongiæ
Cœlenterata,
or Zoophytes. c. Cnidaria Hydrozoa
(stinging Polyps
Animals without
body-cavity, animals)
Medusæ
blood, or anus. Platodaria
d. Platodes Turbullaria
(flat-worms) Trematoda
Cestoda
II a. Vermalia Rotatoria
Cœlomaria or (worm-like) Strongylaria
Bilaterals.

Animals with Prosopygia
body-cavity and Frontonia
anus, and Cochlides
generally b. Molluscs Conchades
blood. Teuthodes
Annelida
c. Articulates Crustacea
Tracheata
Monorchonia
d. Echinoderms
Pentorchonia
Copelata
e. Tunicates Ascidiæ
Thalidiæ
f. Vertebrates I. Acrania-Lancelet
(without skull)
II. Craniota
(with skull)
a. Cyclostomes
(“round-
mouthed”)
Selachii
Ganoids
b. Fishes
Teleosts
Dipneusts
c. Amphibia
d. Reptiles
e. Birds
f. Mammal Monotremes
Marsupials
Placentals:
Rodents
Edentates
Ungulates
Cetacea
Sirenia
Insectivora

Cheiroptera
Carnassia
Primates
(This classification is given for the purpose of explaining Haeckel’s use of terms
in this volume. The general reader should bear in mind that it differs very
considerably from more recent schemes of classification. He should compare the
scheme framed by Professor E. Ray Lankester.)

THE EVOLUTION OF MAN

Chapter I.
THE FUNDAMENTAL LAW OF ORGANIC
EVOLUTION

The field of natural phenomena into which I would introduce my readers
in the following chapters has a quite peculiar place in the broad realm of
scientific inquiry. There is no object of investigation that touches man more
closely, and the knowledge of which should be more acceptable to him,
than his own frame. But among all the various branches of the natural
history of mankind, or anthropology, the story of his development by
natural means must excite the most lively interest. It gives us the key of the
great world-riddles at which the human mind has been working for
thousands of years. The problem of the nature of man, or the question of
man’s place in nature, and the cognate inquiries as to the past, the earliest
history, the present situation, and the future of humanity—all these most
important questions are directly and intimately connected with that branch
of study which we call the science of the evolution of man, or, in one word,
“Anthropogeny” (the genesis of man). Yet it is an astonishing fact that the
science of the evolution of man does not even yet form part of the scheme
of general education. In fact, educated people even in our day are for the
most part quite ignorant of the important truths and remarkable phenomena
which anthropogeny teaches us.
As an illustration of this curious state of things, it may be pointed out that
most of what are considered to be “educated” people do not know that
every human being is developed from an egg, or ovum, and that this egg is
one simple cell, like any other plant or animal egg. They are equally
ignorant that in the course of the development of this tiny, round egg-cell
there is first formed a body that is totally different from the human frame,
and has not the remotest resemblance to it. Most of them have never seen
such a human embryo in the earlier period of its development, and do not

know that it is quite indistinguishable from other animal embryos. At first
the embryo is no more than a round cluster of cells, then it becomes a
simple hollow sphere, the wall of which is composed of a layer of cells.
Later it approaches very closely, at one period, to the anatomic structure of
the lancelet, afterwards to that of a fish, and again to the typical build of the
amphibia and mammals. As it continues to develop, a form appears which is
like those we find at the lowest stage of mammal-life (such as the duck-
bills), then a form that resembles the marsupials, and only at a late stage a
form that has a resemblance to the ape; until at last the definite human form
emerges and closes the series of transformations. These suggestive facts are,
as I said, still almost unknown to the general public—so completely
unknown that, if one casually mentions them, they are called in question or
denied outright as fairy-tales. Everybody knows that the butterfly emerges
from the pupa, and the pupa from a quite different thing called a larva, and
the larva from the butterfly’s egg. But few besides medical men are aware
that man, in the course of his individual formation, passes through a series
of transformations which are not less surprising and wonderful than the
familiar metamorphoses of the butterfly.
The mere description of these remarkable changes through which man
passes during his embryonic life should arouse considerable interest. But
the mind will experience a far keener satisfaction when we trace these
curious facts to their causes, and when we learn to behold in them natural
phenomena which are of the highest importance throughout the whole field
of human knowledge. They throw light first of all on the “natural history of
creation,” then on psychology, or “the science of the soul,” and through this
on the whole of philosophy. And as the general results of every branch of
inquiry are summed up in philosophy, all the sciences come in turn to be
touched and influenced more or less by the study of the evolution of man.
But when I say that I propose to present here the most important features
of these phenomena and trace them to their causes, I take the term, and I
interpret my task, in a very much wider sense than is usual. The lectures
which have been delivered on this subject in the universities during the last
half-century are almost exclusively adapted to medical men. Certainly, the
medical man has the greatest interest in studying the origin of the human
body, with which he is daily occupied. But I must not give here this special
description of the embryonic processes such as it has hitherto been given, as
most of my readers have not studied anatomy, and are not likely to be

entrusted with the care of the adult organism. I must content myself with
giving some parts of the subject only in general outline, and must not enter
upon all the marvellous, but very intricate and not easily described, details
that are found in the story of the development of the human frame. To
understand these fully a knowledge of anatomy is needed. I will endeavour
to be as plain as possible in dealing with this branch of science. Indeed, a
sufficient general idea of the course of the embryonic development of man
can be obtained without going too closely into the anatomic details. I trust
we may be able to arouse the same interest in this delicate field of inquiry
as has been excited already in other branches of science; though we shall
meet more obstacles here than elsewhere.
The story of the evolution of man, as it has hitherto been expounded to
medical students, has usually been confined to embryology—more
correctly, ontogeny—or the science of the development of the individual
human organism. But this is really only the first part of our task, the first
half of the story of the evolution of man in that wider sense in which we
understand it here. We must add as the second half—as another and not less
important and interesting branch of the science of the evolution of the
human stem—phylogeny: this may be described as the science of the
evolution of the various animal forms from which the human organism has
been developed in the course of countless ages. Everybody now knows of
the great scientific activity that was occasioned by the publication of
Darwin’s Origin of Species in 1859. The chief direct consequence of this
publication was to provoke a fresh inquiry into the origin of the human
race, and this has proved beyond question our gradual evolution from the
lower species. We give the name of “Phylogeny” to the science which
describes this ascent of man from the lower ranks of the animal world. The
chief source that it draws upon for facts is “Ontogeny,” or embryology, the
science of the development of the individual organism. Moreover, it derives
a good deal of support from paleontology, or the science of fossil remains,
and even more from comparative anatomy, or morphology.
These two branches of our science—on the one side ontogeny or
embryology, and on the other phylogeny, or the science of race-evolution—
are most vitally connected. The one cannot be understood without the other.
It is only when the two branches fully co-operate and supplement each
other that “Biogeny” (or the science of the genesis of life in the widest
sense) attains to the rank of a philosophic science. The connection between

them is not external and superficial, but profound, intrinsic, and causal. This
is a discovery made by recent research, and it is most clearly and correctly
expressed in the comprehensive law which I have called “the fundamental
law of organic evolution,” or “the fundamental law of biogeny.” This
general law, to which we shall find ourselves constantly recurring, and on
the recognition of which depends one’s whole insight into the story of
evolution, may be briefly expressed in the phrase: “The history of the fœtus
is a recapitulation of the history of the race”; or, in other words, “Ontogeny
is a recapitulation of phylogeny.” It may be more fully stated as follows:
The series of forms through which the individual organism passes during its
development from the ovum to the complete bodily structure is a brief,
condensed repetition of the long series of forms which the animal ancestors
of the said organism, or the ancestral forms of the species, have passed
through from the earliest period of organic life down to the present day.
The causal character of the relation which connects embryology with
stem-history is due to the action of heredity and adaptation. When we have
rightly understood these, and recognised their great importance in the
formation of organisms, we can go a step further and say: Phylogenesis is
the mechanical cause of ontogenesis.[1] In other words, the development of
the stem, or race, is, in accordance with the laws of heredity and adaptation,
the cause of all the changes which appear in a condensed form in the
evolution of the fœtus.
[1] The term “genesis,” which occurs throughout, means, of course, “birth” or
origin. From this we get: Biogeny = the origin of life (bios); Anthropogeny = the
origin of man (anthropos); Ontogeny = the origin of the individual (on);
Phylogeny = the origin of the species (phulon); and so on. In each case the term
may refer to the process itself, or to the science describing the process.—
Translator.

The chain of manifold animal forms which represent the ancestry of each
higher organism, or even of man, according to the theory of descent, always
form a connected whole. We may designate this uninterrupted series of
forms with the letters of the alphabet: A, B, C, D, E, etc., to Z. In apparent
contradiction to what I have said, the story of the development of the
individual, or the ontogeny of most organisms, only offers to the observer a
part of these forms; so that the defective series of embryonic forms would
run: A, B, D, F, H, K, M, etc.; or, in other cases, B, D, H, L, M, N, etc.
Here, then, as a rule, several of the evolutionary forms of the original series

have fallen out. Moreover, we often find—to continue with our illustration
from the alphabet—one or other of the original letters of the ancestral series
represented by corresponding letters from a different alphabet. Thus,
instead of the Roman B and D, we often have the Greek Β and Δ. In this
case the text of the biogenetic law has been corrupted, just as it had been
abbreviated in the preceding case. But, in spite of all this, the series of
ancestral forms remains the same, and we are in a position to discover its
original complexion.
In reality, there is always a certain parallel between the two evolutionary
series. But it is obscured from the fact that in the embryonic succession
much is wanting that certainly existed in the earlier ancestral succession. If
the parallel of the two series were complete, and if this great fundamental
law affirming the causal connection between ontogeny and phylogeny in the
proper sense of the word were directly demonstrable, we should only have
to determine, by means of the microscope and the dissecting knife, the
series of forms through which the fertilised ovum passes in its development;
we should then have before us a complete picture of the remarkable series
of forms which our animal ancestors have successively assumed from the
dawn of organic life down to the appearance of man. But such a repetition
of the ancestral history by the individual in its embryonic life is very rarely
complete. We do not often find our full alphabet. In most cases the
correspondence is very imperfect, being greatly distorted and falsified by
causes which we will consider later. We are thus, for the most part, unable
to determine in detail, from the study of its embryology, all the different
shapes which an organism’s ancestors have assumed; we usually—and
especially in the case of the human fœtus—encounter many gaps. It is true
that we can fill up most of these gaps satisfactorily with the help of
comparative anatomy, but we cannot do so from direct embryological
observation. Hence it is important that we find a large number of lower
animal forms to be still represented in the course of man’s embryonic
development. In these cases we may draw our conclusions with the utmost
security as to the nature of the ancestral form from the features of the form
which the embryo momentarily assumes.
To give a few examples, we can infer from the fact that the human ovum
is a simple cell that the first ancestor of our species was a tiny unicellular
being, something like the amœba. In the same way, we know, from the fact
that the human fœtus consists, at the first, of two simple cell-layers (the

gastrula), that the gastræa, a form with two such layers, was certainly in
the line of our ancestry. A later human embryonic form (the chordula)
points just as clearly to a worm-like ancestor (the prochordonia), the nearest
living relation of which is found among the actual ascidiæ. To this succeeds
a most important embryonic stage (acrania), in which our headless fœtus
presents, in the main, the structure of the lancelet. But we can only
indirectly and approximately, with the aid of comparative anatomy and
ontogeny, conjecture what lower forms enter into the chain of our ancestry
between the gastræa and the chordula, and between this and the lancelet. In
the course of the historical development many intermediate structures have
gradually fallen out, which must certainly have been represented in our
ancestry. But, in spite of these many, and sometimes very appreciable, gaps,
there is no contradiction between the two successions. In fact, it is the chief
purpose of this work to prove the real harmony and the original parallelism
of the two. I hope to show, on a substantial basis of facts, that we can draw
most important conclusions as to our genealogical tree from the actual and
easily-demonstrable series of embryonic changes. We shall then be in a
position to form a general idea of the wealth of animal forms which have
figured in the direct line of our ancestry in the lengthy history of organic
life.
In this evolutionary appreciation of the facts of embryology we must, of
course, take particular care to distinguish sharply and clearly between the
primitive, palingenetic (or ancestral) evolutionary processes and those due
to cenogenesis.[2] By palingenetic processes, or embryonic recapitulations,
we understand all those phenomena in the development of the individual
which are transmitted from one generation to another by heredity, and
which, on that account, allow us to draw direct inferences as to
corresponding structures in the development of the species. On the other
hand, we give the name of cenogenetic processes, or embryonic variations,
to all those phenomena in the fœtal development that cannot be traced to
inheritance from earlier species, but are due to the adaptation of the fœtus,
or the infant-form, to certain conditions of its embryonic development.
These cenogenetic phenomena are foreign or later additions; they allow us
to draw no direct inference whatever as to corresponding processes in our
ancestral history, but rather hinder us from doing so.
[2] Palingenesis = new birth, or re-incarnation (palin = again, genesis or genea =
development); hence its application to the phenomena which are recapitulated by

heredity from earlier ancestral forms. Cenogenesis = foreign or negligible
development (kenos and genea); hence, those phenomena which come later in
the story of life to disturb the inherited structure, by a fresh adaptation to
environment.—Translator.

This careful discrimination between the primary or palingenetic
processes and the secondary or cenogenetic is of great importance for the
purposes of the scientific history of a species, which has to draw
conclusions from the available facts of embryology, comparative anatomy,
and paleontology, as to the processes in the formation of the species in the
remote past. It is of the same importance to the student of evolution as the
careful distinction between genuine and spurious texts in the works of an
ancient writer, or the purging of the real text from interpolations and
alterations, is for the student of philology. It is true that this distinction has
not yet been fully appreciated by many scientists. For my part, I regard it as
the first condition for forming any just idea of the evolutionary process, and
I believe that we must, in accordance with it, divide embryology into two
sections—palingenesis, or the science of recapitulated forms; and
cenogenesis, or the science of supervening structures.
To give at once a few examples from the science of man’s origin in
illustration of this important distinction, I may instance the following
processes in the embryology of man, and of all the higher vertebrates, as
palingenetic: the formation of the two primary germinal layers and of the
primitive gut, the undivided structure of the dorsal nerve-tube, the
appearance of a simple axial rod between the medullary tube and the gut,
the temporary formation of the gill-clefts and arches, the primitive kidneys,
and so on.[3] All these, and many other important structures, have clearly
been transmitted by a steady heredity from the early ancestors of the
mammal, and are, therefore, direct indications of the presence of similar
structures in the history of the stem. On the other hand, this is certainly not
the case with the following embryonic forms, which we must describe as
cenogenetic processes: the formation of the yelk-sac, the allantois, the
placenta, the amnion, the serolemma, and the chorion—or, generally
speaking, the various fœtal membranes and the corresponding changes in
the blood vessels. Further instances are: the dual structure of the heart
cavity, the temporary division of the plates of the primitive vertebræ and
lateral plates, the secondary closing of the ventral and intestinal walls, the
formation of the navel, and so on. All these and many other phenomena are

certainly not traceable to similar structures in any earlier and completely-
developed ancestral form, but have arisen simply by adaptation to the
peculiar conditions of embryonic life (within the fœtal membranes). In view
of these facts, we may now give the following more precise expression to
our chief law of biogeny: The evolution of the fœtus (or ontogenesis) is a
condensed and abbreviated recapitulation of the evolution of the stem (or
phylogenesis); and this recapitulation is the more complete in proportion as
the original development (or palingenesis) is preserved by a constant
heredity; on the other hand, it becomes less complete in proportion as a
varying adaptation to new conditions increases the disturbing factors in the
development (or cenogenesis).
[3] All these, and the following structures, will be fully described in later
chapters.—Translator.

The cenogenetic alterations or distortions of the original palingenetic
course of development take the form, as a rule, of a gradual displacement of
the phenomena, which is slowly effected by adaptation to the changed
conditions of embryonic existence during the course of thousands of years.
This displacement may take place as regards either the position or the time
of a phenomenon.
The great importance and strict regularity of the time-variations in
embryology have been carefully studied recently by Ernest Mehnert, in his
Biomechanik (Jena, 1898). He contends that our biogenetic law has not
been impaired by the attacks of its opponents, and goes on to say: “Scarcely
any piece of knowledge has contributed so much to the advance of
embryology as this; its formulation is one of the most signal services to
general biology. It was not until this law passed into the flesh and blood of
investigators, and they had accustomed themselves to see a reminiscence of
ancestral history in embryonic structures, that we witnessed the great
progress which embryological research has made in the last two decades.”
The best proof of the correctness of this opinion is that now the most
fruitful work is done in all branches of embryology with the aid of this
biogenetic law, and that it enables students to attain every year thousands of
brilliant results that they would never have reached without it.
It is only when one appreciates the cenogenetic processes in relation to
the palingenetic, and when one takes careful account of the changes which
the latter may suffer from the former, that the radical importance of the

biogenetic law is recognised, and it is felt to be the most illuminating
principle in the science of evolution. In this task of discrimination it is the
silver thread in relation to which we can arrange all the phenomena of this
realm of marvels—the “Ariadne thread,” which alone enables us to find our
way through this labyrinth of forms. Hence the brothers Sarasin, the
zoologists, could say with perfect justice, in their study of the evolution of
the Ichthyophis, that “the great biogenetic law is just as important for the
zoologist in tracing long-extinct processes as spectrum analyses is for the
astronomer.”
Even at an earlier period, when a correct acquaintance with the evolution
of the human and animal frame was only just being obtained—and that is
scarcely eighty years ago!—the greatest astonishment was felt at the
remarkable similarity observed between the embryonic forms, or stages of
fœtal development, in very different animals; attention was called even then
to their close resemblance to certain fully-developed animal forms
belonging to some of the lower groups. The older scientists (Oken,
Treviranus, and others) knew perfectly well that these lower forms in a
sense illustrated and fixed, in the hierarchy of the animal world, a
temporary stage in the evolution of higher forms. The famous anatomist
Meckel spoke in 1821 of a “similarity between the development of the
embryo and the series of animals.” Baer raised the question in 1828 how
far, within the vertebrate type, the embryonic forms of the higher animals
assume the permanent shapes of members of lower groups. But it was
impossible fully to understand and appreciate this remarkable resemblance
at that time. We owe our capacity to do this to the theory of descent; it is
this that puts in their true light the action of heredity on the one hand and
adaptation on the other. It explains to us the vital importance of their
constant reciprocal action in the production of organic forms. Darwin was
the first to teach us the great part that was played in this by the ceaseless
struggle for existence between living things, and to show how, under the
influence of this (by natural selection), new species were produced and
maintained solely by the interaction of heredity and adaptation. It was thus
Darwinism that first opened our eyes to a true comprehension of the
supremely important relations between the two parts of the science of
organic evolution—Ontogeny and Phylogeny.
Heredity and adaptation are, in fact, the two constructive physiological
functions of living things; unless we understand these properly we can

make no headway in the study of evolution. Hence, until the time of Darwin
no one had a clear idea of the real nature and causes of embryonic
development. It was impossible to explain the curious series of forms
through which the human embryo passed; it was quite unintelligible why
this strange succession of animal-like forms appeared in the series at all. It
had previously been generally assumed that the man was found complete in
all his parts in the ovum, and that the development consisted only in an
unfolding of the various parts, a simple process of growth. This is by no
means the case. On the contrary, the whole process of the development of
the individual presents to the observer a connected succession of different
animal-forms; and these forms display a great variety of external and
internal structure. But why each individual human being should pass
through this series of forms in the course of his embryonic development it
was quite impossible to say until Lamarck and Darwin established the
theory of descent. Through this theory we have at last detected the real
causes, the efficient causes, of the individual development; we have learned
that these mechanical causes suffice of themselves to effect the formation
of the organism, and that there is no need of the final causes which were
formerly assumed. It is true that in the academic philosophies of our time
these final causes still figure very prominently; in the new philosophy of
nature we can entirely replace them by efficient causes. We shall see, in the
course of our inquiry, how the most wonderful and hitherto insoluble
enigmas in the human and animal frame have proved amenable to a
mechanical explanation, by causes acting without prevision, through
Darwin’s reform of the science of evolution. We have everywhere been able
to substitute unconscious causes, acting from necessity, for conscious,
purposive causes.[4]
[4] The monistic or mechanical philosophy of nature holds that only
unconscious, necessary, efficient causes are at work in the whole field of nature,
in organic life as well as in inorganic changes. On the other hand, the dualist or
vitalist philosophy of nature affirms that unconscious forces are only at work in
the inorganic world, and that we find conscious, purposive, or final causes in
organic nature.

If the new science of evolution had done no more than this, every
thoughtful man would have to admit that it had accomplished an immense
advance in knowledge. It means that in the whole of philosophy that
tendency which we call monistic, in opposition to the dualistic, which has

hitherto prevailed, must be accepted.[5] At this point the science of human
evolution has a direct and profound bearing on the foundations of
philosophy. Modern anthropology has, by its astounding discoveries during
the second half of the nineteenth century, compelled us to take a completely
monistic view of life. Our bodily structure and its life, our embryonic
development and our evolution as a species, teach us that the same laws of
nature rule in the life of man as in the rest of the universe. For this reason, if
for no others, it is desirable, nay, indispensable, that every man who wishes
to form a serious and philosophic view of life, and, above all, the expert
philosopher, should acquaint himself with the chief facts of this branch of
science.
[5] Monism is neither purely materialistic nor purely spiritualistic, but a
reconciliation of these two principles, since it regards the whole of nature as one,
and sees only efficient causes at work in it. Dualism, on the contrary, holds that
nature and spirit, matter and force, the world and God, inorganic and organic
nature, are separate and independent existences. Cf. The Riddle of the Universe,
chap. xii.

The facts of embryology have so great and obvious a significance in this
connection that even in recent years dualist and teleological philosophers
have tried to rid themselves of them by simply denying them. This was
done, for instance, as regards the fact that man is developed from an egg,
and that this egg or ovum is a simple cell, as in the case of other animals.
When I had explained this pregnant fact and its significance in my History
of Creation, it was described in many of the theological journals as a
dishonest invention of my own. The fact that the embryos of man and the
dog are, at a certain stage of their development, almost indistinguishable
was also denied. When we examine the human embryo in the third or fourth
week of its development, we find it to be quite different in shape and
structure from the full-grown human being, but almost identical with that of
the ape, the dog, the rabbit, and other mammals, at the same stage of
ontogeny. We find a bean-shaped body of very simple construction, with a
tail below and a pair of fins at the sides, something like those of a fish, but
very different from the limbs of man and the mammals. Nearly the whole
front half of the body is taken up by a shapeless head without face, at the
sides of which we find gill-clefts and arches as in the fish. At this stage of
its development the human embryo does not differ in any essential detail
from that of the ape, dog, horse, ox, etc., at a corresponding period. This

important fact can easily be verified at any moment by a comparison of the
embryos of man, the dog, rabbit, etc. Nevertheless, the theologians and
dualist philosophers pronounced it to be a materialistic invention; even
scientists, to whom the facts should be known, have sought to deny them.
There could not be a clearer proof of the profound importance of these
embryological facts in favour of the monistic philosophy than is afforded by
these efforts of its opponents to get rid of them by silence or denial. The
truth is that these facts are most inconvenient for them, and are quite
irreconcilable with their views. We must be all the more pressing on our
side to put them in their proper light. I fully agree with Huxley when he
says, in his Man’s Place in Nature: “Though these facts are ignored by
several well-known popular leaders, they are easy to prove, and are
accepted by all scientific men; on the other hand, their importance is so
great that those who have once mastered them will, in my opinion, find few
other biological discoveries to astonish them.”
We shall make it our chief task to study the evolution of man’s bodily
frame and its various organs in their external form and internal structures.
But I may observe at once that this is accompanied step by step with a study
of the evolution of their functions. These two branches of inquiry are
inseparably united in the whole of anthropology, just as in zoology (of
which the former is only a section) or general biology. Everywhere the
peculiar form of the organism and its structures, internal and external, is
directly related to the special physiological functions which the organism or
organ has to execute. This intimate connection of structure and function, or
of the instrument and the work done by it, is seen in the science of
evolution and all its parts. Hence the story of the evolution of structures,
which is our immediate concern, is also the history of the development of
functions; and this holds good of the human organism as of any other.
At the same time, I must admit that our knowledge of the evolution of
functions is very far from being as complete as our acquaintance with the
evolution of structures. One might say, in fact, that the whole science of
evolution has almost confined itself to the study of structures; the evolution
of functions hardly exists even in name. That is the fault of the
physiologists, who have as yet concerned themselves very little about
evolution. It is only in recent times that physiologists like W. Engelmann,

W. Preyer, M. Verworn, and a few others, have attacked the evolution of
functions.
It will be the task of some future physiologist to engage in the study of
the evolution of functions with the same zeal and success as has been done
for the evolution of structures in morphogeny (the science of the genesis of
forms). Let me illustrate the close connection of the two by a couple of
examples. The heart in the human embryo has at first a very simple
construction, such as we find in permanent form among the ascidiæ and
other low organisms; with this is associated a very simple system of
circulation of the blood. Now, when we find that with the full-grown heart
there comes a totally different and much more intricate circulation, our
inquiry into the development of the heart becomes at once, not only an
anatomical, but also a physiological, study. Thus it is clear that the
ontogeny of the heart can only be understood in the light of its phylogeny
(or development in the past), both as regards function and structure. The
same holds true of all the other organs and their functions. For instance, the
science of the evolution of the alimentary canal, the lungs, or the sexual
organs, gives us at the same time, through the exact comparative
investigation of structure-development, most important information with
regard to the evolution of the functions of these organs.
This significant connection is very clearly seen in the evolution of the
nervous system. This system is in the economy of the human body the
medium of sensation, will, and even thought, the highest of the psychic
functions; in a word, of all the various functions which constitute the proper
object of psychology. Modern anatomy and physiology have proved that
these psychic functions are immediately dependent on the fine structure and
the composition of the central nervous system, or the internal texture of the
brain and spinal cord. In these we find the elaborate cell-machinery, of
which the psychic or soul-life is the physiological function. It is so intricate
that most men still look upon the mind as something supernatural that
cannot be explained on mechanical principles.
But embryological research into the gradual appearance and the
formation of this important system of organs yields the most astounding and
significant results. The first sketch of a central nervous system in the human
embryo presents the same very simple type as in the other vertebrates. A
spinal tube is formed in the external skin of the back, and from this first

comes a simple spinal cord without brain, such as we find to be the
permanent psychic organ in the lowest type of vertebrate, the amphioxus.
Not until a later stage is a brain formed at the anterior end of this cord, and
then it is a brain of the most rudimentary kind, such as we find permanently
among the lower fishes. This simple brain develops step by step,
successively assuming forms which correspond to those of the amphibia,
the reptiles, the duck-bills, and the lemurs. Only in the last stage does it
reach the highly organised form which distinguishes the apes from the other
vertebrates, and which attains its full development in man.
Comparative physiology discovers a precisely similar growth. The
function of the brain, the psychic activity, rises step by step with the
advancing development of its structure.
Thus we are enabled, by this story of the evolution of the nervous system,
to understand at length the natural development of the human mind and its
gradual unfolding. It is only with the aid of embryology that we can grasp
how these highest and most striking faculties of the animal organism have
been historically evolved. In other words, a knowledge of the evolution of
the spinal cord and brain in the human embryo leads us directly to a
comprehension of the historic development (or phylogeny) of the human
mind, that highest of all faculties, which we regard as something so
marvellous and supernatural in the adult man. This is certainly one of the
greatest and most pregnant results of evolutionary science. Happily our
embryological knowledge of man’s central nervous system is now so
adequate, and agrees so thoroughly with the complementary results of
comparative anatomy and physiology, that we are thus enabled to obtain a
clear insight into one of the highest problems of philosophy, the phylogeny
of the soul, or the ancestral history of the mind of man. Our chief support in
this comes from the embryological study of it, or the ontogeny of the soul.
This important section of psychology owes its origin especially to W.
Preyer, in his interesting works, such as The Mind of the Child. The
Biography of a Baby (1900), of Milicent Washburn Shinn, also deserves
mention. [See also Preyer’s Mental Development in the Child (translation),
and Sully’s Studies of Childhood and Children’s Ways.]
In this way we follow the only path along which we may hope to reach
the solution of this difficult problem.

Thirty-six years have now elapsed since, in my General Morphology, I
established phylogeny as an independent science and showed its intimate
causal connection with ontogeny; thirty years have passed since I gave in
my gastræa-theory the proof of the justice of this, and completed it with the
theory of germinal layers. When we look back on this period we may ask,
What has been accomplished during it by the fundamental law of biogeny?
If we are impartial, we must reply that it has proved its fertility in hundreds
of sound results, and that by its aid we have acquired a vast fund of
knowledge which we should never have obtained without it.
There has been no dearth of attacks—often violent attacks—on my
conception of an intimate causal connection between ontogenesis and
phylogenesis; but no other satisfactory explanation of these important
phenomena has yet been offered to us. I say this especially with regard to
Wilhelm His’s theory of a “mechanical evolution,” which questions the
truth of phylogeny generally, and would explain the complicated embryonic
processes without going beyond by simple physical changes—such as the
bending and folding of leaves by electricity, the origin of cavities through
unequal strain of the tissues, the formation of processes by uneven growth,
and so on. But the fact is that these embryological phenomena themselves
demand explanation in turn, and this can only be found, as a rule, in the
corresponding changes in the long ancestral series, or in the physiological
functions of heredity and adaptation.

Chapter II.
THE OLDER EMBRYOLOGY

It is in many ways useful, on entering upon the study of any science, to
cast a glance at its historical development. The saying that “everything is
best understood in its growth” has a distinct application to science. While
we follow its gradual development we get a clearer insight into its aims and
objects. Moreover, we shall see that the present condition of the science of
human evolution, with all its characteristics, can only be rightly understood
when we examine its historical growth. This task will, however, not detain
us long. The study of man’s evolution is one of the latest branches of
natural science, whether you consider the embryological or the
phylogenetic section of it.
Apart from the few germs of our science which we find in classical
antiquity, and which we shall notice presently, we may say that it takes its
definite rise, as a science, in the year 1759, when one of the greatest
German scientists, Caspar Friedrich Wolff, published his Theoria
generationis. That was the foundation-stone of the science of animal
embryology. It was not until fifty years later, in 1809, that Jean Lamarck
published his Philosophie Zoologique—the first effort to provide a base for
the theory of evolution; and it was another half-century before Darwin’s
work appeared (in 1859), which we may regard as the first scientific
attainment of this aim. But before we go further into this solid
establishment of evolution, we must cast a brief glance at that famous
philosopher and scientist of antiquity, who stood alone in this, as in many
other branches of science, for more than 2000 years: the “father of Natural
History,” Aristotle.
The extant scientific works of Aristotle deal with many different sides of
biological research; the most comprehensive of them is his famous History
of Animals. But not less interesting is the smaller work, On the Generation

of Animals (Peri zoon geneseos). This work treats especially of embryonic
development, and it is of great interest as being the earliest of its kind and
the only one that has come down to us in any completeness from classical
antiquity.
Aristotle studied embryological questions in various classes of animals,
and among the lower groups he learned many most remarkable facts which
we only rediscovered between 1830 and 1860. It is certain, for instance, that
he was acquainted with the very peculiar mode of propagation of the
cuttlefishes, or cephalopods, in which a yelk-sac hangs out of the mouth of
the fœtus. He knew, also, that embryos come from the eggs of the bee even
when they have not been fertilised. This “parthenogenesis” (or virgin-birth)
of the bees has only been established in our time by the distinguished
zoologist of Munich, Siebold. He discovered that male bees come from the
unfertilised, and female bees only from the fertilised, eggs. Aristotle further
states that some kinds of fishes (of the genus serranus) are hermaphrodites,
each individual having both male and female organs and being able to
fertilise itself; this, also, has been recently confirmed. He knew that the
embryo of many fishes of the shark family is attached to the mother’s body
by a sort of placenta, or nutritive organ very rich in blood; apart from these,
such an arrangement is only found among the higher mammals and man.
This placenta of the shark was looked upon as legendary for a long time,
until Johannes Müller proved it to be a fact in 1839. Thus a number of
remarkable discoveries were found in Aristotle’s embryological work,
proving a very good acquaintance of the great scientist—possibly helped by
his predecessors—with the facts of ontogeny, and a great advance upon
succeeding generations in this respect.
In the case of most of these discoveries he did not merely describe the
fact, but added a number of observations on its significance. Some of these
theoretical remarks are of particular interest, because they show a correct
appreciation of the nature of the embryonic processes. He conceives the
development of the individual as a new formation, in the course of which
the various parts of the body take shape successively. When the human or
animal frame is developed in the mother’s body, or separately in an egg, the
heart—which he regards as the starting-point and centre of the organism—
must appear first. Once the heart is formed the other organs arise, the
internal ones before the external, the upper (those above the diaphragm)
before the lower (or those beneath the diaphragm). The brain is formed at

an early stage, and the eyes grow out of it. These observations are quite
correct. And, if we try to form some idea from these data of Aristotle’s
general conception of the embryonic process, we find a dim prevision of the
theory which Wolff showed 2000 years afterwards to be the correct view. It
is significant, for instance, that Aristotle denied the eternity of the
individual in any respect. He said that the species or genus, the group of
similar individuals, might be eternal, but the individual itself is temporary.
It comes into being in the act of procreation, and passes away at death.
During the 2000 years after Aristotle no progress whatever was made in
general zoology, or in embryology in particular. People were content to
read, copy, translate, and comment on Aristotle. Scarcely a single
independent effort at research was made in the whole of the period. During
the Middle Ages the spread of strong religious beliefs put formidable
obstacles in the way of independent scientific investigation. There was no
question of resuming the advance of biology. Even when human anatomy
began to stir itself once more in the sixteenth century, and independent
research was resumed into the structure of the developed body, anatomists
did not dare to extend their inquiries to the unformed body, the embryo, and
its development. There were many reasons for the prevailing horror of such
studies. It is natural enough, when we remember that a Bull of Boniface
VIII excommunicated every man who ventured to dissect a human corpse.
If the dissection of a developed body were a crime to be thus punished, how
much more dreadful must it have seemed to deal with the embryonic body
still enclosed in the womb, which the Creator himself had decently veiled
from the curiosity of the scientist! The Christian Church, then putting many
thousands to death for unbelief, had a shrewd presentiment of the menace
that science contained against its authority. It was powerful enough to see
that its rival did not grow too quickly.
It was not until the Reformation broke the power of the Church, and a
refreshing breath of the spirit dissolved the icy chains that bound science,
that anatomy and embryology, and all the other branches of research, could
begin to advance once more. However, embryology lagged far behind
anatomy. The first works on embryology appear at the beginning of the
sixteenth century. The Italian anatomist, Fabricius ab Aquapendente, a
professor at Padua, opened the advance. In his two books (De formato fœtu,
1600, and De formatione fœtus, 1604) he published the older illustrations
and descriptions of the embryos of man and other mammals, and of the hen.

Similar imperfect illustrations were given by Spigelius (De formato fœtu,
1631), and by Needham (1667) and his more famous compatriot, Harvey
(1652), who discovered the circulation of the blood in the animal body and
formulated the important principle, Omne vivum ex vivo (all life comes
from pre-existing life). The Dutch scientist, Swammerdam, published in his
Bible of Nature the earliest observations on the embryology of the frog and
the division of its egg-yelk. But the most important embryological studies
in the sixteenth century were those of the famous Italian, Marcello
Malpighi, of Bologna, who led the way both in zoology and botany. His
treatises, De formatione pulli and De ovo incubato (1687), contain the first
consistent description of the development of the chick in the fertilised egg.
Here I ought to say a word about the important part played by the chick
in the growth of our science. The development of the chick, like that of the
young of all other birds, agrees in all its main features with that of the other
chief vertebrates, and even of man. The three highest classes of vertebrates
—mammals, birds, and reptiles (lizards, serpents, tortoises, etc.)—have
from the beginning of their embryonic development so striking a
resemblance in all the chief points of structure, and especially in their first
forms, that for a long time it is impossible to distinguish between them. We
have known now for some time that we need only examine the embryo of a
bird, which is the easiest to get at, in order to learn the typical mode of
development of a mammal (and therefore of man). As soon as scientists
began to study the human embryo, or the mammal-embryo generally, in its
earlier stages about the middle and end of the seventeenth century, this
important fact was very quickly discovered. It is both theoretically and
practically of great value. As regards the theory of evolution, we can draw
the most weighty inferences from this similarity between the embryos of
widely different classes of animals. But for the practical purposes of
embryological research the discovery is invaluable, because we can fill up
the gaps in our imperfect knowledge of the embryology of the mammals
from the more thoroughly studied embryology of the bird. Hens’ eggs are
easily to be had in any quantity, and the development of the chick may be
followed step by step in artificial incubation. The development of the
mammal is much more difficult to follow, because here the embryo is not
detached and enclosed in a large egg, but the tiny ovum remains in the
womb until the growth is completed. Hence, it is very difficult to keep up
sustained observation of the various stages in any great extent, quite apart

from such extrinsic considerations as the cost, the technical difficulties, and
many other obstacles which we encounter when we would make an
extensive study of the fertilised mammal. The chicken has, therefore,
always been the chief object of study in this connection. The excellent
incubators we now have enable us to observe it in any quantity and at any
stage of development, and so follow the whole course of its formation step
by step.
By the end of the seventeenth century Malpighi had advanced as far as it
was possible to do with the imperfect microscope of his time in the
embryological study of the chick. Further progress was arrested until the
instrument and the technical methods should be improved. The vertebrate
embryos are so small and delicate in their earlier stages that you cannot go
very far into the study of them without a good microscope and other
technical aid. But this substantial improvement of the microscope and the
other apparatus did not take place until the beginning of the nineteenth
century.
Embryology made scarcely any advance in the first half of the eighteenth
century, when the systematic natural history of plants and animals received
so great an impulse through the publication of Linné’s famous Systema
Naturæ. Not until 1759 did the genius arise who was to give it an entirely
new character, Caspar Friedrich Wolff. Until then embryology had been
occupied almost exclusively in unfortunate and misleading efforts to build
up theories on the imperfect empirical material then available.
The theory which then prevailed, and remained in favour throughout
nearly the whole of the eighteenth century, was commonly called at that
time “the evolution theory”; it is better to describe it as “the preformation
theory.”[6] Its chief point is this: There is no new formation of structures in
the embryonic development of any organism, animal or plant, or even of
man; there is only a growth, or unfolding, of parts which have been
constructed or pre-formed from all eternity, though on a very small scale
and closely packed together. Hence, every living germ contains all the
organs and parts of the body, in the form and arrangement they will present
later, already within it, and thus the whole embryological process is merely
an evolution in the literal sense of the word, or an unfolding, of parts that
were pre-formed and folded up in it. So, for instance, we find in the hen’s
egg not merely a simple cell, that divides and subdivides and forms

germinal layers, and at last, after all kinds of variation and cleavage and
reconstruction, brings forth the body of the chick; but there is in every egg
from the first a complete chicken, with all its parts made and neatly packed.
These parts are so small or so transparent that the microscope cannot detect
them. In the hatching, these parts merely grow larger, and spread out in the
normal way.
[6] This theory is usually known as the “evolution theory” in Germany, in
contradistinction to the “epigenesis theory.” But as it is the latter that is called the
“evolution theory” in England, France, and Italy, and “evolution” and
“epigenesis” are taken to be synonymous, it seems better to call the first the
“pre-formation theory.”

When this theory is consistently developed it becomes a “scatulation
theory.”[7] According to its teaching, there was made in the beginning one
couple or one individual of each species of animal or plant; but this one
individual contained the germs of all the other individuals of the same
species who should ever come to life. As the age of the earth was generally
believed at that time to be fixed by the Bible at 5000 or 6000 years, it
seemed possible to calculate how many individuals of each species had
lived in the period, and so had been packed inside the first being that was
created. The theory was consistently extended to man, and it was affirmed
that our common parent Eve had had stored in her ovary the germs of all the
children of men.
[7] “Packing theory” would be the literal translation. Scatula is the Latin for a
case or box.—Translator.

The theory at first took the form of a belief that it was the females who
were thus encased in the first being. One couple of each species was
created, but the female contained in her ovary all the future individuals of
the species, of either sex. However, this had to be altered when the Dutch
microscopist, Leeuwenhoek, discovered the male spermatozoa in 1690, and
showed that an immense number of these extremely fine and mobile thread-
like beings exist in the male sperm (this will be explained in Chapter VII).
This astonishing discovery was further advanced when it was proved that
these living bodies, swimming about in the seminal fluid, were real
animalcules, and, in fact, were the pre-formed germs of the future
generation. When the male and female procreative elements came together
at conception, these thread-like spermatozoa (“seed-animals”) were

supposed to penetrate into the fertile body of the ovum and begin to develop
there, as the plant seed does in the fruitful earth. Hence, every
spermatozoon was regarded as a homunculus, a tiny complete man; all the
parts were believed to be pre-formed in it, and merely grew larger when it
reached its proper medium in the female ovum. This theory, also, was
consistently developed in the sense that in each of these thread-like bodies
the whole of its posterity was supposed to be present in the minutest form.
Adam’s sexual glands were thought to have contained the germs of the
whole of humanity.
This “theory of male scatulation” found itself at once in keen opposition
to the prevailing “female” theory. The two rival theories at once opened a
very lively campaign, and the physiologists of the eighteenth century were
divided into two great camps—the Animalculists and the Ovulists—which
fought vigorously. The animalculists held that the spermatozoa were the
true germs, and appealed to the lively movements and the structure of these
bodies. The opposing party of the Ovulists, who clung to the older
“evolution theory,” affirmed that the ovum is the real germ, and that the
spermatozoa merely stimulate it at conception to begin its growth; all the
future generations are stored in the ovum. This view was held by the great
majority of the biologists of the eighteenth century, in spite of the fact that
Wolff proved it in 1759 to be without foundation. It owed its prestige
chiefly to the circumstance that the most weighty authorities in the biology
and philosophy of the day decided in favour of it, especially Haller, Bonnet,
and Leibnitz.
Albrecht Haller, professor at Göttingen, who is often called the father of
physiology, was a man of wide and varied learning, but he does not occupy
a very high position in regard to insight into natural phenomena. He made a
vigorous defence of the “evolutionary theory” in his famous work,
Elementa physiologiae, affirming: “There is no such thing as formation
(nulla est epigenesis). No part of the animal frame is made before another;
all were made together.” He thus denied that there was any evolution in the
proper sense of the word, and even went so far as to say that the beard
existed in the new-born child and the antlers in the hornless fawn; all the
parts were there in advance, and were merely hidden from the eye of man
for the time being. Haller even calculated the number of human beings that
God must have created on the sixth day and stored away in Eve’s ovary. He
put the number at 200,000 millions, assuming the age of the world to be

6000 years, the average age of a human being to be thirty years, and the
population of the world at that time to be 1000 millions. And the famous
Haller maintained all this nonsense, in spite of its ridiculous consequences,
even after Wolff had discovered the real course of embryonic development
and established it by direct observation!
Among the philosophers of the time the distinguished Leibnitz was the
chief defender of the “preformation theory,” and by his authority and
literary prestige won many adherents to it. Supported by his system of
monads, according to which body and soul are united in inseparable
association and by their union form the individual, or the “monad,” Leibnitz
consistently extended the “scatulation theory” to the soul, and held that this
was no more evolved than the body. He says, for instance, in his Théodicée:
“I mean that these souls, which one day are to be the souls of men, are
present in the seed, like those of other species; in such wise that they
existed in our ancestors as far back as Adam, or from the beginning of the
world, in the forms of organised bodies.”
The theory seemed to receive considerable support from the observations
of one of its most zealous supporters, Bonnet. In 1745 he discovered, in the
plant-louse, a case of parthenogenesis, or virgin-birth, an interesting form of
reproduction that has lately been found by Siebold and others among
various classes of the articulata, especially crustacea and insects. Among
these and other animals of certain lower species the female may reproduce
for several generations without having been fertilised by the male. These
ova that do not need fertilisation are called “false ova,” pseudova or spores.
Bonnet saw that a female plant-louse, which he had kept in cloistral
isolation, and rigidly removed from contact with males, had on the eleventh
day (after forming a new skin for the fourth time) a living daughter, and
during the next twenty days ninety-four other daughters; and that all of
them went on to reproduce in the same way without any contact with males.
It seemed as if this furnished an irrefutable proof of the truth of the
scatulation theory, as it was held by the Ovulists; it is not surprising to find
that the theory then secured general acceptance.
This was the condition of things when suddenly, in 1759, Caspar
Friedrich Wolff appeared, and dealt a fatal blow at the whole preformation
theory with his new theory of epigenesis. Wolff, the son of a Berlin tailor,
was born in 1733, and went through his scientific and medical studies, first

at Berlin under the famous anatomist Meckel, and afterwards at Halle. Here
he secured his doctorate in his twenty-sixth year, and in his academic
dissertation (November 28th, 1759), the Theoria generationis, expounded
the new theory of a real development on a basis of epigenesis. This treatise
is, in spite of its smallness and its obscure phraseology, one of the most
valuable in the whole range of biological literature. It is equally
distinguished for the mass of new and careful observations it contains, and
the far-reaching and pregnant ideas which the author everywhere extracts
from his observations and builds into a luminous and accurate theory of
generation. Nevertheless, it met with no success at the time. Although
scientific studies were then assiduously cultivated owing to the impulse
given by Linné—although botanists and zoologists were no longer counted
by dozens, but by hundreds, hardly any notice was taken of Wolff’s theory.
Even when he established the truth of epigenesis by the most rigorous
observations, and demolished the airy structure of the preformation theory,
the “exact” scientist Haller proved one of the most strenuous supporters of
the old theory, and rejected Wolff’s correct view with a dictatorial “There is
no such thing as evolution.” He even went on to say that religion was
menaced by the new theory! It is not surprising that the whole of the
physiologists of the second half of the eighteenth century submitted to the
ruling of this physiological pontiff, and attacked the theory of epigenesis as
a dangerous innovation. It was not until more than fifty years afterwards
that Wolff’s work was appreciated. Only when Meckel translated into
German in 1812 another valuable work of Wolff’s on The Formation of the
Alimentary Canal (written in 1768), and called attention to its great
importance, did people begin to think of him once more; yet this obscure
writer had evinced a profounder insight into the nature of the living
organism than any other scientist of the eighteenth century.
Wolff’s idea led to an appreciable advance over the whole field of
biology. There is such a vast number of new and important observations and
pregnant thoughts in his writings that we have only gradually learned to
appreciate them rightly in the course of the nineteenth century. He opened
up the true path for research in many directions. In the first place, his theory
of epigenesis gave us our first real insight into the nature of embryonic
development. He showed convincingly that the development of every
organism consists of a series of new formations, and that there is no trace
whatever of the complete form either in the ovum or the spermatozoon. On

the contrary, these are quite simple bodies, with a very different purport.
The embryo which is developed from them is also quite different, in its
internal arrangement and outer configuration, from the complete organism.
There is no trace whatever of preformation or in-folding of organs. To-day
we can scarcely call epigenesis a theory, because we are convinced it is a
fact, and can demonstrate it at any moment with the aid of the microscope.
Wolff furnished the conclusive empirical proof of his theory in his classic
dissertation on The Formation of the Alimentary Canal (1768). In its
complete state the alimentary canal of the hen is a long and complex tube,
with which the lungs, liver, salivary glands, and many other small glands,
are connected. Wolff showed that in the early stages of the embryonic chick
there is no trace whatever of this complicated tube with all its dependencies,
but instead of it only a flat, leaf-shaped body; that, in fact, the whole
embryo has at first the appearance of a flat, oval-shaped leaf. When we
remember how difficult the exact observation of so fine and delicate a
structure as the early leaf-shaped body of the chick must have been with the
poor microscopes then in use, we must admire the rare faculty for
observation which enabled Wolff to make the most important discoveries in
this most difficult part of embryology. By this laborious research he reached
the correct opinion that the embryonic body of all the higher animals, such
as the birds, is for some time merely a flat, thin, leaf-shaped disk—
consisting at first of one layer, but afterwards of several. The lowest of
these layers is the alimentary canal, and Wolff followed its development
from its commencement to its completion. He showed how this leaf-shaped
structure first turns into a groove, then the margins of this groove fold
together and form a closed canal, and at length the two external openings of
the tube (the mouth and anus) appear.
Moreover, the important fact that the other systems of organs are
developed in the same way, from tubes formed out of simple layers, did not
escape Wolff. The nerveless system, muscular system, and vascular (blood-
vessel) system, with all the organs appertaining thereto, are, like the
alimentary system, developed out of simple leaf-shaped structures. Hence,
Wolff came to the view by 1768 which Pander developed in the Theory of
Germinal Layers fifty years afterwards. His principles are not literally
correct; but he comes as near to the truth in them as was possible at that
time, and could be expected of him.

Our admiration of this gifted genius increases when we find that he was
also the precursor of Goethe in regard to the metamorphosis of plants and of
the famous cellular theory. Wolff had, as Huxley showed, a clear
presentiment of this cardinal theory, since he recognised small microscopic
globules as the elementary parts out of which the germinal layers arose.
Finally, I must invite special attention to the mechanical character of the
profound philosophic reflections which Wolff always added to his
remarkable observations. He was a great monistic philosopher, in the best
meaning of the word. It is unfortunate that his philosophic discoveries were
ignored as completely as his observations for more than half a century. We
must be all the more careful to emphasise the fact of their clear monistic
tendency.

Chapter III.
MODERN EMBRYOLOGY

We may distinguish three chief periods in the growth of our science of
human embryology. The first has been considered in the preceding chapter;
it embraces the whole of the preparatory period of research, and extends
from Aristotle to Caspar Friedrich Wolff, or to the year 1759, in which the
epoch-making Theoria generationis was published. The second period, with
which we have now to deal, lasts about a century—that is to say, until the
appearance of Darwin’s Origin of Species, which brought about a change in
the very foundations of biology, and, in particular, of embryology. The third
period begins with Darwin. When we say that the second period lasted a full
century, we must remember that Wolff’s work had remained almost
unnoticed during half the time—namely, until the year 1812. During the
whole of these fifty-three years not a single book that appeared followed up
the path that Wolff had opened, or extended his theory of embryonic
development. We merely find his views—perfectly correct views, based on
extensive observations of fact—mentioned here and there as erroneous;
their opponents, who adhered to the dominant theory of preformation, did
not even deign to reply to them. This unjust treatment was chiefly due to the
extraordinary authority of Albrecht von Haller; it is one of the most
astonishing instances of a great authority, as such, preventing for a long
time the recognition of established facts.
The general ignorance of Wolff’s work was so great that at the beginning
of the nineteenth century two scientists of Jena, Oken (1806) and Kieser
(1810), began independent research into the development of the alimentary
canal of the chick, and hit upon the right clue to the embryonic puzzle,
without knowing a word about Wolff’s important treatise on the same
subject. They were treading in his very footsteps without suspecting it. This
can be easily proved from the fact that they did not travel as far as Wolff. It

was not until Meckel translated into German Wolff’s book on the
alimentary system, and pointed out its great importance, that the eyes of
anatomists and physiologists were suddenly opened. At once a number of
biologists instituted fresh embryological inquiries, and began to confirm
Wolff’s theory of epigenesis.
This resuscitation of embryology and development of the epigenesis-
theory was chiefly connected with the university of Würtzburg. One of the
professors there at that time was Döllinger, an eminent biologist, and father
of the famous Catholic historian who later distinguished himself by his
opposition to the new dogma of papal infallibility. Döllinger was both a
profound thinker and an accurate observer. He took the keenest interest in
embryology, and worked at it a good deal. However, he is not himself
responsible for any important result in this field. In 1816 a young medical
doctor, whom we may at once designate as Wolff’s chief successor, Karl
Ernst von Baer, came to Würtzburg. Baer’s conversations with Döllinger on
embryology led to a fresh series of most extensive investigations. Döllinger
had expressed a wish that some young scientist should begin again under
his guidance an independent inquiry into the development of the chick
during the hatching of the egg. As neither he nor Baer had money enough to
pay for an incubator and the proper control of the experiments, and for a
competent artist to illustrate the various stages observed, the lead of the
enterprise was given to Christian Pander, a wealthy friend of Baer’s who
had been induced by Baer to come to Würtzburg. An able engraver, Dalton,
was engaged to do the copper-plates. In a short time the embryology of the
chick, in which Baer was taking the greatest indirect interest, was so far
advanced that Pander was able to sketch the main features of it on the
ground of Wolff’s theory in the dissertation he published in 1817. He
clearly enunciated the theory of germinal layers which Wolff had
anticipated, and established the truth of Wolff’s idea of a development of
the complicated systems of organs out of simple leaf-shaped primitive
structures. According to Pander, the leaf-shaped object in the hen’s egg
divides, before the incubation has proceeded twelve hours, into two
different layers, an external serous layer and an internal mucous layer;
between the two there develops later a third layer, the vascular (blood-
vessel) layer.[8]
[8] The technical terms which are bound to creep into this chapter will be fully
understood later on.—Translator.

Karl Ernst von Baer, who had set afoot Pander’s investigation, and had
shown the liveliest interest in it after Pander’s departure from Würtzburg,
began his own much more comprehensive research in 1819. He published
the mature result nine years afterwards in his famous work, Animal
Embryology: Observation and Reflection (not translated). This classic work
still remains a model of careful observation united to profound philosophic
speculation. The first part appeared in 1828, the second in 1837. The book
proved to be the foundation on which the whole science of embryology has
built down to our own day. It so far surpassed its predecessors, and Pander
in particular, that it has become, after Wolff’s work, the chief base of
modern embryology.
Baer was one of the greatest scientists of the nineteenth century, and
exercised considerable influence on other branches of biology as well. He
built up the theory of germinal layers, as a whole and in detail, so clearly
and solidly that it has been the starting-point of embryological research ever
since. He taught that in all the vertebrates first two and then four of these
germinal layers are formed; and that the earliest rudimentary organs of the
body arise by the conversion of these layers into tubes. He described the
first appearance of the vertebrate embryo, as it may be seen in the globular
yelk of the fertilised egg, as an oval disk which first divides into two layers.
From the upper or animal layer are developed all the organs which
accomplish the phenomena of animal life—the functions of sensation and
motion, and the covering of the body. From the lower or vegetative layer
come the organs which effect the vegetative life of the organism—nutrition,
digestion, blood-formation, respiration, secretion, reproduction, etc.
Each of these original layers divides, according to Baer, into two thinner
and superimposed layers or plates. He calls the two plates of the animal
layer, the skin-stratum and muscle-stratum. From the upper of these plates,
the skin-stratum, the external skin, or outer covering of the body, the central
nervous system, and the sense-organs, are formed. From the lower, or
muscle-stratum, the muscles, or fleshy parts and the bony skeleton—in a
word, the motor organs—are evolved. In the same way, Baer said, the lower
or vegetative layer splits into two plates, which he calls the vascular-stratum
and the mucous-stratum. From the outer of the two (the vascular) the heart,
blood-vessels, spleen, and the other vascular glands, the kidneys, and sexual
glands, are formed. From the fourth or mucous layer, in fine, we get the
internal and digestive lining of the alimentary canal and all its

dependencies, the liver, lungs, salivary glands, etc. Baer had, in the main,
correctly judged the significance of these four secondary embryonic layers,
and he followed the conversion of them into the tube-shaped primitive
organs with great perspicacity. He first solved the difficult problem of the
transformation of this four-fold, flat, leaf-shaped, embryonic disk into the
complete vertebrate body, through the conversion of the layers or plates into
tubes. The flat leaves bend themselves in obedience to certain laws of
growth; the borders of the curling plates approach nearer and nearer; until at
last they come into actual contact. Thus out of the flat gut-plate is formed a
hollow gut-tube, out of the flat spinal plate a hollow nerve-tube, from the
skin-plate a skin-tube, and so on.
Among the many great services which Baer rendered to embryology,
especially vertebrate embryology, we must not forget his discovery of the
human ovum. Earlier scientists had, as a rule, of course, assumed that man
developed out of an egg, like the other animals. In fact, the preformation
theory held that the germs of the whole of humanity were stored already in
Eve’s ova. But the real ovum escaped detection until the year 1827. This
ovum is extremely small, being a tiny round vesicle about the 1/120 of an
inch in diameter; it can be seen under very favourable circumstances with
the naked eye as a tiny particle, but is otherwise quite invisible. This
particle is formed in the ovary inside a much larger globule, which takes the
name of the Graafian follicle, from its discoverer, Graaf, and had previously
been regarded as the true ovum. However, in 1827 Baer proved that it was
not the real ovum, which is much smaller, and is contained within the
follicle. (Compare the end of Chapter XXIX.)
Baer was also the first to observe what is known as the segmentation
sphere of the vertebrate; that is to say, the round vesicle which first
develops out of the impregnated ovum, and the thin wall of which is made
up of a single layer of regular, polygonal (many-cornered) cells (see the
illustration in Chapter XII). Another discovery of his that was of great
importance in constructing the vertebrate stem and the characteristic
organisation of this extensive group (to which man belongs) was the
detection of the axial rod, or the chorda dorsalis. There is a long, round,
cylindrical rod of cartilage which runs down the longer axis of the
vertebrate embryo; it appears at an early stage, and is the first sketch of the
spinal column, the solid skeletal axis of the vertebrate. In the lowest of the
vertebrates, the amphioxus, the internal skeleton consists only of this cord

throughout life. But even in the case of man and all the higher vertebrates it
is round this cord that the spinal column and the brain are afterwards
formed.
However, important as these and many other discoveries of Baer’s were
in vertebrate embryology, his researches were even more influential, from
the circumstance that he was the first to employ the comparative method in
studying the development of the animal frame. Baer occupied himself
chiefly with the embryology of vertebrates (especially the birds and fishes).
But he by no means confined his attention to these, gradually taking the
various groups of the invertebrates into his sphere of study. As the general
result of his comparative embryological research, Baer distinguished four
different modes of development and four corresponding groups in the
animal world. These chief groups or types are: 1, the vertebrata; 2, the
articulata; 3, the mollusca; and 4, all the lower groups which were then
wrongly comprehended under the general name of the radiata. Georges
Cuvier had been the first to formulate this distinction, in 1812. He showed
that these groups present specific differences in their whole internal
structure, and the connection and disposal of their systems of organs; and
that, on the other hand, all the animals of the same type—say, the
vertebrates—essentially agreed in their inner structure, in spite of the
greatest superficial differences. But Baer proved that these four groups are
also quite differently developed from the ovum; and that the series of
embryonic forms is the same throughout for animals of the same type, but
different in the case of other animals. Up to that time the chief aim in the
classification of the animal kingdom was to arrange all the animals from
lowest to highest, from the infusorium to man, in one long and continuous
series. The erroneous idea prevailed nearly everywhere that there was one
uninterrupted chain of evolution from the lowest animal to the highest.
Cuvier and Baer proved that this view was false, and that we must
distinguish four totally different types of animals, on the ground of
anatomic structure and embryonic development.
Baer’s epoch-making works aroused an extraordinary and widespread
interest in embryological research. Immediately afterwards we find a great
number of observers at work in the newly opened field, enlarging it in a
very short time with great energy by their various discoveries in detail. Next
to Baer’s comes the admirable work of Heinrich Rathke, of Königsberg
(died 1860); he made an extensive study of the embryology, not only of the

invertebrates (crustaceans, insects, molluscs), but also, and particularly, of
the vertebrates (fishes, tortoises, serpents, crocodiles, etc.). We owe the first
comprehensive studies of mammal embryology to the careful research of
Wilhelm Bischoff, of Munich; his embryology of the rabbit (1840), the dog
(1842), the guinea-pig (1852), and the doe (1854), still form classical
studies. About the same time a great impetus was given to the embryology
of the invertebrates. The way was opened through this obscure province by
the studies of the famous Berlin zoologist, Johannes Müller, on the
echinoderms. He was followed by Albert Kölliker, of Würtzburg, writing on
the cuttlefish (or the cephalopods), Siebold and Huxley on worms and
zoophytes, Fritz Muller (Desterro) on the crustacea, Weismann on insects,
and so on. The number of workers in this field has greatly increased of late,
and a quantity of new and astonishing discoveries have been made. One
notices, in several of these recent works on embryology, that their authors
are too little acquainted with comparative anatomy and classification.
Paleontology is, unfortunately, altogether neglected by many of these new
workers, although this interesting science furnishes most important facts for
phylogeny, and thus often proves of very great service in ontogeny.
A very important advance was made in our science in 1839, when the
cellular theory was established, and a new field of inquiry bearing on
embryology was suddenly opened. When the famous botanist, M.
Schleiden, of Jena, showed in 1838, with the aid of the microscope, that
every plant was made up of innumerable elementary parts, which we call
cells, a pupil of Johannes Müller at Berlin, Theodor Schwann, applied the
discovery at once to the animal organism. He showed that in the animal
body as well, when we examine its tissues in the microscope, we find these
cells everywhere to be the elementary units. All the different tissues of the
organism, especially the very dissimilar tissues of the nerves, muscles,
bones, external skin, mucous lining, etc., are originally formed out of cells;
and this is also true of all the tissues of the plant. These cells are separate
living beings; they are the citizens of the State which the entire
multicellular organism seems to be. This important discovery was bound to
be of service to embryology, as it raised a number of new questions. What is
the relation of the cells to the germinal layers? Are the germinal layers
composed of cells, and what is their relation to the cells of the tissues that
form later? How does the ovum stand in the cellular theory? Is the ovum

itself a cell, or is it composed of cells? These important questions were now
imposed on the embryologist by the cellular theory.
The most notable effort to answer these questions—which were attacked
on all sides by different students—is contained in the famous work,
Inquiries into the Development of the Vertebrates (not translated) of Robert
Remak, of Berlin (1851). This gifted scientist succeeded in mastering, by a
complete reform of the science, the great difficulties which the cellular
theory had at first put in the way of embryology. A Berlin anatomist, Carl
Boguslaus Reichert, had already attempted to explain the origin of the
tissues. But this attempt was bound to miscarry, since its not very clear-
headed author lacked a sound acquaintance with embryology and the cell
theory, and even with the structure and development of the tissue in
particular. Remak at length brought order into the dreadful confusion that
Reichert had caused; he gave a perfectly simple explanation of the origin of
the tissues. In his opinion the animal ovum is always a simple cell : the
germinal layers which develop out of it are always composed of cells; and
these cells that constitute the germinal layers arise simply from the
continuous and repeated cleaving (segmentation) of the original solitary
cell. It first divides into two and then into four cells; out of these four cells
are born eight, then sixteen, thirty-two, and so on. Thus, in the embryonic
development of every animal and plant there is formed first of all out of the
simple egg cell, by a repeated subdivision, a cluster of cells, as Kölliker had
already stated in connection with the cephalopods in 1844. The cells of this
group spread themselves out flat and form leaves or plates; each of these
leaves is formed exclusively out of cells. The cells of different layers
assume different shapes, increase, and differentiate; and in the end there is a
further cleavage (differentiation) and division of work of the cells within
the layers, and from these all the different tissues of the body proceed.
These are the simple foundations of histogeny, or the science that treats
of the development of the tissues ( hista), as it was established by Remak
and Kölliker. Remak, in determining more closely the part which the
different germinal layers play in the formation of the various tissues and
organs, and in applying the theory of evolution to the cells and the tissues
they compose, raised the theory of germinal layers, at least as far as it
regards the vertebrates, to a high degree of perfection.

Remak showed that three layers are formed out of the two germinal
layers which compose the first simple leaf-shaped structure of the
vertebrate body (or the “germinal disk”), as the lower layer splits into two
plates. These three layers have a very definite relation to the various tissues.
First of all, the cells which form the outer skin of the body (the epidermis),
with its various dependencies (hairs, nails, etc.)—that is to say, the entire
outer envelope of the body—are developed out of the outer or upper layer;
but there are also developed in a curious way out of the same layer the cells
which form the central nervous system, the brain and the spinal cord. In the
second place, the inner or lower germinal layer gives rise only to the cells
which form the epithelium (the whole inner lining) of the alimentary canal
and all that depends on it (the lungs, liver, pancreas, etc.), or the tissues that
receive and prepare the nourishment of the body. Finally, the middle layer
gives rise to all the other tissues of the body, the muscles, blood, bones,
cartilage, etc. Remak further proved that this middle layer, which he calls
“the motor-germinative layer,” proceeds to subdivide into two secondary
layers. Thus we find once more the four layers which Baer had indicated.
Remak calls the outer secondary leaf of the middle layer (Baer’s “muscular
layer”) the “skin layer” (it would be better to say, skin-fibre layer); it forms
the outer wall of the body (the true skin, the muscles, etc.). To the inner
secondary leaf (Baer’s “vascular layer”) he gave the name of the
“alimentary-fibre layer”; this forms the outer envelope of the alimentary
canal, with the mesentery, the heart, the blood-vessels, etc.
On this firm foundation provided by Remak for histogeny, or the science
of the formation of the tissues, our knowledge has been gradually built up
and enlarged in detail. There have been several attempts to restrict and even
destroy Remak’s principles. The two anatomists, Reichert (of Berlin) and
Wilhelm His (of Leipzic), especially, have endeavoured in their works to
introduce a new conception of the embryonic development of the
vertebrate, according to which the two primary germinal layers would not
be the sole sources of formation. But these efforts were so seriously marred
by ignorance of comparative anatomy, an imperfect acquaintance with
ontogenesis, and a complete neglect of phylogenesis, that they could not
have more than a passing success. We can only explain how these curious
attacks of Reichert and His came to be regarded for a time as advances by
the general lack of discrimination and of grasp of the true object of
embryology.

Wilhelm His published, in 1868, his extensive Researches into the
Earliest Form of the Vertebrate Body,[9] one of the curiosities of
embryological literature. The author imagines that he can build a
“mechanical theory of embryonic development” by merely giving an exact
description of the embryology of the chick, without any regard to
comparative anatomy and phylogeny, and thus falls into an error that is
almost without parallel in the history of biological literature. As the final
result of his laborious investigations, His tells us “that a comparatively
simple law of growth is the one essential thing in the first development.
Every formation, whether it consist in cleavage of layers, or folding, or
complete division, is a consequence of this fundamental law.”
Unfortunately, he does not explain what this “law of growth” is; just as
other opponents of the theory of selection, who would put in its place a
great “law of evolution,” omit to tell us anything about the nature of this.
Nevertheless, it is quite clear from His’s works that he imagines
constructive Nature to be a sort of skilful tailor. The ingenious operator
succeeds in bringing into existence, by “evolution,” all the various forms of
living things by cutting up in different ways the germinal layers, bending
and folding, tugging and splitting, and so on.
[9] None of His’s works have been translated into English.

His’s embryological theories excited a good deal of interest at the time of
publication, and have evoked a fair amount of literature in the last few
decades. He professed to explain the most complicated parts of organic
construction (such as the development of the brain) in the simplest way on
mechanical principles, and to derive them immediately from simple
physical processes (such as unequal distribution of strain in an elastic
plate). It is quite true that a mechanical or monistic explanation (or a
reduction of natural processes) is the ideal of modern science, and this ideal
would be realised if we could succeed in expressing these formative
processes in mathematical formulæ. His has, therefore, inserted plenty of
numbers and measurements in his embryological works, and given them an
air of “exact” scholarship by putting in a quantity of mathematical tables.
Unfortunately, they are of no value, and do not help us in the least in
forming an “exact” acquaintance with the embryonic phenomena. Indeed,
they wander from the true path altogether by neglecting the phylogenetic
method; this, he thinks, is “a mere by-path,” and is “not necessary at all for

the explanation of the facts of embryology,” which are the direct
consequence of physiological principles. What His takes to be a simple
physical process—for instance, the folding of the germinal layers (in the
formation of the medullary tube, alimentary tube, etc.)—is, as a matter of
fact, the direct result of the growth of the various cells which form those
organic structures; but these growth-motions have themselves been
transmitted by heredity from parents and ancestors, and are only the
hereditary repetition of countless phylogenetic changes which have taken
place for thousands of years in the race-history of the said ancestors. Each
of these historical changes was, of course, originally due to adaptation; it
was, in other words, physiological, and reducible to mechanical causes. But
we have, naturally, no means of observing them now. It is only by the
hypotheses of the science of evolution that we can form an approximate
idea of the organic links in this historic chain.
All the best recent research in animal embryology has led to the
confirmation and development of Baer and Remak’s theory of the germinal
layers. One of the most important advances in this direction of late was the
discovery that the two primary layers out of which is built the body of all
vertebrates (including man) are also present in all the invertebrates, with the
sole exception of the lowest group, the unicellular protozoa. Huxley had
detected them in the medusa in 1849. He showed that the two layers of cells
from which the body of this zoophyte is developed correspond, both
morphologically and physiologically, to the two original germinal layers of
the vertebrate. The outer layer, from which come the external skin and the
muscles, was then called by Allman (1853) the “ectoderm” (outer layer, or
skin); the inner layer, which forms the alimentary and reproductory organs,
was called the “entoderm” (= inner layer). In 1867 and the following years
the discovery of the germinal layers was extended to other groups of the
invertebrates. In particular, the indefatigable Russian zoologist,
Kowalevsky, found them in all the most diverse sections of the
invertebrates—the worms, tunicates, echinoderms, molluscs, articulates,
etc.
In my monograph on the sponges (1872) I proved that these two primary
germinal layers are also found in that group, and that they may be traced
from it right up to man, through all the various classes, in identical form.
This “homology of the two primary germinal layers” extends through the
whole of the metazoa, or tissue-forming animals; that is to say, through the

whole animal kingdom, with the one exception of its lowest section, the
unicellular beings, or protozoa. These lowly organised animals do not form
germinal layers, and therefore do not succeed in forming true tissue. Their
whole body consists of a single cell (as is the case with the amœbæ and
infusoria), or of a loose aggregation of only slightly differentiated cells,
though it may not even reach the full structure of a single cell (as with the
monera). But in all other animals the ovum first grows into two primary
layers, the outer or animal layer (the ectoderm, epiblast, or ectoblast), and
the inner or vegetal layer (the entoderm, hypoblast, or endoblast); and from
these the tissues and organs are formed. The first and oldest organ of all
these metazoa is the primitive gut (or progaster) and its opening, the
primitive mouth (prostoma). The typical embryonic form of the metazoa, as
it is presented for a time by this simple structure of the two-layered body, is
called the gastrula ; it is to be conceived as the hereditary reproduction of
some primitive common ancestor of the metazoa, which we call the
gastræa. This applies to the sponges and other zoophyta, and to the worms,
the mollusca, echinoderma, articulata, and vertebrata. All these animals may
be comprised under the general heading of “gut animals,” or metazoa, in
contradistinction to the gutless protozoa.
I have pointed out in my Study of the Gastræa Theory [not translated]
(1873) the important consequences of this conception in the morphology
and classification of the animal world. I also divided the realm of metazoa
into two great groups, the lower and higher metazoa. In the first are
comprised the cœlenterata (also called zoophytes, or plant-animals). In the
lower forms of this group the body consists throughout life merely of the
primary germinal layers, with the cells sometimes more and sometimes less
differentiated. But with the higher forms of the cœlentarata (the corals,
higher medusæ, ctenophoræ, and platodes) a middle layer, or mesoderm,
often of considerable size, is developed between the other two layers; but
blood and an internal cavity are still lacking.
To the second great group of the metazoa I gave the name of the
cœlomaria, or bilaterata (or the bilateral higher forms). They all have a
cavity within the body (cœloma), and most of them have blood and blood-
vessels. In this are comprised the six higher stems of the animal kingdom,
the annulata and their descendants, the mollusca, echinoderma, articulata,
tunicata, and vertebrata. In all these bilateral organisms the two-sided body
is formed out of four secondary germinal layers, of which the inner two

construct the wall of the alimentary canal, and the outer two the wall of the
body. Between the two pairs of layers lies the cavity (cœloma).
Although I laid special stress on the great morphological importance of
this cavity in my Study of the Gastræa Theory, and endeavoured to prove
the significance of the four secondary germinal layers in the organisation of
the cœlomaria, I was unable to deal satisfactorily with the difficult question
of the mode of their origin. This was done eight years afterwards by the
brothers Oscar and Richard Hertwig in their careful and extensive
comparative studies. In their masterly Cœlum Theory: An Attempt to
Explain the Middle Germinal Layer [not translated] (1881) they showed
that in most of the metazoa, especially in all the vertebrates, the body-cavity
arises in the same way, by the outgrowth of two sacs from the inner layer.
These two cœlom-pouches proceed from the rudimentary mouth of the
gastrula, between the two primary layers. The inner plate of the two-layered
cœlom-pouch (the visceral layer) joins itself to the entoderm; the outer plate
(parietal layer) unites with the ectoderm. Thus are formed the double-
layered gut-wall within and the double-layered body-wall without; and
between the two is formed the cavity of the cœlom, by the blending of the
right and left cœlom-sacs. We shall see this more fully in Chapter X.
The many new points of view and fresh ideas suggested by my gastræa
theory and Hertwig’s cœlom theory led to the publication of a number of
writings on the theory of germinal layers. Most of them set out to oppose it
at first, but in the end the majority supported it. Of late years both theories
are accepted in their essential features by nearly every competent man of
science, and light and order have been introduced into this once dark and
contradictory field of research. A further cause of congratulation for this
solution of the great embryological controversy is that it brought with it a
recognition of the need for phylogenetic study and explanation.
Interest and practice in embryological research have been remarkably
stimulated during the past thirty years by this appreciation of phylogenetic
methods. Hundreds of assiduous and able observers are now engaged in the
development of comparative embryology and its establishment on a basis of
evolution, whereas they numbered only a few dozen not many decades ago.
It would take too long to enumerate even the most important of the
countless valuable works which have enriched embryological literature

since that time. References to them will be found in the latest manuals of
embryology of Kölliker, Balfour, Hertwig, Kollman, Korschelt, and Heider.
Kölliker’s Entwickelungsgeschichte des Menschen und der höherer
Thiere, the first edition of which appeared forty-two years ago, had the rare
merit at that time of gathering into presentable form the scattered
attainments of the science, and expounding them in some sort of unity on
the basis of the cellular theory and the theory of germinal layers.
Unfortunately, the distinguished Würtzburg anatomist, to whom
comparative anatomy, histology, and ontogeny owe so much, is opposed to
the theory of descent generally and to Darwinism in particular. All the other
manuals I have mentioned take a decided stand on evolution. Francis
Balfour has carefully collected and presented with discrimination, in his
Manual of Comparative Embryology (1880), the very scattered and
extensive literature of the subject; he has also widened the basis of the
gastræa theory by a comparative description of the rise of the organs from
the germinal layers in all the chief groups of the animal kingdom, and has
given a most thorough empirical support to the principles I have
formulated. A comparison of his work with the excellent Text-book of the
Embryology of the Vertebrates (1890) [translation 1895] of Korschelt and
Heider shows what astonishing progress has been made in the science in the
course of ten years. I would especially recommend the manuals of Julius
Kollmann and Oscar Hertwig to those readers who are stimulated to further
study by these chapters on human embryology. Kollmann’s work is
commendable for its clear treatment of the subject and very fine original
illustrations; its author adheres firmly to the biogenetic law, and uses it
throughout with considerable profit. That is not the case in Oscar Hertwig’s
recent Text-book of the Embryology of Man and the Mammals [translations
1892 and 1899] (seventh edition 1902). This able anatomist has of late often
been quoted as an opponent of the biogenetic law, although he himself had
demonstrated its great value thirty years ago. His recent vacillation is partly
due to the timidity which our “exact” scientists have with regard to
hypotheses; though it is impossible to make any headway in the explanation
of facts without them. However, the purely descriptive part of embryology
in Hertwig’s Text-book is very thorough and reliable.
A new branch of embryological research has been studied very
assiduously in the last decade of the nineteenth century—namely,
“experimental embryology.” The great importance which has been attached

to the application of physical experiments to the living organism for the last
hundred years, and the valuable results that it has given to physiology in the
study of the vital phenomena, have led to its extension to embryology. I was
the first to make experiments of this kind during a stay of four months on
the Canary Island, Lanzerote, in 1866. I there made a thorough
investigation of the almost unknown embryology of the siphonophoræ. I cut
a number of the embryos of these animals (which develop freely in the
water, and pass through a very curious transformation), at an early stage,
into several pieces, and found that a fresh organism (more or less complete,
according to the size of the piece) was developed from each particle. More
recently some of my pupils have made similar experiments with the
embryos of vertebrates (especially the frog) and some of the invertebrates.
Wilhelm Roux, in particular, has made extensive experiments, and based on
them a special “mechanical embryology,” which has given rise to a good
deal of discussion and controversy. Roux has published a special journal for
these subjects since 1895, the Archiv für Entwickelungsmechanik. The
contributions to it are very varied in value. Many of them are valuable
papers on the physiology and pathology of the embryo. Pathological
experiments—the placing of the embryo in abnormal conditions—have
yielded many interesting results; just as the physiology of the normal body
has for a long time derived assistance from the pathology of the diseased
organism. Other of these mechanical-embryological articles return to the
erroneous methods of His, and are only misleading. This must be said of the
many contributions of mechanical embryology which take up a position of
hostility to the theory of descent and its chief embryological foundation—
the biogenetic law. This law, however, when rightly understood, is not
opposed to, but is the best and most solid support of, a sound mechanical
embryology. Impartial reflection and a due attention to paleontology and
comparative anatomy should convince these one-sided mechanicists that the
facts they have discovered—and, indeed, the whole embryological process
—cannot be fully understood without the theory of descent and the
biogenetic law.

Chapter IV.
THE OLDER PHYLOGENY

The embryology of man and the animals, the history of which we have
reviewed in the last two chapters, was mainly a descriptive science forty
years ago. The earlier investigations in this province were chiefly directed
to the discovery, by careful observation, of the wonderful facts of the
embryonic development of the animal body from the ovum. Forty years ago
no one dared attack the question of the causes of these phenomena. For
fully a century, from the year 1759, when Wolff’s solid Theoria
generationis appeared, until 1859, when Darwin published his famous
Origin of Species, the real causes of the embryonic processes were quite
unknown. No one thought of seeking the agencies that effected this
marvellous succession of structures. The task was thought to be so difficult
as almost to pass beyond the limits of human thought. It was reserved for
Charles Darwin to initiate us into the knowledge of these causes. This
compels us to recognise in this great genius, who wrought a complete
revolution in the whole field of biology, a founder at the same time of a new
period in embryology. It is true that Darwin occupied himself very little
with direct embryological research, and even in his chief work he only
touches incidentally on the embryonic phenomena; but by his reform of the
theory of descent and the founding of the theory of selection he has given
us the means of attaining to a real knowledge of the causes of embryonic
formation. That is, in my opinion, the chief feature in Darwin’s incalculable
influence on the whole science of evolution.
When we turn our attention to this latest period of embryological
research, we pass into the second division of organic evolution—stem-
evolution, or phylogeny. I have already indicated in Chapter I the important
and intimate causal connection between these two sections of the science of
evolution—between the evolution of the individual and that of his

ancestors. We have formulated this connection in the biogenetic law; the
shorter evolution, that of the individual, or ontogenesis, is a rapid and
summary repetition, a condensed recapitulation, of the larger evolution, or
that of the species. In this principle we express all the essential points
relating to the causes of evolution; and we shall seek throughout this work
to confirm this principle and lend it the support of facts. When we look to
its causal significance, perhaps it would be better to formulate the
biogenetic law thus: “The evolution of the species and the stem ( phylon)
shows us, in the physiological functions of heredity and adaptation, the
conditioning causes on which the evolution of the individual depends”; or,
more briefly: “Phylogenesis is the mechanical cause of ontogenesis.”
But before we examine the great achievement by which Darwin revealed
the causes of evolution to us, we must glance at the efforts of earlier
scientists to attain this object. Our historical inquiry into these will be even
shorter than that into the work done in the field of ontogeny. We have very
few names to consider here. At the head of them we find the great French
naturalist, Jean Lamarck, who first established evolution as a scientific
theory in 1809. Even before his time, however, the chief philosopher, Kant,
and the chief poet, Goethe, of Germany had occupied themselves with the
subject. But their efforts passed almost without recognition in the
eighteenth century. A “philosophy of nature” did not arise until the
beginning of the nineteenth century. In the whole of the time before this no
one had ventured to raise seriously the question of the origin of species,
which is the culminating point of phylogeny. On all sides it was regarded as
an insoluble enigma.
The whole science of the evolution of man and the other animals is
intimately connected with the question of the nature of species, or with the
problem of the origin of the various animals which we group together under
the name of species. Thus the definition of the species becomes important.
It is well known that this definition was given by Linné, who, in his famous
Systema Naturæ (1735), was the first to classify and name the various
groups of animals and plants, and drew up an orderly scheme of the species
then known. Since that time “species” has been the most important and
indispensable idea in descriptive natural history, in zoological and botanical
classification; although there have been endless controversies as to its real
meaning.

What, then, is this “organic species”? Linné himself appealed directly to
the Mosaic narrative; he believed that, as it is stated in Genesis, one pair of
each species of animals and plants was created in the beginning, and that all
the individuals of each species are the descendants of these created couples.
As for the hermaphrodites (organisms that have male and female organs in
one being), he thought it sufficed to assume the creation of one sole
individual, since this would be fully competent to propagate its species.
Further developing these mystic ideas, Linné went on to borrow from
Genesis the account of the deluge and of Noah’s ark as a ground for a
science of the geographical and topographical distribution of organisms. He
accepted the story that all the plants, animals, and men on the earth were
swept away in a universal deluge, except the couples preserved with Noah
in the ark, and ultimately landed on Mount Ararat. This mountain seemed to
Linné particularly suitable for the landing, as it reaches a height of more
than 16,000 feet, and thus provides in its higher zones the several climates
demanded by the various species of animals and plants: the animals that
were accustomed to a cold climate could remain at the summit; those used
to a warm climate could descend to the foot; and those requiring a
temperate climate could remain half-way down. From this point the re-
population of the earth with animals and plants could proceed.
It was impossible to have any scientific notion of the method of evolution
in Linné’s time, as one of the chief sources of information, paleontology,
was still wholly unknown. This science of the fossil remains of extinct
animals and plants is very closely bound up with the whole question of
evolution. It is impossible to explain the origin of living organisms without
appealing to it. But this science did not rise until a much later date. The real
founder of scientific paleontology was Georges Cuvier, the most
distinguished zoologist who, after Linné, worked at the classification of the
animal world, and effected a complete revolution in systematic zoology at
the beginning of the nineteenth century. In regard to the nature of the
species he associated himself with Linné and the Mosaic story of creation,
though this was more difficult for him with his acquaintance with fossil
remains. He clearly showed that a number of quite different animal
populations have lived on the earth; and he claimed that we must
distinguish a number of stages in the history of our planet, each of which
was characterised by a special population of animals and plants. These
successive populations were, he said, quite independent of each other, and

therefore the supernatural creative act, which was demanded as the origin of
the animals and plants by the dominant creed, must have been repeated
several times. In this way a whole series of different creative periods must
have succeeded each other; and in connection with these he had to assume
that stupendous revolutions or cataclysms—something like the legendary
deluge—must have taken place repeatedly. Cuvier was all the more
interested in these catastrophes or cataclysms as geology was just beginning
to assert itself, and great progress was being made in our knowledge of the
structure and formation of the earth’s crust. The various strata of the crust
were being carefully examined, especially by the famous geologist Werner
and his school, and the fossils found in them were being classified; and
these researches also seemed to point to a variety of creative periods. In
each period the earth’s crust, composed of the various strata, seemed to be
differently constituted, just like the population of animals and plants that
then lived on it. Cuvier combined this notion with the results of his own
paleontological and zoological research; and in his effort to get a consistent
view of the whole process of the earth’s history he came to form the theory
which is known as “the catastrophic theory,” or the theory of terrestrial
revolutions. According to this theory, there have been a series of mighty
cataclysms on the earth, and these have suddenly destroyed the whole
animal and plant population then living on it; after each cataclysm there
was a fresh creation of living things throughout the earth. As this creation
could not be explained by natural laws, it was necessary to appeal to an
intervention on the part of the Creator. This catastrophic theory, which
Cuvier described in a special work, was soon generally accepted, and
retained its position in biology for half a century.
However, Cuvier’s theory was completely overthrown sixty years ago by
the geologists, led by Charles Lyell, the most distinguished worker in this
field of science. Lyell proved in his famous Principles of Geology (1830)
that the theory was false, in so far as it concerned the crust of the earth; that
it was totally unnecessary to bring in supernatural agencies or general
catastrophes in order to explain the structure and formation of the
mountains; and that we can explain them by the familiar agencies which are
at work to-day in altering and reconstructing the surface of the earth. These
causes are—the action of the atmosphere and water in its various forms
(snow, ice, fog, rain, the wear of the river, and the stormy ocean), and the
volcanic action which is exerted by the molten central mass. Lyell

convincingly proved that these natural causes are quite adequate to explain
every feature in the build and formation of the crust. Hence Cuvier’s theory
of cataclysms was very soon driven out of the province of geology, though
it remained for another thirty years in undisputed authority in biology. All
the zoologists and botanists who gave any thought to the question of the
origin of organisms adhered to Cuvier’s erroneous idea of revolutions and
new creations.
In order to illustrate the complete stagnancy of biology from 1830 to
1859 on the question of the origin of the various species of animals and
plants, I may say, from my own experience, that during the whole of my
university studies I never heard a single word said about this most
important problem of the science. I was fortunate enough at that time
(1852–1857) to have the most distinguished masters for every branch of
biological science. Not one of them ever mentioned this question of the
origin of species. Not a word was ever said about the earlier efforts to
understand the formation of living things, nor about Lamarck’s Philosophie
Zoologique which had made a fresh attack on the problem in 1809. Hence it
is easy to understand the enormous opposition that Darwin encountered
when he took up the question for the first time. His views seemed to float in
the air, without a single previous effort to support them. The whole question
of the formation of living things was considered by biologists, until 1859,
as pertaining to the province of religion and transcendentalism; even in
speculative philosophy, in which the question had been approached from
various sides, no one had ventured to give it serious treatment. This was due
to the dualistic system of Immanuel Kant, who taught a natural system of
evolution as far as the inorganic world was concerned; but, on the whole,
adopted a supernaturalist system as regards the origin of living things. He
even went so far as to say: “It is quite certain that we cannot even
satisfactorily understand, much less explain, the nature of an organism and
its internal forces on purely mechanical principles; it is so certain, indeed,
that we may confidently say: ‘It is absurd for a man to imagine even that
some day a Newton will arise who will explain the origin of a single blade
of grass by natural laws not controlled by design’—such a hope is entirely
forbidden us.” In these words Kant definitely adopts the dualistic and
teleological point of view for biological science.
Nevertheless, Kant deserted this point of view at times, particularly in
several remarkable passages which I have dealt with at length in my

Natural History of Creation (chap. v), where he expresses himself in the
opposite, or monistic, sense. In fact, these passages would justify one, as I
showed, in claiming his support for the theory of evolution. However, these
monistic passages are only stray gleams of light; as a rule, Kant adheres in
biology to the obscure dualistic ideas, according to which the forces at work
in inorganic nature are quite different from those of the organic world. This
dualistic system prevails in academic philosophy to-day—most of our
philosophers still regarding these two provinces as totally distinct. They put,
on the one side, the inorganic or “lifeless” world, in which there are at work
only mechanical laws, acting necessarily and without design; and, on the
other, the province of organic nature, in which none of the phenomena can
be properly understood, either as regards their inner nature or their origin,
except in the light of preconceived design, carried out by final or purposive
causes.
The prevalence of this unfortunate dualistic prejudice prevented the
problem of the origin of species, and the connected question of the origin of
man, from being regarded by the bulk of people as a scientific question at
all until 1859. Nevertheless, a few distinguished students, free from the
current prejudice, began, at the commencement of the nineteenth century, to
make a serious attack on the problem. The merit of this attaches particularly
to what is known as “the older school of natural philosophy,” which has
been so much misrepresented, and which included Jean Lamarck, Buffon,
Geoffroy St. Hilaire, and Blainville in France; Wolfgang Goethe, Reinhold
Treviranus, Schelling, and Lorentz Oken in Germany [and Erasmus Darwin
in England].
The gifted natural philosopher who treated this difficult question with the
greatest sagacity and comprehensiveness was Jean Lamarck. He was born at
Bazentin, in Picardy, on August 1st, 1744; he was the son of a clergyman,
and was destined for the Church. But he turned to seek glory in the army,
and eventually devoted himself to science.
His Philosophie Zoologique was the first scientific attempt to sketch the
real course of the origin of species, the first “natural history of creation” of
plants, animals, and men. But, as in the case of Wolff’s book, this
remarkably able work had no influence whatever; neither one nor the other
could obtain any recognition from their prejudiced contemporaries. No man
of science was stimulated to take an interest in the work, and to develop the

germs it contained of the most important biological truths. The most
distinguished botanists and zoologists entirely rejected it, and did not even
deign to reply to it. Cuvier, who lived and worked in the same city, has not
thought fit to devote a single syllable to this great achievement in his
memoir on progress in the sciences, in which the pettiest observations
found a place. In short, Lamarck’s Philosophie Zoologique shared the fate
of Wolff’s theory of development, and was for half a century ignored and
neglected. The German scientists, especially Oken and Goethe, who were
occupied with similar speculations at the same time, seem to have known
nothing about Lamarck’s work. If they had known it, they would have been
greatly helped by it, and might have carried the theory of evolution much
farther than they found it possible to do.
To give an idea of the great importance of the Philosophie Zoologique, I
will briefly explain Lamarck’s leading thought. He held that there was no
essential difference between living and lifeless beings. Nature is one united
and connected system of phenomena; and the forces which fashion the
lifeless bodies are the only ones at work in the kingdom of living things. We
have, therefore, to use the same method of investigation and explanation in
both provinces. Life is only a physical phenomenon. All the plants and
animals, with man at their head, are to be explained, in structure and life, by
mechanical or efficient causes, without any appeal to final causes, just as in
the case of minerals and other inorganic bodies. This applies equally to the
origin of the various species. We must not assume any original creation, or
repeated creations (as in Cuvier’s theory), to explain this, but a natural,
continuous, and necessary evolution. The whole evolutionary process has
been uninterrupted. All the different kinds of animals and plants which we
see to-day, or that have ever lived, have descended in a natural way from
earlier and different species; all come from one common stock, or from a
few common ancestors. These remote ancestors must have been quite
simple organisms of the lowest type, arising by spontaneous generation
from inorganic matter. The succeeding species have been constantly
modified by adaptation to their varying environment (especially by use and
habit), and have transmitted their modifications to their successors by
heredity.
Lamarck was the first to formulate as a scientific theory the natural origin
of living things, including man, and to push the theory to its extreme
conclusions—the rise of the earliest organisms by spontaneous generation

(or abiogenesis) and the descent of man from the nearest related mammal,
the ape. He sought to explain this last point, which is of especial interest to
us here, by the same agencies which he found at work in the natural origin
of the plant and animal species. He considered use and habit (adaptation) on
the one hand, and heredity on the other, to be the chief of these agencies.
The most important modifications of the organs of plants and animals are
due, in his opinion, to the function of these very organs, or to the use or
disuse of them. To give a few examples, the woodpecker and the humming-
bird have got their peculiarly long tongues from the habit of extracting their
food with their tongues from deep and narrow folds or canals; the frog has
developed the web between his toes by his own swimming; the giraffe has
lengthened his neck by stretching up to the higher branches of trees, and so
on. It is quite certain that this use or disuse of organs is a most important
factor in organic development, but it is not sufficient to explain the origin of
species.
To adaptation we must add heredity as the second and not less important
agency, as Lamarck perfectly recognised. He said that the modification of
the organs in any one individual by use or disuse was slight, but that it was
increased by accumulation in passing by heredity from generation to
generation. But he missed altogether the principle which Darwin afterwards
found to be the chief factor in the theory of transformation—namely, the
principle of natural selection in the struggle for existence. It was partly
owing to his failure to detect this supremely important element, and partly
to the poor condition of all biological science at the time, that Lamarck did
not succeed in establishing more firmly his theory of the common descent
of man and the other animals.
Independently of Lamarck, the older German school of natural
philosophy, especially Reinhold Treviranus, in his Biologie (1802), and
Lorentz Oken, in his Naturphilosophie (1809), turned its attention to the
problem of evolution about the end of the eighteenth and beginning of the
nineteenth century. I have described its work in my History of Creation
(chap. iv). Here I can only deal with the brilliant genius whose evolutionary
ideas are of special interest—the greatest of German poets, Wolfgang
Goethe. With his keen eye for the beauties of nature, and his profound
insight into its life, Goethe was early attracted to the study of various
natural sciences. It was the favourite occupation of his leisure hours
throughout life. He gave particular and protracted attention to the theory of

colours. But the most valuable of his scientific studies are those which
relate to that “living, glorious, precious thing,” the organism. He made
profound research into the science of structures or morphology (morphæ =
forms). Here, with the aid of comparative anatomy, he obtained the most
brilliant results, and went far in advance of his time. I may mention, in
particular, his vertebral theory of the skull, his discovery of the pineal gland
in man, his system of the metamorphosis of plants, etc. These
morphological studies led Goethe on to research into the formation and
modification of organic structures which we must count as the first germ of
the science of evolution. He approaches so near to the theory of descent that
we must regard him, after Lamarck, as one of its earliest founders. It is true
that he never formulated a complete scientific theory of evolution, but we
find a number of remarkable suggestions of it in his splendid miscellaneous
essays on morphology. Some of them are really among the very basic ideas
of the science of evolution. He says, for instance (1807): “When we
compare plants and animals in their most rudimentary forms, it is almost
impossible to distinguish between them. But we may say that the plants and
animals, beginning with an almost inseparable closeness, gradually advance
along two divergent lines, until the plant at last grows in the solid, enduring
tree and the animal attains in man to the highest degree of mobility and
freedom.” That Goethe was not merely speaking in a poetical, but in a
literal genealogical, sense of this close affinity of organic forms is clear
from other remarkable passages in which he treats of their variety in
outward form and unity in internal structure. He believes that every living
thing has arisen by the interaction of two opposing formative forces or
impulses. The internal or “centripetal” force, the type or “impulse to
specification,” seeks to maintain the constancy of the specific forms in the
succession of generations: this is heredity. The external or “centrifugal”
force, the element of variation or “impulse to metamorphosis,” is
continually modifying the species by changing their environment: this is
adaptation. In these significant conceptions Goethe approaches very close
to a recognition of the two great mechanical factors which we now assign as
the chief causes of the formation of species.
However, in order to appreciate Goethe’s views on morphology, one must
associate his decidedly monistic conception of nature with his pantheistic
philosophy. The warm and keen interest with which he followed, in his last
years, the controversies of contemporary French scientists, and especially

the struggle between Cuvier and Geoffroy St. Hilaire (see chap. iv of The
History of Creation), is very characteristic. It is also necessary to be
familiar with his style and general tenour of thought in order to appreciate
rightly the many allusions to evolution found in his writings. Otherwise,
one is apt to make serious errors.
He approached so close, at the end of the eighteenth century, to the
principles of the science of evolution that he may well be described as the
first forerunner of Darwin, although he did not go so far as to formulate
evolution as a scientific system, as Lamarck did.

Chapter V.
THE MODERN SCIENCE OF EVOLUTION

We owe so much of the progress of scientific knowledge to Darwin’s
Origin of Species that its influence is almost without parallel in the history
of science. The literature of Darwinism grows from day to day, not only on
the side of academic zoology and botany, the sciences which were chiefly
affected by Darwin’s theory, but in a far wider circle, so that we find
Darwinism discussed in popular literature with a vigour and zest that are
given to no other scientific conception. This remarkable success is due
chiefly to two circumstances. In the first place, all the sciences, and
especially biology, have made astounding progress in the last half-century,
and have furnished a very vast quantity of proofs of the theory of evolution.
In striking contrast to the failure of Lamarck and the older scientists to
attract attention to their effort to explain the origin of living things and of
man, we have this second and successful effort of Darwin, which was able
to gather to its support a large number of established facts. Availing himself
of the progress already made, he had very different scientific proofs to
allege than Lamarck, or St. Hilaire, or Goethe, or Treviranus had had. But,
in the second place, we must acknowledge that Darwin had the special
distinction of approaching the subject from an entirely new side, and of
basing the theory of descent on a consistent system, which now goes by the
name of Darwinism.
Lamarck had unsuccessfully attempted to explain the modification of
organisms that descend from a common form chiefly by the action of habit
and the use of organs, though with the aid of heredity. But Darwin’s success
was complete when he independently sought to give a mechanical
explanation, on a quite new ground, of this modification of plant and animal
structures by adaptation and heredity. He was impelled to his theory of
selection on the following grounds. He compared the origin of the various

kinds of animals and plants which we modify artificially—by the action of
artificial selection in horticulture and among domestic animals—with the
origin of the species of animals and plants in their natural state. He then
found that the agencies which we employ in the modification of forms by
artificial selection are also at work in Nature. The chief of these agencies he
held to be “the struggle for life.” The gist of this peculiarly Darwinian idea
is given in this formula: The struggle for existence produces new species
without premeditated design in the life of Nature, in the same way that the
will of man consciously selects new races in artificial conditions. The
gardener or the farmer selects new forms as he wills for his own profit, by
ingeniously using the agency of heredity and adaptation for the
modification of structures; so, in the natural state, the struggle for life is
always unconsciously modifying the various species of living things. This
struggle for life, or competition of organisms in securing the means of
subsistence, acts without any conscious design, but it is none the less
effective in modifying structures. As heredity and adaptation enter into the
closest reciprocal action under its influence, new structures, or alterations of
structure, are produced; and these are purposive in the sense that they serve
the organism when formed, but they were produced without any pre-
conceived aim.
This simple idea is the central thought of Darwinism, or the theory of
selection. Darwin conceived this idea at an early date, and then, for more
than twenty years, worked at the collection of empirical evidence in support
of it before he published his theory. His grandfather, Erasmus Darwin, was
an able scientist of the older school of natural philosophy, who published a
number of natural-philosophic works about the end of the eighteenth
century. The most important of them is his Zoonomia, published in 1794, in
which he expounds views similar to those of Goethe and Lamarck, without
really knowing anything of the work of these contemporaries. However, in
the writings of the grandfather the plastic imagination rather outran the
judgment, while in Charles Darwin the two were better balanced.
Darwin did not publish any account of his theory until 1858, when Alfred
Russel Wallace, who had independently reached the same theory of
selection, published his own work. In the following year appeared the
Origin of Species, in which he develops it at length and supports it with a
mass of proof. Wallace had reached the same conclusion, but he had not so
clear a perception as Darwin of the effectiveness of natural selection in

forming species, and did not develop the theory so fully. Nevertheless,
Wallace’s writings, especially those on mimicry, etc., and an admirable
work on The Geographical Distribution of Animals, contain many fine
original contributions to the theory of selection. Unfortunately, this gifted
scientist has since devoted himself to spiritism.[10]
[10] Darwin and Wallace arrived at the theory quite independently. Vide
Wallace’s Contributions to the Theory of Natural Selection (1870) and
Darwinism (1891).

Darwin’s Origin of Species had an extraordinary influence, though not at
first on the experts of the science. It took zoologists and botanists several
years to recover from the astonishment into which they had been thrown
through the revolutionary idea of the work. But its influence on the special
sciences with which we zoologists and botanists are concerned has
increased from year to year; it has introduced a most healthy fermentation
in every branch of biology, especially in comparative anatomy and
ontogeny, and in zoological and botanical classification. In this way it has
brought about almost a revolution in the prevailing views.
However, the point which chiefly concerns us here—the extension of the
theory to man—was not touched at all in Darwin’s first work in 1859. It
was believed for several years that he had no thought of applying his
principles to man, but that he shared the current idea of man holding a
special position in the universe. Not only ignorant laymen (especially
several theologians), but also a number of men of science, said very naively
that Darwinism in itself was not to be opposed; that it was quite right to use
it to explain the origin of the various species of plants and animals, but that
it was totally inapplicable to man.
In the meantime, however, it seemed to a good many thoughtful people,
laymen as well as scientists, that this was wrong; that the descent of man
from some other animal species, and immediately from some ape-like
mammal, followed logically and necessarily from Darwin’s reformed theory
of evolution. Many of the acuter opponents of the theory saw at once the
justice of this position, and, as this consequence was intolerable, they
wanted to get rid of the whole theory.
The first scientific application of the Darwinian theory to man was made
by Huxley, the greatest zoologist in England. This able and learned
scientist, to whom zoology owes much of its progress, published in 1863 a

small work entitled Evidence as to Man’s Place in Nature. In the extremely
important and interesting lectures which made up this work he proved
clearly that the descent of man from the ape followed necessarily from the
theory of descent. If that theory is true, we are bound to conceive the
animals which most closely resemble man as those from which humanity
has been gradually evolved. About the same time Carl Vogt published a
larger work on the same subject. We must also mention Gustav Jaeger and
Friedrich Rolle among the zoologists who accepted and taught the theory of
evolution immediately after the publication of Darwin’s book, and
maintained that the descent of man from the lower animals logically
followed from it. The latter published, in 1866, a work on the origin and
position of man.
About the same time I attempted, in the second volume of my General
Morphology (1866), to apply the theory of evolution to the whole organic
kingdom, including man.[11] I endeavoured to sketch the probable ancestral
trees of the various classes of the animal world, the protists, and the plants,
as it seemed necessary to do on Darwinian principles, and as we can
actually do now with a high degree of confidence. If the theory of descent,
which Lamarck first clearly formulated and Darwin thoroughly established,
is true, we should be able to draw up a natural classification of plants and
animals in the light of their genealogy, and to conceive the large and small
divisions of the system as the branches and twigs of an ancestral tree. The
eight genealogical tables which I inserted in the second volume of the
General Morphology are the first sketches of their kind. In Chapter 27,
particularly, I trace the chief stages in man’s ancestry, as far as it is possible
to follow it through the vertebrate stem. I tried especially to determine, as
well as one could at that time, the position of man in the classification of
the mammals and its genealogical significance. I have greatly improved this
attempt, and treated it in a more popular form, in chaps. xxvi–xxviii of my
History of Creation (1868).[12]
[11] Huxley spoke of this “as one of the greatest scientific works ever
published.”—Translator.

[12] Of which Darwin said that the Descent of Man would probably never have
been written if he had seen it earlier.—Translator.

It was not until 1871, twelve years after the appearance of The Origin of
Species, that Darwin published the famous work which made the much-

contested application of his theory to man, and crowned the splendid
structure of his system. This important work was The Descent of Man, and
Selection in Relation to Sex. In this Darwin expressly drew the conclusion,
with rigorous logic, that man also must have been developed out of lower
species, and described the important part played by sexual selection in the
elevation of man and the other higher animals. He showed that the careful
selection which the sexes exercise on each other in regard to sexual
relations and procreation, and the æsthetic feeling which the higher animals
develop through this, are of the utmost importance in the progressive
development of forms and the differentiation of the sexes. The males
choosing the handsomest females in one class of animals, and the females
choosing only the finest-looking males in another, the special features and
the sexual characteristics are increasingly accentuated. In fact, some of the
higher animals develop in this connection a finer taste and judgment than
man himself. But, even as regards man, it is to this sexual selection that we
owe the family-life, which is the chief foundation of civilisation. The rise of
the human race is due for the most part to the advanced sexual selection
which our ancestors exercised in choosing their mates.
Darwin accepted in the main the general outlines of man’s ancestral tree,
as I gave it in the General Morphology and the History of Creation, and
admitted that his studies led him to the same conclusion. That he did not at
once apply the theory to man in his first work was a commendable piece of
discretion; such a sequel was bound to excite the strongest opposition to the
whole theory. The first thing to do was to establish it as regards the animal
and plant worlds. The subsequent extension to man was bound to be made
sooner or later.
It is important to understand this very clearly. If all living things come
from a common root, man must be included in the general scheme of
evolution. On the other hand, if the various species were separately created,
man, too, must have been created, and not evolved. We have to choose
between these two alternatives. This cannot be too frequently or too
strongly emphasised. Either all the species of animals and plants are of
supernatural origin—created, not evolved—and in that case man also is the
outcome of a creative act, as religion teaches, or the different species have
been evolved from a few common, simple ancestral forms, and in that case
man is the highest fruit of the tree of evolution.

We may state this briefly in the following principle—The descent of man
from the lower animals is a special deduction which inevitably follows from
the general inductive law of the whole theory of evolution. In this principle
we have a clear and plain statement of the matter. Evolution is in reality
nothing but a great induction, which we are compelled to make by the
comparative study of the most important facts of morphology and
physiology. But we must draw our conclusion according to the laws of
induction, and not attempt to determine scientific truths by direct
measurement and mathematical calculation. In the study of living things we
can scarcely ever directly and fully, and with mathematical accuracy,
determine the nature of phenomena, as is done in the simpler study of the
inorganic world—in chemistry, physics, mineralogy, and astronomy. In the
latter, especially, we can always use the simplest and absolutely safest
method—that of mathematical determination. But in biology this is quite
impossible for various reasons; one very obvious reason being that most of
the facts of the science are very complicated and much too intricate to allow
a direct mathematical analysis. The greater part of the phenomena that
biology deals with are complicated historical processes, which are related
to a far-reaching past, and as a rule can only be approximately estimated.
Hence we have to proceed by induction—that is to say, to draw general
conclusions, stage by stage, and with proportionate confidence, from the
accumulation of detailed observations. These inductive conclusions cannot
command absolute confidence, like mathematical axioms; but they
approach the truth, and gain increasing probability, in proportion as we
extend the basis of observed facts on which we build. The importance of
these inductive laws is not diminished from the circumstance that they are
looked upon merely as temporary acquisitions of science, and may be
improved to any extent in the progress of scientific knowledge. The same
may be said of the attainments of many other sciences, such as geology or
archeology. However much they may be altered and improved in detail in
the course of time, these inductive truths may retain their substance
unchanged.
Now, when we say that the theory of evolution in the sense of Lamarck
and Darwin is an inductive law—in fact, the greatest of all biological
inductions—we rely, in the first place, on the facts of paleontology. This
science gives us some direct acquaintance with the historical phenomena of
the changes of species. From the situations in which we find the fossils in

the various strata of the earth we gather confidently, in the first place, that
the living population of the earth has been gradually developed, as clearly
as the earth’s crust itself; and that, in the second place, several different
populations have succeeded each other in the various geological periods.
Modern geology teaches that the formation of the earth has been gradual,
and unbroken by any violent revolutions. And when we compare together
the various kinds of animals and plants which succeed each other in the
history of our planet, we find, in the first place, a constant and gradual
increase in the number of species from the earliest times until the present
day; and, in the second place, we notice that the forms in each great group
of animals and plants also constantly improve as the ages advance. Thus, of
the vertebrates there are at first only the lower fishes; then come the higher
fishes, and later the amphibia. Still later appear the three higher classes of
vertebrates—the reptiles, birds, and mammals, for the first time; only the
lowest and least perfect forms of the mammals are found at first; and it is
only at a very late period that placental mammals appear, and man belongs
to the latest and youngest branch of these. Thus perfection of form increases
as well as variety from the earliest to the latest stage. That is a fact of the
greatest importance. It can only be explained by the theory of evolution,
with which it is in perfect harmony. If the different groups of plants and
animals do really descend from each other, we must expect to find this
increase in their number and perfection under the influence of natural
selection, just as the succession of fossils actually discloses it to us.
Comparative anatomy furnishes a second series of facts which are of
great importance for the forming of our inductive law. This branch of
morphology compares the adult structures of living things, and seeks in the
great variety of organic forms the stable and simple law of organisation, or
the common type or structure. Since Cuvier founded this science at the
beginning of the nineteenth century it has been a favourite study of the most
distinguished scientists. Even before Cuvier’s time Goethe had been greatly
stimulated by it, and induced to take up the study of morphology.
Comparative osteology, or the philosophic study and comparison of the
bony skeleton of the vertebrates—one of its most interesting sections—
especially fascinated him, and led him to form the theory of the skull which
I mentioned before. Comparative anatomy shows that the internal structure
of the animals of each stem and the plants of each class is the same in its
essential features, however much they differ in external appearance. Thus

man has so great a resemblance in the chief features of his internal
organisation to the other mammals that no comparative anatomist has ever
doubted that he belongs to this class. The whole internal structure of the
human body, the arrangement of its various systems of organs, the
distribution of the bones, muscles, blood-vessels, etc., and the whole
structure of these organs in the larger and the finer scale, agree so closely
with those of the other mammals (such as the apes, rodents, ungulates,
cetacea, marsupials, etc.) that their external differences are of no account
whatever. We learn further from comparative anatomy that the chief
features of animal structure are so similar in the various classes (fifty to
sixty in number altogether) that they may all be comprised in from eight to
twelve great groups. But even in these groups, the stem-forms or animal
types, certain organs (especially the alimentary canal) can be proved to have
been originally the same for all. We can only explain by the theory of
evolution this essential unity in internal structure of all these animal forms
that differ so much in outward appearance. This wonderful fact can only be
really understood and explained when we regard the internal resemblance
as an inheritance from common-stem forms, and the external differences as
the effect of adaptation to different environments.
In recognising this, comparative anatomy has itself advanced to a higher
stage. Gegenbaur, the most distinguished of recent students of this science,
says that with the theory of evolution a new period began in comparative
anatomy, and that the theory in turn found a touch stone in the science. “Up
to now there is no fact in comparative anatomy that is inconsistent with the
theory of evolution; indeed, they all lead to it. In this way the theory
receives back from the science all the service it rendered to its method.”
Until then students had marvelled at the wonderful resemblance of living
things in their inner structure without being able to explain it. We are now
in a position to explain the causes of this, by showing that this remarkable
agreement is the necessary consequence of the inheriting of common stem-
forms; while the striking difference in outward appearance is a result of
adaptation to changes of environment. Heredity and adaptation alone
furnish the true explanation.
But one special part of comparative anatomy is of supreme interest and
of the utmost philosophic importance in this connection. This is the science
of rudimentary or useless organs; I have given it the name of “dysteleology”
in view of its philosophic consequences. Nearly every organism (apart from

the very lowest), and especially every highly-developed animal or plant,
including man, has one or more organs which are of no use to the body
itself, and have no share in its functions or vital aims. Thus we all have, in
various parts of our frame, muscles which we never use, as, for instance, in
the shell of the ear and adjoining parts. In most of the mammals, especially
those with pointed ears, these internal and external ear-muscles are of great
service in altering the shell of the ear, so as to catch the waves of sound as
much as possible. But in the case of man and other short-eared mammals
these muscles are useless, though they are still present. Our ancestors
having long abandoned the use of them, we cannot work them at all to-day.
In the inner corner of the eye we have a small crescent-shaped fold of skin;
this is the last relic of a third inner eye-lid, called the nictitating (winking)
membrane. This membrane is highly developed and of great service in some
of our distant relations, such as fishes of the shark type and several other
vertebrates; in us it is shrunken and useless. In the intestines we have a
process that is not only quite useless, but may be very harmful—the
vermiform appendage. This small intestinal appendage is often the cause of
a fatal illness. If a cherry-stone or other hard body is unfortunately squeezed
through its narrow aperture during digestion, a violent inflammation is set
up, and often proves fatal. This appendix has no use whatever now in our
frame; it is a dangerous relic of an organ that was much larger and was of
great service in our vegetarian ancestors. It is still large and important in
many vegetarian animals, such as apes and rodents.
There are similar rudimentary organs in all parts of our body, and in all
the higher animals. They are among the most interesting phenomena to
which comparative anatomy introduces us; partly because they furnish one
of the clearest proofs of evolution, and partly because they most strikingly
refute the teleology of certain philosophers. The theory of evolution enables
us to give a very simple explanation of these phenomena.
We have to look on them as organs which have fallen into disuse in the
course of many generations. With the decrease in the use of its function, the
organ itself shrivels up gradually, and finally disappears. There is no other
way of explaining rudimentary organs. Hence they are also of great interest
in philosophy; they show clearly that the monistic or mechanical view of
the organism is the only correct one, and that the dualistic or teleological
conception is wrong. The ancient legend of the direct creation of man
according to a pre-conceived plan and the empty phrases about “design” in

the organism are completely shattered by them. It would be difficult to
conceive a more thorough refutation of teleology than is furnished by the
fact that all the higher animals have these rudimentary organs.
The theory of evolution finds its broadest inductive foundation in the
natural classification of living things, which arranges all the various forms
in larger and smaller groups, according to their degree of affinity. These
groupings or categories of classification—the varieties, species, genera,
families, orders, classes, etc.—show such constant features of coordination
and subordination that we are bound to look on them as genealogical, and
represent the whole system in the form of a branching tree. This is the
genealogical tree of the variously related groups; their likeness in form is
the expression of a real affinity. As it is impossible to explain in any other
way the natural tree-like form of the system of organisms, we must regard it
at once as a weighty proof of the truth of evolution. The careful
construction of these genealogical trees is, therefore, not an amusement, but
the chief task of modern classification.
Among the chief phenomena that bear witness to the inductive law of
evolution we have the geographical distribution of the various species of
animals and plants over the surface of the earth, and their topographical
distribution on the summits of mountains and in the depths of the ocean.
The scientific study of these features—the “science of distribution,” or
chorology (chora = a place)—has been pursued with lively interest since
the discoveries made by Alexander von Humboldt. Until Darwin’s time the
work was confined to the determination of the facts of the science, and
chiefly aimed at settling the spheres of distribution of the existing large and
small groups of living things. It was impossible at that time to explain the
causes of this remarkable distribution, or the reasons why one group is
found only in one locality and another in a different place, and why there is
this manifold distribution at all. Here, again, the theory of evolution has
given us the solution of the problem. It furnishes the only possible
explanation when it teaches that the various species and groups of species
descend from common stem-forms, whose ever-branching offspring have
gradually spread themselves by migration over the earth. For each group of
species we must admit a “centre of production,” or common home; this is
the original habitat in which the ancestral form was developed, and from
which its descendants spread out in every direction. Several of these
descendants became in their turn the stem-forms for new groups of species,

and these also scattered themselves by active and passive migration, and so
on. As each migrating organism found a different environment in its new
home, and adapted itself to it, it was modified, and gave rise to new forms.
This very important branch of science that deals with active and passive
migration was founded by Darwin, with the aid of the theory of evolution;
and at the same time he advanced the true explanation of the remarkable
relation or similarity of the living population in any locality to the fossil
forms found in it. Moritz Wagner very ably developed his idea under the
title of “the theory of migration.” In my opinion, this famous traveller has
rather over-estimated the value of his theory of migration when he takes it
to be an indispensable condition of the formation of new species and
opposes the theory of selection. The two theories are not opposed in their
main features. Migration (by which the stem-form of a new species is
isolated) is really only a special case of selection. The striking and
interesting facts of chorology can be explained only by the theory of
evolution, and therefore we must count them among the most important of
its inductive bases.
The same must be said of all the remarkable phenomena which we
perceive in the economy of the living organism. The many and various
relations of plants and animals to each other and to their environment,
which are treated in bionomy (from nomos, law or norm, and bios, life), the
interesting facts of parasitism, domesticity, care of the young, social habits,
etc., can only be explained by the action of heredity and adaptation.
Formerly people saw only the guidance of a beneficent Providence in these
phenomena; to-day we discover in them admirable proofs of the theory of
evolution. It is impossible to understand them except in the light of this
theory and the struggle for life.
Finally, we must, in my opinion, count among the chief inductive bases
of the theory of evolution the fœtal development of the individual organism,
the whole science of embryology or ontogeny. But as the later chapters will
deal with this in detail, I need say nothing further here. I shall endeavour in
the following pages to show, step by step, how the whole of the embryonic
phenomena form a massive chain of proof for the theory of evolution; for
they can be explained in no other way. In thus appealing to the close causal
connection between ontogenesis and phylogenesis, and taking our stand

throughout on the biogenetic law, we shall be able to prove, stage by stage,
from the facts of embryology, the evolution of man from the lower animals.
The general adoption of the theory of evolution has definitely closed the
controversy as to the nature or definition of the species. The word has no
absolute meaning whatever, but is only a group-name, or category of
classification, with a purely relative value. In 1857, it is true, a famous and
gifted, but inaccurate and dogmatic, scientist, Louis Agassiz, attempted to
give an absolute value to these “categories of classification.” He did this in
his Essay on Classification, in which he turns upside down the phenomena
of organic nature, and, instead of tracing them to their natural causes,
examines them through a theological prism. The true species (bona species)
was, he said, an “incarnate idea of the Creator.” Unfortunately, this pretty
phrase has no more scientific value than all the other attempts to save the
absolute or intrinsic value of the species.
The dogma of the fixity and creation of species lost its last great
champion when Agassiz died in 1873. The opposite theory, that all the
different species descend from common stem-forms, encounters no serious
difficulty to-day. All the endless research into the nature of the species, and
the possibility of several species descending from a common ancestor, has
been closed to-day by the removal of the sharp limits that had been set up
between species and varieties on the one hand, and species and genera on
the other. I gave an analytic proof of this in my monograph on the sponges
(1872), having made a very close study of variability in this small but
highly instructive group, and shown the impossibility of making any
dogmatic distinction of species. According as the classifier takes his ideas
of genus, species, and variety in a broader or in a narrower sense, he will
find in the small group of the sponges either one genus with three species,
or three genera with 238 species, or 113 genera with 591 species. Moreover,
all these forms are so connected by intermediate forms that we can
convincingly prove the descent of all the sponges from a common stem-
form, the olynthus.
Here, I think, I have given an analytic solution of the problem of the
origin of species, and so met the demand of certain opponents of evolution
for an actual instance of descent from a stem-form. Those who are not
satisfied with the synthetic proofs of the theory of evolution which are
provided by comparative anatomy, embryology, paleontology, dysteleology,

chorology, and classification, may try to refute the analytic proof given in
my treatise on the sponge, the outcome of five years of assiduous study. I
repeat: It is now impossible to oppose evolution on the ground that we have
no convincing example of the descent of all the species of a group from a
common ancestor. The monograph on the sponges furnishes such a proof,
and, in my opinion, an indisputable proof. Any man of science who will
follow the protracted steps of my inquiry and test my assertions will find
that in the case of the sponges we can follow the actual evolution of species
in a concrete case. And if this is so, if we can show the origin of all the
species from a common form in one single class, we have the solution of
the problem of man’s origin, because we are in a position to prove clearly
his descent from the lower animals.
At the same time, we can now reply to the often-repeated assertion, even
heard from scientists of our own day, that the descent of man from the
lower animals, and proximately from the apes, still needs to be “proved
with certainty.” These “certain proofs” have been available for a long time;
one has only to open one’s eyes to see them. It is a mistake to seek them in
the discovery of intermediate forms between man and the ape, or the
conversion of an ape into a human being by skilful education. The proofs lie
in the great mass of empirical material we have already collected. They are
furnished in the strongest form by the data of comparative anatomy and
embryology, completed by paleontology. It is not a question now of
detecting new proofs of the evolution of man, but of examining and
understanding the proofs we already have.
I was almost alone thirty-six years ago when I made the first attempt, in
my General Morphology, to put organic science on a mechanical foundation
through Darwin’s theory of descent. The association of ontogeny and
phylogeny and the proof of the intimate causal connection between these
two sections of the science of evolution, which I expounded in my work,
met with the most spirited opposition on nearly all sides. The next ten years
were a terrible “struggle for life” for the new theory. But for the last twenty-
five years the tables have been turned. The phylogenetic method has met
with so general a reception, and found so prolific a use in every branch of
biology, that it seems superfluous to treat any further here of its validity and
results. The proof of it lies in the whole morphological literature of the last
three decades. But no other science has been so profoundly modified in its
leading thoughts by this adoption, and been forced to yield such far-

reaching consequences, as that science which I am now seeking to establish
—monistic anthropogeny.
This statement may seem to be rather audacious, since the very next
branch of biology, anthropology in the stricter sense, makes very little use
of these results of anthropogeny, and sometimes expressly opposes them.[13]
This applies especially to the attitude which has characterised the German
Anthropological Society (the Deutsche Gesellschaft fur Anthropologie) for
some thirty years. Its powerful president, the famous pathologist, Rudolph
Virchow, is chiefly responsible for this. Until his death (September 5th,
1902) he never ceased to reject the theory of descent as unproven, and to
ridicule its chief consequence—the descent of man from a series of
mammal ancestors—as a fantastic dream. I need only recall his well-known
expression at the Anthropological Congress at Vienna in 1894, that “it
would be just as well to say man came from the sheep or the elephant as
from the ape.”
[13] This does not apply to English anthropologists, who are almost all
evolutionists.

Virchow’s assistant, the secretary of the German Anthropological Society,
Professor Johannes Ranke of Munich, has also indefatigably opposed
transformism: he has succeeded in writing a work in two volumes (Der
Mensch), in which all the facts relating to his organisation are explained in a
sense hostile to evolution. This work has had a wide circulation, owing to
its admirable illustrations and its able treatment of the most interesting facts
of anatomy and physiology—exclusive of the sexual organs! But, as it has
done a great deal to spread erroneous views among the general public, I
have included a criticism of it in my History of Creation, as well as met
Virchow’s attacks on anthropogeny.
Neither Virchow, nor Ranke, nor any other “exact” anthropologist, has
attempted to give any other natural explanation of the origin of man. They
have either set completely aside this “question of questions” as a
transcendental problem, or they have appealed to religion for its solution.
We have to show that this rejection of the rational explanation is totally
without justification. The fund of knowledge which has accumulated in the
progress of biology in the nineteenth century is quite adequate to furnish a
rational explanation, and to establish the theory of the evolution of man on
the solid facts of his embryology.

Chapter VI.
THE OVUM AND THE AMŒBA

In order to understand clearly the course of human embryology, we must
select the more important of its wonderful and manifold processes for fuller
explanation, and then proceed from these to the innumerable features of less
importance. The most important feature in this sense, and the best starting-
point for ontogenetic study, is the fact that man is developed from an ovum,
and that this ovum is a simple cell. The human ovum does not materially
differ in form and composition from that of the other mammals, whereas
there is a distinct difference between the fertilised ovum of the mammal and
that of any other animal.

Fig. 1—The human ovum. The globular mass of yelk (b) is enclosed by
a transparent membrane (the ovolemma or zona pellucida [a]), and contains
a noncentral nucleus (the germinal vesicle, c). Cf. Fig. 14.

This fact is so important that few should be unaware of its extreme
significance; yet it was quite unknown in the first quarter of the nineteenth
century. As we have seen, the human and mammal ovum was not
discovered until 1827, when Carl Ernst von Baer detected it. Up to that time
the larger vesicles, in which the real and much smaller ovum is contained,
had been wrongly regarded as ova. The important circumstance that this
mammal ovum is a simple cell, like the ovum of other animals, could not,
of course, be recognised until the cell theory was established. This was not
done, by Schleiden for the plant and Schwann for the animal, until 1838. As
we have seen, this cell theory is of the greatest service in explaining the

human frame and its embryonic development. Hence we must say a few
words about the actual condition of the theory and the significance of the
views it has suggested.
In order properly to appreciate the cellular theory, the most important
element in our science, it is necessary to understand in the first place that
the cell is a unified organism, a self-contained living being. When we
anatomically dissect the fully-formed animal or plant into its various
organs, and then examine the finer structure of these organs with the
microscope, we are surprised to find that all these different parts are
ultimately made up of the same structural element or unit. This common
unit of structure is the cell. It does not matter whether we thus dissect a leaf,
flower, or fruit, or a bone, muscle, gland, or bit of skin, etc.; we find in
every case the same ultimate constituent, which has been called the cell
since Schleiden’s discovery. There are many opinions as to its real nature,
but the essential point in our view of the cell is to look upon it as a self-
contained or independent living unit. It is, in the words of Brucke, “an
elementary organism.” We may define it most precisely as the ultimate
organic unit, and, as the cells are the sole active principles in every vital
function, we may call them the “plastids,” or “formative elements.” This
unity is found in both the anatomic structure and the physiological function.
In the case of the protists, the entire organism usually consists of a single
independent cell throughout life. But in the tissue-forming animals and
plants, which are the great majority, the organism begins its career as a
simple cell, and then grows into a cell-community, or, more correctly, an
organised cell-state. Our own body is not really the simple unity that it is
generally supposed to be. On the contrary, it is a very elaborate social
system of countless microscopic organisms, a colony or commonwealth,
made up of innumerable independent units, or very different tissue-cells.
In reality, the term “cell,” which existed long before the cell theory was
formulated, is not happily chosen. Schleiden, who first brought it into
scientific use in the sense of the cell theory, gave this name to the
elementary organisms because, when you find them in the dissected plant,
they generally have the appearance of chambers, like the cells in a bee-hive,
with firm walls and a fluid or pulpy content. But some cells, especially
young ones, are entirely without the enveloping membrane, or stiff wall.
Hence we now generally describe the cell as a living, viscous particle of
protoplasm, enclosing a firmer nucleus in its albuminoid body. There may

be an enclosing membrane, as there actually is in the case of most of the
plants; but it may be wholly lacking, as is the case with most of the animals.
There is no membrane at all in the first stage. The young cells are usually
round, but they vary much in shape later on. Illustrations of this will be
found in the cells of the various parts of the body shown in Figs. 3–7.
Hence the essential point in the modern idea of the cell is that it is made
up of two different active constituents—an inner and an outer part. The
smaller and inner part is the nucleus (or caryon or cytoblastus, Fig. 1c and
Fig. 2k). The outer and larger part, which encloses the other, is the body of
the cell (celleus, cytos, or cytosoma). The soft living substance of which the
two are composed has a peculiar chemical composition, and belongs to the
group of the albuminoid plasma-substances (“formative matter”), or
protoplasm. The essential and indispensable element of the nucleus is called
nuclein (or caryoplasm); that of the cell body is called plastin (or
cytoplasm). In the most rudimentary cases both substances seem to be quite
simple and homogeneous, without any visible structure. But, as a rule, when
we examine them under a high power of the microscope, we find a certain
structure in the protoplasm. The chief and most common form of this is the
fibrous or net-like “thready structure” (Frommann) and the frothy
“honeycomb structure” (Bütschli).

Fig. 2—Stem-cell of one of the echinoderms (cytula, or “first
segmentation-cell” = fertilised ovum), after Hertwig. k is the nucleus or
caryon.

The shape or outer form of the cell is infinitely varied, in accordance
with its endless power of adapting itself to the most diverse activities or
environments. In its simplest form the cell is globular (Fig. 2). This normal
round form is especially found in cells of the simplest construction, and
those that are developed in a free fluid without any external pressure. In
such cases the nucleus also is not infrequently round, and located in the

centre of the cell-body (Fig. 2k). In other cases, the cells have no definite
shape; they are constantly changing their form owing to their automatic
movements. This is the case with the amœbæ (Fig. 15 and 16) and the
amœboid travelling cells (Fig. 11), and also with very young ova (Fig.
13).However, as a rule, the cell assumes a definite form in the course of its
career. In the tissues of the multicellular organism, in which a number of
similar cells are bound together in virtue of certain laws of heredity, the
shape is determined partly by the form of their connection and partly by
their special functions. Thus, for instance, we find in the mucous lining of
our tongue very thin and delicate flat cells of roundish shape (Fig. 3). In the
outer skin we find similar, but harder, covering cells, joined together by
saw-like edges (Fig. 4). In the liver and other glands there are thicker and
softer cells, linked together in rows (Fig. 5).
The last-named tissues (Figs. 3–5) belong to the simplest and most
primitive type, the group of the “covering-tissues,” or epithelia. In these
“primary tissues” (to which the germinal layers belong) simple cells of the
same kind are arranged in layers. The arrangement and shape are more
complicated in the “secondary tissues,” which are gradually developed out
of the primary, as in the tissues of the muscles, nerves, bones, etc. In the
bones, for instance, which belong to the group of supporting or connecting
organs, the cells (Fig. 6) are star-shaped, and are joined together by
numbers of net-like interlacing processes; so, also, in the tissues of the teeth
(Fig. 7), and in other forms of supporting-tissue, in which a soft or hard
substance (intercellular matter, or base) is inserted between the cells.

Fig. 3—Three epithelial cells from the mucous lining of the tongue.
Fig. 4—Five spiny or grooved cells, with edges joined, from the outer skin
(epidermis): one of them (b) is isolated.
Fig. 5—Ten liver-cells: one of them (b) has two nuclei.

The cells also differ very much in size. The great majority of them are
invisible to the naked eye, and can be seen only through the microscope
(being as a rule between 1/2500 and 1/250 inch in diameter). There are
many of the smaller plastids—such as the famous bacteria—which only
come into view with a very high magnifying power. On the other hand,
many cells attain a considerable size, and run occasionally to several inches
in diameter, as do certain kinds of rhizopods among the unicellular protists
(such as the radiolaria and thalamophora). Among the tissue-cells of the
animal body many of the muscular fibres and nerve fibres are more than
four inches, and sometimes more than a yard, in length. Among the largest
cells are the yelk-filled ova; as, for instance, the yellow “yolk” in the hen’s
egg, which we shall describe later (Fig. 15).
Cells also vary considerably in structure. In this connection we must first
distinguish between the active and passive components of the cell. It is only
the former, or active parts of the cell, that really live, and effect that
marvellous world of phenomena to which we give the name of “organic
life.” The first of these is the inner nucleus (caryoplasm), and the second
the body of the cell (cytoplasm). The passive portions come third; these are
subsequently formed from the others, and I have given them the name of
“plasma-products.” They are partly external (cell-membranes and
intercellular matter) and partly internal (cell-sap and cell-contents).
The nucleus (or caryon), which is usually of a simple roundish form, is
quite structureless at first (especially in very young cells), and composed of
homogeneous nuclear matter or caryoplasm (Fig. 2k). But, as a rule, it
forms a sort of vesicle later on, in which we can distinguish a more solid
nuclear base (caryobasis) and a softer or fluid nuclear sap (caryolymph). In
a mesh of the nuclear network (or it may be on the inner side of the nuclear
envelope) there is, as a rule, a dark, very opaque, solid body, called the
nucleolus. Many of the nuclei contain several of these nucleoli (as, for
instance, the germinal vesicle of the ova of fishes and amphibia). Recently a
very small, but particularly important, part of the nucleus has been
distinguished as the central body (centrosoma)—a tiny particle that is
originally found in the nucleus itself, but is usually outside it, in the
cytoplasm; as a rule, fine threads stream out from it in the cytoplasm. From
the position of the central body with regard to the other parts it seems
probable that it has a high physiological importance as a centre of
movement; but it is lacking in many cells.

The cell-body also consists originally, and in its simplest form, of a
homogeneous viscid plasmic matter. But, as a rule, only the smaller part of
it is formed of the living active cell-substance (protoplasm); the greater part
consists of dead, passive plasma-products (metaplasm). It is useful to
distinguish between the inner and outer of these. External plasma-products
(which are thrust out from the protoplasm as solid “structural matter”) are
the cell-membranes and the intercellular matter. The internal plasma-
products are either the fluid cell-sap or hard structures. As a rule, in mature
and differentiated cells these various parts are so arranged that the
protoplasm (like the caryoplasm in the round nucleus) forms a sort of
skeleton or framework. The spaces of this network are filled partly with the
fluid cell-sap and partly by hard structural products.

Fig. 6—Nine star-shaped bone-cells, with interlaced branches.

The simple round ovum, which we take as the starting-point of our study
(Figs. 1 and 2), has in many cases the vague, indifferent features of the
typical primitive cell. As a contrast to it, and as an instance of a very highly
differentiated plastid, we may consider for a moment a large nerve-cell, or
ganglionic cell, from the brain. The ovum stands potentially for the entire
organism—in other words, it has the faculty of building up out of itself the
whole multicellular body. It is the common parent of all the countless
generations of cells which form the different tissues of the body; it unites all
their powers in itself, though only potentially or in germ. In complete
contrast to this, the neural cell in the brain (Fig. 9) develops along one rigid
line. It cannot, like the ovum, beget endless generations of cells, of which

some will become skin-cells, others muscle-cells, and others again bone-
cells. But, on the other hand, the nerve-cell has become fitted to discharge
the highest functions of life; it has the powers of sensation, will, and
thought. It is a real soul-cell, or an elementary organ of the psychic activity.
It has, therefore, a most elaborate and delicate structure.

Fig. 7—Eleven star-shaped cells from the enamel of a tooth, joined
together by their branchlets.

Numbers of extremely fine threads, like the electric wires at a large
telegraphic centre, cross and recross in the delicate protoplasm of the nerve
cell, and pass out in the branching processes which proceed from it and put
it in communication with other nerve-cells or nerve-fibres (a, b). We can
only partly follow their intricate paths in the fine matter of the body of the
cell.
Here we have a most elaborate apparatus, the delicate structure of which
we are just beginning to appreciate through our most powerful microscopes,
but whose significance is rather a matter of conjecture than knowledge. Its
intricate structure corresponds to the very complicated functions of the
mind. Nevertheless, this elementary organ of psychic activity—of which
there are thousands in our brain—is nothing but a single cell. Our whole
mental life is only the joint result of the combined activity of all these
nerve-cells, or soul-cells. In the centre of each cell there is a large
transparent nucleus, containing a small and dark nuclear body. Here, as
elsewhere, it is the nucleus that determines the individuality of the cell; it
proves that the whole structure, in spite of its intricate composition,
amounts to only a single cell.

Fig. 8—Unfertilised ovum of an echinoderm (from Hertwig). The
vesicular nucleus (or “germinal vesicle”) is globular, half the size of the
round ovum, and encloses a nuclear framework, in the central knot of which
there is a dark nucleolus (the “germinal spot”).

In contrast with this very elaborate and very strictly differentiated
psychic cell (Fig. 9), we have our ovum (Figs. 1 and 2), which has hardly
any structure at all. But even in the case of the ovum we must infer from its
properties that its protoplasmic body has a very complicated chemical
composition and a fine molecular structure which escapes our observation.
This presumed molecular structure of the plasm is now generally admitted;
but it has never been seen, and, indeed, lies far beyond the range of
microscopic vision. It must not be confused—as is often done—with the
structure of the plasm (the fibrous network, groups of granules, honey-
comb, etc.) which does come within the range of the microscope.
But when we speak of the cells as the elementary organisms, or structural
units, or “ultimate individualities,” we must bear in mind a certain
restriction of the phrases. I mean, that the cells are not, as is often supposed,
the very lowest stage of organic individuality. There are yet more
elementary organisms to which I must refer occasionally. These are what
we call the “cytodes” (cytos = cell), certain living, independent beings,
consisting only of a particle of plasson—an albuminoid substance, which is
not yet differentiated into caryoplasm and cytoplasm, but combines the
properties of both. Those remarkable beings called the monera—especially
the chromacea and bacteria—are specimens of these simple cytodes.
(Compare Chapter XIX.) To be quite accurate, then, we must say: the
elementary organism, or the ultimate individual, is found in two different
stages. The first and lower stage is the cytode, which consists merely of a
particle of plasson, or quite simple plasm. The second and higher stage is
the cell, which is already divided or differentiated into nuclear matter and

cellular matter. We comprise both kinds—the cytodes and the cells—under
the name of plastids (“formative particles”), because they are the real
builders of the organism. However, these cytodes are not found, as a rule, in
the higher animals and plants; here we have only real cells with a nucleus.
Hence, in these tissue-forming organisms (both plant and animal) the
organic unit always consists of two chemically and anatomically different
parts—the outer cell-body and the inner nucleus.
In order to convince oneself that this cell is really an independent
organism, we have only to observe the development and vital phenomena of
one of them. We see then that it performs all the essential functions of life—
both vegetal and animal—which we find in the entire organism. Each of
these tiny beings grows and nourishes itself independently. It takes its food
from the surrounding fluid; sometimes, even, the naked cells take in solid
particles at certain points of their surface—in other words, “eat” them—
without needing any special mouth and stomach for the purpose (cf. Fig.
19).
Further, each cell is able to reproduce itself. This multiplication, in most
cases, takes the form of a simple cleavage, sometimes direct, sometimes
indirect; the simple direct (or “amitotic”) division is less common, and is
found, for instance, in the blood cells (Fig. 10). In these the nucleus first
divides into two equal parts by constriction. The indirect (or “mitotic”)
cleavage is much more frequent; in this the caryoplasm of the nucleus and
the cytoplasm of the cell-body act upon each other in a peculiar way, with a
partial dissolution (caryolysis), the formation of knots and loops (mitosis),
and a movement of the halved plasma-particles towards two mutually
repulsive poles of attraction (caryokinesis, Fig. 11.)

Fig. 9—A large branching nerve-cell, or “soul-cell”, from the brain of
an electric fish (Torpedo). In the middle of the cell is the large transparent
round nucleus, one nucleolus, and, within the latter again, a nucleolinus.
The protoplasm of the cell is split into innumerable fine threads (or fibrils),
which are embedded in intercellular matter, and are prolonged into the
branching processes of the cell (b). One branch (a) passes into a nerve-
fibre. (From Max Schultze.)

Fig. 10—Blood-cells, multiplying by direct division, from the blood of
the embryo of a stag. Originally, each blood-cell has a nucleus and is round
(a). When it is going to multiply, the nucleus divides into two (b, c, d). Then
the protoplasmic body is constricted between the two nuclei, and these
move away from each other (e). Finally, the constriction is complete, and
the cell splits into two daughter-cells (f). (From Frey.)

The intricate physiological processes which accompany this “mitosis”
have been very closely studied of late years. The inquiry has led to the
detection of certain laws of evolution which are of extreme importance in
connection with heredity. As a rule, two very different parts of the nucleus
play an important part in these changes. They are: the chromatin, or
coloured nuclear substance, which has a peculiar property of tingeing itself
deeply with certain colouring matters (carmine, hæmatoxylin, etc.), and the
achromin (or linin, or achromatin), a colourless nuclear substance that lacks
this property. The latter generally forms in the dividing cell a sort of
spindle, at the poles of which there is a very small particle, also colourless,
called the “central body” (centrosoma). This acts as the centre or focus in a
“sphere of attraction” for the granules of protoplasm in the surrounding
cell-body, and assumes a star-like appearance (the cell-star, or monaster).
The two central bodies, standing opposed to each other at the poles of the
nuclear spindle, form “the double-star” (or amphiaster, Fig. 11, B C). The
chromatin often forms a long, irregularly-wound thread—“the coil”
(spirema, Fig. A). At the commencement of the cleavage it gathers at the
equator of the cell, between the stellar poles, and forms a crown of U-
shaped loops (generally four or eight, or some other definite number). The
loops split lengthwise into two halves (B), and these back away from each
other towards the poles of the spindle (C). Here each group forms a crown
once more, and this, with the corresponding half of the divided spindle,
forms a fresh nucleus (D). Then the protoplasm of the cell-body begins to

contract in the middle, and gather about the new daughter-nuclei, and at last
the two daughter-cells become independent beings.
Between this common mitosis, or indirect cell-division—which is the
normal cleavage-process in most cells of the higher animals and plants—
and the simple direct division (Fig. 10) we find every grade of
segmentation; in some circumstances even one kind of division may be
converted into another.
The plastid is also endowed with the functions of movement and
sensation. The single cell can move and creep about, when it has space for
free movement and is not prevented by a hard envelope; it then thrusts out
at its surface processes like fingers, and quickly withdraws them again, and
thus changes its shape (Fig. 12). Finally, the young cell is sensitive, or more
or less responsive to stimuli; it makes certain movements on the application
of chemical and mechanical irritation. Hence we can ascribe to the
individual cell all the chief functions which we comprehend under the
general heading of “life”—sensation, movement, nutrition, and
reproduction. All these properties of the multicellular and highly developed
animal are also found in the single animal-cell, at least in its younger stages.
There is no longer any doubt about this, and so we may regard it as a solid
and important base of our physiological conception of the elementary
organism.
Without going any further here into these very interesting phenomena of
the life of the cell, we will pass on to consider the application of the cell
theory to the ovum. Here comparative research yields the important result
that every ovum is at first a simple cell. I say this is very important, because
our whole science of embryology now resolves itself into the problem:
“How does the multicellular organism arise from the unicellular?” Every
organic individual is at first a simple cell, and as such an elementary
organism, or a unit of individuality. This cell produces a cluster of cells by
segmentation, and from these develops the multicellular organism, or
individual of higher rank.

A. Mother-cell
(Knot, spirema)
1. Nuclear threads (chromosomata) (coloured nuclear matter, chromatin)
2. Nuclear membrane
3. Nuclear sap
4. Cytosoma
5. Protoplasm of the cell-body

B. Mother-star, the loops beginning to split lengthways (nuclear membrane
gone)
1. Star-like appearance in cytoplasm
2. Centrosoma (sphere of attraction)
3. Nuclear spindle (achromin, colourless matter)
4. Nuclear loops (chromatin, coloured matter)

C. The two daughter-stars,
produced by the breaking of the loops of the mother-star (moving away)
1. Upper daughter-crown
2. Connecting threads of the two crowns (achromin)
3. Lower daughter-crown
4. Double-star (amphiaster)

D. The two daughter-cells,
produced by the complete division of the two nuclear halves (cytosomata
still connected at the equator) (Double-knot, Dispirema)
1. Upper daughter-nucleus
2. Equatorial constriction of the cell-body
3. Lower daughter-nucleus.
Fig. 11—Indirect or mitotic cell-division (with caryolysis and
caryokinesis) from the skin of the larva of a salamander. (From Rabl.).

When we examine a little closer the original features of the ovum, we
notice the extremely significant fact that in its first stage the ovum is just
the same simple and indefinite structure in the case of man and all the
animals (Fig. 13). We are unable to detect any material difference between
them, either in outer shape or internal constitution. Later, though the ova
remain unicellular, they differ in size and shape, enclose various kinds of
yelk-particles, have different envelopes, and so on. But when we examine
them at their birth, in the ovary of the female animal, we find them to be
always of the same form in the first stages of their life. In the beginning
each ovum is a very simple, roundish, naked, mobile cell, without a
membrane; it consists merely of a particle of cytoplasm enclosing a nucleus
(Fig. 13). Special names have been given to these parts of the ovum; the
cell-body is called the yelk (vitellus), and the cell-nucleus the germinal
vesicle. As a rule, the nucleus of the ovum is soft, and looks like a small
pimple or vesicle. Inside it, as in many other cells, there is a nuclear
skeleton or frame and a third, hard nuclear body (the nucleolus). In the
ovum this is called the germinal spot. Finally, we find in many ova (but not
in all) a still further point within the germinal spot, a “nucleolin,” which

goes by the name of the germinal point. The latter parts (germinal spot and
germinal point) have, apparently, a minor importance, in comparison with
the other two (the yelk and germinal vesicle). In the yelk we must
distinguish the active formative yelk (or protoplasm = first plasm) from the
passive nutritive yelk (or deutoplasm = second plasm).

Fig. 12—Mobile cells from the inflamed eye of a frog (from the watery
fluid of the eye, the humor aqueus). The naked cells creep freely about, by
(like the amœba or rhizopods) protruding fine processes from the uncovered
protoplasmic body. These bodies vary continually in number, shape, and
size. The nucleus of these amœboid lymph-cells (“travelling cells,” or
planocytes) is invisible, because concealed by the numbers of fine granules
which are scattered in the protoplasm. (From Frey.)

In many of the lower animals (such as sponges, polyps, and medusæ) the
naked ova retain their original simple appearance until impregnation. But in
most animals they at once begin to change; the change consists partly in the
formation of connections with the yelk, which serve to nourish the ovum,
and partly of external membranes for their protection (the ovolemma, or
prochorion). A membrane of this sort is formed in all the mammals in the
course of the embryonic process. The little globule is surrounded by a thick
capsule of glass-like transparency, the zona pellucida, or ovolemma
pellucidum (Fig. 14). When we examine it closely under the microscope, we
see very fine radial streaks in it, piercing the zona, which are really very
narrow canals. The human ovum, whether fertilised or not, cannot be
distinguished from that of most of the other mammals. It is nearly the same
everywhere in form, size, and composition. When it is fully formed, it has a
diameter of (on an average) about 1/120 of an inch. When the mammal
ovum has been carefully isolated, and held against the light on a glass-plate,

it may be seen as a fine point even with the naked eye. The ova of most of
the higher mammals are about the same size. The diameter of the ovum is
almost always between 1/250 to 1/125 inch. It has always the same globular
shape; the same characteristic membrane; the same transparent germinal
vesicle with its dark germinal spot. Even when we use the most powerful
microscope with its highest power, we can detect no material difference
between the ova of man, the ape, the dog, and so on. I do not mean to say
that there are no differences between the ova of these different mammals.
On the contrary, we are bound to assume that there are such, at least as
regards chemical composition. Even the ova of different men must differ
from each other; otherwise we should not have a different individual from
each ovum. It is true that our crude and imperfect apparatus cannot detect
these subtle individual differences, which are probably in the molecular
structure. However, such a striking resemblance of their ova in form, so
great as to seem to be a complete similarity, is a strong proof of the
common parentage of man and the other mammals. From the common
germ-form we infer a common stem-form. On the other hand, there are
striking peculiarities by which we can easily distinguish the fertilised ovum
of the mammal from the fertilised ovum of the birds, amphibia, fishes, and
other vertebrates (see the close of Chap. XXIX).
The fertilised bird-ovum (Fig. 15) is notably different. It is true that in its
earliest stage (Fig. 13 E) this ovum also is very like that of the mammal
(Fig. 13 F). But afterwards, while still within the oviduct, it takes up a
quantity of nourishment and works this into the familiar large yellow yelk.
When we examine a very young ovum in the hen’s oviduct, we find it to be
a simple, small, naked, amœboid cell, just like the young ova of other
animals (Fig. 13). But it then grows to the size we are familiar with in the
round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is
thus pressed right to the surface of the globular ovum, and is embedded
there in a small quantity of transparent matter, the so-called white yelk. This
forms a round white spot, which is known as the “tread” (cicatricula) (Fig.
15 b). From the tread a thin column of the white yelk penetrates through the
yellow yelk to the centre of the globular cell, where it swells into a small,
central globule (wrongly called the yelk-cavity, or latebra, Fig. 15 d′). The
yellow yelk-matter which surrounds this white yelk has the appearance in
the egg (when boiled hard) of concentric layers (c). The yellow yelk is also
enclosed in a delicate structureless membrane (the membrana vitellina, a).

Fig. 13—Ova of various animals, executing amœboid movements,
magnified. All the ova are naked cells of varying shape. In the dark fine-
grained protoplasm (yelk) is a large vesicular nucleus (the germinal
vesicle), and in this is seen a nuclear body (the germinal spot), in which
again we often see a germinal point. Figs. A1–A4 represent the ovum of a
sponge (Leuculmis echinus) in four successive movements. B1–B8 are the
ovum of a parasitic crab (Chondracanthus cornutus), in eight successive
movements. (From Edward von Beneden.) C1–C5 show the ovum of the cat
in various stages of movement (from Pflüger); Fig. D the ovum of a trout; E
the ovum of a chicken; F a human ovum.

As the large yellow ovum of the bird attains a diameter of several inches
in the bigger birds, and encloses round yelk-particles, there was formerly a
reluctance to consider it as a simple cell. This was a mistake. Every animal
that has only one cell-nucleus, every amœba, every gregarina, every
infusorium, is unicellular, and remains unicellular whatever variety of
matter it feeds on. So the ovum remains a simple cell, however much
yellow yelk it afterwards accumulates within its protoplasm. It is, of course,
different, with the bird’s egg when it has been fertilised. The ovum then
consists of as many cells as there are nuclei in the tread. Hence, in the
fertilised egg which we eat daily, the yellow yelk is already a multicellular

body. Its tread is composed of several cells, and is now commonly called
the germinal disc. We shall return to this discogastrula in Chap. IX.

Fig. 14—The human ovum, taken from the female ovary, magnified.
The whole ovum is a simple round cell. The chief part of the globular mass
is formed by the nuclear yelk (deutoplasm), which is evenly distributed in
the active protoplasm, and consists of numbers of fine yelk-granules. In the
upper part of the yelk is the transparent round germinal vesicle, which
corresponds to the nucleus. This encloses a darker granule, the germinal
spot, which shows a nucleolus. The globular yelk is surrounded by the thick
transparent germinal membrane (ovolemma, or zona pellucida). This is
traversed by numbers of lines as fine as hairs, which are directed radially
towards the centre of the ovum. These are called the pore-canals; it is
through these that the moving spermatozoa penetrate into the yelk at
impregnation.

When the mature bird-ovum has left the ovary and been fertilised in the
oviduct, it covers itself with various membranes which are secreted from
the wall of the oviduct. First, the large clear albuminous layer is deposited
around the yellow yelk; afterwards, the hard external shell, with a fine inner
skin. All these gradually forming envelopes and processes are of no
importance in the formation of the embryo; they serve merely for the
protection of the original simple ovum. We sometimes find extraordinarily
large eggs with strong envelopes in the case of other animals, such as fishes
of the shark type. Here, also, the ovum is originally of the same character as
it is in the mammal; it is a perfectly simple and naked cell. But, as in the
case of the bird, a considerable quantity of nutritive yelk is accumulated

inside the original yelk as food for the developing embryo; and various
coverings are formed round the egg. The ovum of many other animals has
the same internal and external features. They have, however, only a
physiological, not a morphological, importance; they have no direct
influence on the formation of the fœtus. They are partly consumed as food
by the embryo, and partly serve as protective envelopes. Hence we may
leave them out of consideration altogether here, and restrict ourselves to
material points—to the substantial identity of the original ovum in man and
the rest of the animals (Fig. 13).
Now, let us for the first time make use of our biogenetic law; and directly
apply this fundamental law of evolution to the human ovum. We reach a
very simple, but very important, conclusion. From the fact that the human
ovum and that of all other animals consists of a single cell, it follows
immediately, according to the biogenetic law, that all the animals, including
man, descend from a unicellular organism. If our biogenetic law is true, if
the embryonic development is a summary or condensed recapitulation of
the stem-history—and there can be no doubt about it—we are bound to
conclude, from the fact that all the ova are at first simple cells, that all the
multicellular organisms originally sprang from a unicellular being. And as
the original ovum in man and all the other animals has the same simple and
indefinite appearance, we may assume with some probability that this
unicellular stem-form was the common ancestor of the whole animal world,
including man. However, this last hypothesis does not seem to me as
inevitable and as absolutely certain as our first conclusion.

Fig. 15—A fertilised ovum from the oviduct of a hen. The yellow yelk
(c) consists of several concentric layers (d), and is enclosed in a thin yelk-
membrane (a). The nucleus or germinal vesicle is seen above in the cicatrix
or “tread” (b). From that point the white yelk penetrates to the central yelk-
cavity (d′). The two kinds of yelk do not differ very much.

This inference from the unicellular embryonic form to the unicellular
ancestor is so simple, but so important, that we cannot sufficiently
emphasise it. We must, therefore, turn next to the question whether there are
to-day any unicellular organisms, from the features of which we may draw
some approximate conclusion as to the unicellular ancestors of the
multicellular organisms. The answer is: Most certainly there are. There are
assuredly still unicellular organisms which are, in their whole nature, really
nothing more than permanent ova. There are independent unicellular
organisms of the simplest character which develop no further, but reproduce
themselves as such, without any further growth. We know to-day of a great
number of these little beings, such as the gregarinæ, flagellata, acineta,
infusoria, etc. However, there is one of them that has an especial interest for
us, because it at once suggests itself when we raise our question, and it must
be regarded as the unicellular being that approaches nearest to the real
ancestral form. This organism is the Amœba.

Fig. 16—A creeping amœba (highly magnified). The whole organism is
a simple naked cell, and moves about by means of the changing arms which
it thrusts out of and withdraws into its protoplasmic body. Inside it is the
roundish nucleus with its nucleolus.

For a long time now we have comprised under the general name of
amœbæ a number of microscopic unicellular organisms, which are very
widely distributed, especially in fresh-water, but also in the ocean; in fact,
they have lately been discovered in damp soil. There are also parasitic
amœbæ which live inside other animals. When we place one of these
amœbæ in a drop of water under the microscope and examine it with a high
power, it generally appears as a roundish particle of a very irregular and
varying shape (Figs. 16 and 17). In its soft, slimy, semi-fluid substance,
which consists of protoplasm, we see only the solid globular particle it

contains, the nucleus. This unicellular body moves about continually,
creeping in every direction on the glass on which we are examining it. The
movement is effected by the shapeless body thrusting out finger-like
processes at various parts of its surface; and these are slowly but
continually changing, and drawing the rest of the body after them. After a
time, perhaps, the action changes. The amœba suddenly stands still,
withdraws its projections, and assumes a globular shape. In a little while,
however, the round body begins to expand again, thrusts out arms in
another direction, and moves on once more. These changeable processes are
called “false feet,” or pseudopodia, because they act physiologically as feet,
yet are not special organs in the anatomic sense. They disappear as quickly
as they come, and are nothing more than temporary projections of the semi-
fluid and structureless body.

Fig. 17—Division of a unicellular amœba (Amœba polypodia) in six
stages. (From F. E. Schultze.) the dark spot is the nucleus, the lighter spot a
contractile vacuole in the protoplasm. The latter reforms in one of the
daughter-cells.)

If you touch one of these creeping amœbæ with a needle, or put a drop of
acid in the water, the whole body at once contracts in consequence of this
mechanical or physical stimulus. As a rule, the body then resumes its
globular shape. In certain circumstances—for instance, if the impurity of
the water lasts some time—the amœba begins to develop a covering. It

exudes a membrane or capsule, which immediately hardens, and assumes
the appearance of a round cell with a protective membrane. The amœba
either takes its food directly by imbibition of matter floating in the water, or
by pressing into its protoplasmic body solid particles with which it comes in
contact. The latter process may be observed at any moment by forcing it to
eat. If finely ground colouring matter, such as carmine or indigo, is put into
the water, you can see the body of the amœba pressing these coloured
particles into itself, the substance of the cell closing round them. The
amœba can take in food in this way at any point on its surface, without
having any special organs for intussusception and digestion, or a real mouth
or gut.
The amœba grows by thus taking in food and dissolving the particles
eaten in its protoplasm. When it reaches a certain size by this continual
feeding, it begins to reproduce. This is done by the simple process of
cleavage (Fig. 17). First, the nucleus divides into two parts. Then the
protoplasm is separated between the two new nuclei, and the whole cell
splits into two daughter-cells, the protoplasm gathering about each of the
nuclei. The thin bridge of protoplasm which at first connects the daughter-
cells soon breaks. Here we have the simple form of direct cleavage of the
nuclei. Without mitosis, or formation of threads, the homogeneous nucleus
divides into two halves. These move away from each other, and become
centres of attraction for the enveloping matter, the protoplasm. The same
direct cleavage of the nuclei is also witnessed in the reproduction of many
other protists, while other unicellular organisms show the indirect division
of the cell.
Hence, although the amœba is nothing but a simple cell, it is evidently
able to accomplish all the functions of the multicellular organism. It moves,
feels, nourishes itself, and reproduces. Some kinds of these amœbæ can be
seen with the naked eye, but most of them are microscopically small. It is
for the following reasons that we regard the amœbæ as the unicellular
organisms which have special phylogenetic (or evolutionary) relations to
the ovum. In many of the lower animals the ovum retains its original naked
form until fertilisation, develops no membranes, and is then often
indistinguishable from the ordinary amœba. Like the amœbæ, these naked
ova may thrust out processes, and move about as travelling cells. In the
sponges these mobile ova move about freely in the maternal body like
independent amœbæ (Fig. 17). They had been observed by earlier scientists,

but described as foreign bodies—namely, parasitic amœbæ, living
parasitically on the body of the sponge. Later, however, it was discovered
that they were not parasites, but the ova of the sponge. We also find this
remarkable phenomenon among other animals, such as the graceful, bell-
shaped zoophytes, which we call polyps and medusæ. Their ova remain
naked cells, which thrust out amœboid projections, nourish themselves, and
move about. When they have been fertilised, the multicellular organism is
formed from them by repeated segmentation.
It is, therefore, no audacious hypothesis, but a perfectly sound
conclusion, to regard the amœba as the particular unicellular organism
which offers us an approximate illustration of the ancient common
unicellular ancestor of all the metazoa, or multicellular animals. The simple
naked amœba has a less definite and more original character than any other
cell. Moreover, there is the fact that recent research has discovered such
amœba-like cells everywhere in the mature body of the multicellular
animals. They are found, for instance, in the human blood, side by side with
the red corpuscles, as colourless blood-cells; and it is the same with all the
vertebrates. They are also found in many of the invertebrates—for instance,
in the blood of the snail. I showed, in 1859, that these colourless blood-cells
can, like the independent amœbæ, take up solid particles, or “eat” (whence
they are called phagocytes = “eating-cells,” Fig. 19). Lately, it has been
discovered that many different cells may, if they have room enough,
execute the same movements, creeping about and eating. They behave just
like amœbæ (Fig. 12). It has also been shown that these “travelling-cells,”
or planocytes, play an important part in man’s physiology and pathology (as
means of transport for food, infectious matter, bacteria, etc.).
The power of the naked cell to execute these characteristic amœba-like
movements comes from the contractility (or automatic mobility) of its
protoplasm. This seems to be a universal property of young cells. When
they are not enclosed by a firm membrane, or confined in a “cellular
prison,” they can always accomplish these amœboid movements. This is
true of the naked ova as well as of any other naked cells, of the “travelling-
cells,” of various kinds in connective tissue, lymph-cells, mucus-cells, etc.

Fig. 18—Ovum of a sponge (Olynthus). The ovum creeps about in a
body of the sponge by thrusting out ever-changing processes. It is
indistinguishable from the common amœba.)

We have now, by our study of the ovum and the comparison of it with the
amœba, provided a perfectly sound and most valuable foundation for both
the embryology and the evolution of man. We have learned that the human
ovum is a simple cell, that this ovum is not materially different from that of
other mammals, and that we may infer from it the existence of a primitive
unicellular ancestral form, with a substantial resemblance to the amœba.
The statement that the earliest progenitors of the human race were simple
cells of this kind, and led an independent unicellular life like the amœba,
has not only been ridiculed as the dream of a natural philosopher, but also
been violently censured in theological journals as “shameful and immoral.”
But, as I observed in my essay On the Origin and Ancestral Tree of the
Human Race in 1870, this offended piety must equally protest against the
“shameful and immoral” fact that each human individual is developed from
a simple ovum, and that this human ovum is indistinguishable from those of
the other mammals, and in its earliest stage is like a naked amœba. We can
show this to be a fact any day with the microscope, and it is little use to
close one’s eyes to “immoral” facts of this kind. It is as indisputable as the
momentous conclusions we draw from it and as the vertebrate character of
man (see Chap. XI).

Fig. 19—Blood-cells that eat, or phagocytes, from a naked sea-snail
(Thetis), greatly magnified. I was the first to observe in the blood-cells of

this snail the important fact that “the blood-cells of the invertebrates are
unprotected pieces of plasm, and take in food, by means of their peculiar
movements, like the amœbæ.” I had (in Naples, on May 10th, 1859)
injected into the blood-vessels of one of these snails an infusion of water
and ground indigo, and was greatly astonished to find the blood-cells
themselves more or less filled with the particles of indigo after a few hours.
After repeated injections I succeeded in “observing the very entrance of the
coloured particles in the blood-cells, which took place just in the same way
as with the amœba.” I have given further particulars about this in my
Monograph on the Radiolaria.

We now see very clearly how extremely important the cell theory has
been for our whole conception of organic nature. “Man’s place in nature” is
settled beyond question by it. Apart from the cell theory, man is an
insoluble enigma to us. Hence philosophers, and especially physiologists,
should be thoroughly conversant with it. The soul of man can only be really
understood in the light of the cell-soul, and we have the simplest form of
this in the amœba. Only those who are acquainted with the simple psychic
functions of the unicellular organisms and their gradual evolution in the
series of lower animals can understand how the elaborate mind of the
higher vertebrates, and especially of man, was gradually evolved from
them. The academic psychologists who lack this zoological equipment are
unable to do so.
This naturalistic and realistic conception is a stumbling-block to our
modern idealistic metaphysicians and their theological colleagues. Fenced
about with their transcendental and dualistic prejudices, they attack not only
the monistic system we establish on our scientific knowledge, but even the
plainest facts which go to form its foundation. An instructive instance of
this was seen a few years ago, in the academic discourse delivered by a
distinguished theologian, Willibald Beyschlag, at Halle, January 12th, 1900,
on the occasion of the centenary festival. The theologian protested violently
against the “materialistic dustmen of the scientific world who offer our
people the diploma of a descent from the ape, and would prove to them that
the genius of a Shakespeare or a Goethe is merely a distillation from a drop
of primitive mucus.” Another well-known theologian protested against “the
horrible idea that the greatest of men, Luther and Christ, were descended
from a mere globule of protoplasm.” Nevertheless, not a single informed

and impartial scientist doubts the fact that these greatest men were, like all
other men—and all other vertebrates—developed from an impregnated
ovum, and that this simple nucleated globule of protoplasm has the same
chemical constitution in all the mammals.

Chapter VII.
CONCEPTION

The recognition of the fact that every man begins his individual existence
as a simple cell is the solid foundation of all research into the genesis of
man. From this fact we are forced, in virtue of our biogenetic law, to draw
the weighty phylogenetic conclusion that the earliest ancestors of the
human race were also unicellular organisms; and among these protozoa we
may single out the vague form of the amœba as particularly important (cf.
Chapter VI). That these unicellular ancestral forms did once exist follows
directly from the phenomena which we perceive every day in the fertilised
ovum. The development of the multicellular organism from the ovum, and
the formation of the germinal layers and the tissues, follow the same laws in
man and all the higher animals. It will, therefore, be our next task to
consider more closely the impregnated ovum and the process of conception
which produces it.
The process of impregnation or sexual conception is one of those
phenomena that people love to conceal behind the mystic veil of
supernatural power. We shall soon see, however, that it is a purely
mechanical process, and can be reduced to familiar physiological functions.
Moreover, this process of conception is of the same type, and is effected by
the same organs, in man as in all the other mammals. The pairing of the
male and female has in both cases for its main purpose the introduction of
the ripe matter of the male seed or sperm into the female body, in the sexual
canals of which it encounters the ovum. Conception then ensues by the
blending of the two.
We must observe, first, that this important process is by no means so
widely distributed in the animal and plant world as is commonly supposed.
There is a very large number of lower organisms which propagate
unsexually, or by monogamy; these are especially the sexless monera

(chromacea, bacteria, etc.) but also many other protists, such as the amœbæ,
foraminifera, radiolaria, myxomycetæ, etc. In these the multiplication of
individuals takes place by unsexual reproduction, which takes the form of
cleavage, budding, or spore-formation. The copulation of two coalescing
cells, which in these cases often precedes the reproduction, cannot be
regarded as a sexual act unless the two copulating plastids differ in size or
structure. On the other hand, sexual reproduction is the general rule with all
the higher organisms, both animal and plant; very rarely do we find asexual
reproduction among them. There are, in particular, no cases of
parthenogenesis (virginal conception) among the vertebrates.
Sexual reproduction offers an infinite variety of interesting forms in the
different classes of animals and plants, especially as regards the mode of
conception, and the conveyance of the spermatozoon to the ovum. These
features are of great importance not only as regards conception itself, but
for the development of the organic form, and especially for the
differentiation of the sexes. There is a particularly curious correlation of
plants and animals in this respect. The splendid studies of Charles Darwin
and Hermann Müller on the fertilisation of flowers by insects have given us
very interesting particulars of this.[14] This reciprocal service has given rise
to a most intricate sexual apparatus. Equally elaborate structures have been
developed in man and the higher animals, serving partly for the isolation of
the sexual products on each side, partly for bringing them together in
conception. But, however interesting these phenomena are in themselves,
we cannot go into them here, as they have only a minor importance—if any
at all—in the real process of conception. We must, however, try to get a
very clear idea of this process and the meaning of sexual reproduction.
[14] See Darwin’s work, On the Various Contrivances by which Orchids are
Fertilised (1862).

In every act of conception we have, as I said, to consider two different
kinds of cells—a female and a male cell. The female cell of the animal
organism is always called the ovum (or ovulum, egg, or egg-cell); the male
cells are known as the sperm or seed-cells, or the spermatozoa (also
spermium and zoospermium). The ripe ovum is, on the whole, one of the
largest cells we know. It attains colossal dimensions when it absorbs great
quantities of nutritive yelk, as is the case with birds and reptiles and many
of the fishes. In the great majority of the animals the ripe ovum is rich in

yelk and much larger than the other cells. On the other hand, the next cell
which we have to consider in the process of conception, the male sperm-cell
or spermatozoon, is one of the smallest cells in the animal body. Conception
usually consists in the bringing into contact with the ovum of a slimy fluid
secreted by the male, and this may take place either inside or out of the
female body. This fluid is called sperm, or the male seed. Sperm, like saliva
or blood, is not a simple fluid, but a thick agglomeration of innumerable
cells, swimming about in a comparatively small quantity of fluid. It is not
the fluid, but the independent male cells that swim in it, that cause
conception.

Fig. 20—Spermia or spermatozoa of various mammals. The pear-
shaped flattened nucleus is seen from the front in I and sideways in II. k is
the nucleus, m its middle part (protoplasm), s the mobile, serpent-like tail
(or whip); M four human spermatozoa, A four spermatozoa from the ape; K
from the rabbit; H from the mouse; C from the dog; S from the pig.

The spermatozoa of the great majority of animals have two characteristic
features. Firstly, they are extraordinarily small, being usually the smallest
cells in the body; and, secondly, they have, as a rule, a peculiarly lively
motion, which is known as spermatozoic motion. The shape of the cell has a
good deal to do with this motion. In most of the animals, and also in many
of the lower plants (but not the higher) each of these spermatozoa has a
very small, naked cell-body, enclosing an elongated nucleus, and a long
thread hanging from it (Fig. 20). It was long before we could recognise that
these structures are simple cells. They were formerly held to be special
organisms, and were called “seed animals” (spermato-zoa, or spermato-
zoidia); they are now scientifically known as spermia or spermidia, or as

spermatosomata (seed-bodies) or spermatofila (seed threads). It took a
good deal of comparative research to convince us that each of these
spermatozoa is really a simple cell. They have the same shape as in many
other vertebrates and most of the invertebrates. However, in many of the
lower animals they have quite a different shape. Thus, for instance, in the
craw fish they are large round cells, without any movement, equipped with
stiff outgrowths like bristles (Fig. 21 f ). They have also a peculiar form in
some of the worms, such as the thread-worms (filaria); in this case they are
sometimes amœboid and like very small ova (Fig. 21 c to e). But in most of
the lower animals (such as the sponges and polyps) they have the same
pine-cone shape as in man and the other animals (Fig. 21 a, h).

Fig. 21—Spermatozoa or spermidia of various animals. (From Lang).
a of a fish, b of a turbellaria worm (with two side-lashes), c to e of a
nematode worm (amœboid spermatozoa), f from a craw fish (star-shaped), g
from the salamander (with undulating membrane), h of an annelid (a and h
are the usual shape).

When the Dutch naturalist Leeuwenhoek discovered these thread-like
lively particles in 1677 in the male sperm, it was generally believed that
they were special, independent, tiny animalcules, like the infusoria, and that
the whole mature organism existed already, with all its parts, but very small
and packed together, in each spermatozoon (see p.12). We now know that
the mobile spermatozoa are nothing but simple and real cells, of the kind
that we call “ciliated” (equipped with lashes, or cilia). In the previous
illustrations we have distinguished in the spermatozoon a head, trunk, and
tail. The “head” (Fig. 20 k) is merely the oval nucleus of the cell; the body
or middle-part (m) is an accumulation of cell-matter; and the tail (s) is a
thread-like prolongation of the same.

Moreover, we now know that these spermatozoa are not at all a peculiar
form of cell; precisely similar cells are found in various other parts of the
body. If they have many short threads projecting, they are called ciliated; if
only one long, whip-shaped process (or, more rarely, two or four), caudate
(tailed) cells.
Very careful recent examination of the spermia, under a very high
microscopic power (Fig. 22 a, b), has detected some further details in the
finer structure of the ciliated cell, and these are common to man and the
anthropoid ape. The head (k) encloses the elliptic nucleus in a thin envelope
of cytoplasm; it is a little flattened on one side, and thus looks rather pear-
shaped from the front (b). In the central piece (m) we can distinguish a short
neck and a longer connective piece (with central body). The tail consists of
a long main section (h) and a short, very fine tail (e).

Fig. 22—A single human spermatozoon magnified; a shows it from the
broader and b from the narrower side. k head (with nucleus), m middle-
stem, h long-stem, and e tail. (From Retzius.)

The process of fertilisation by sexual conception consists, therefore,
essentially in the coalescence and fusing together of two different cells. The
lively spermatozoon travels towards the ovum by its serpentine movements,
and bores its way into the female cell (Fig. 23). The nuclei of both sexual
cells, attracted by a certain “affinity,” approach each other and melt into
one.

The fertilised cell is quite another thing from the unfertilised cell. For if
we must regard the spermia as real cells no less than the ova, and the
process of conception as a coalescence of the two, we must consider the
resultant cell as a quite new and independent organism. It bears in the cell
and nuclear matter of the penetrating spermatozoon a part of the father’s
body, and in the protoplasm and caryoplasm of the ovum a part of the
mother’s body. This is clear from the fact that the child inherits many
features from both parents. It inherits from the father by means of the
spermatozoon, and from the mother by means of the ovum. The actual
blending of the two cells produces a third cell, which is the germ of the
child, or the new organism conceived. One may also say of this sexual
coalescence that the stem-cell is a simple hermaphrodite; it unites both
sexual substances in itself.

Fig. 23—The fertilisation of the ovum by the spermatozoon (of a
mammal). One of the many thread-like, lively spermidia pierces through a
fine pore-canal into the nuclear yelk. The nucleus of the ovum is invisible.

I think it necessary to emphasise the fundamental importance of this
simple, but often unappreciated, feature in order to have a correct and clear
idea of conception. With that end, I have given a special name to the new
cell from which the child develops, and which is generally loosely called
“the fertilised ovum,” or “the first segmentation sphere.” I call it “the stem-
cell” (cytula). The name “stem-cell” seems to me the simplest and most
suitable, because all the other cells of the body are derived from it, and
because it is, in the strictest sense, the stem-father and stem-mother of all
the countless generations of cells of which the multicellular organism is to
be composed. That complicated molecular movement of the protoplasm
which we call “life” is, naturally, something quite different in this stem-cell
from what we find in the two parent-cells, from the coalescence of which it

has issued. The life of the stem-cell or cytula is the product or resultant of
the paternal life-movement that is conveyed in the spermatozoon and the
maternal life-movement that is contributed by the ovum.
The admirable work done by recent observers has shown that the
individual development, in man and the other animals, commences with the
formation of a simple “stem-cell” of this character, and that this then passes,
by repeated segmentation (or cleavage), into a cluster of cells, known as
“the segmentation sphere” or “segmentation cells.” The process is most
clearly observed in the ova of the echinoderms (star-fishes, sea-urchins,
etc.). The investigations of Oscar and Richard Hertwig were chiefly
directed to these. The main results may be summed up as follows:—
Conception is preceded by certain preliminary changes, which are very
necessary—in fact, usually indispensable—for its occurrence. They are
comprised under the general heading of “Changes prior to impregnation.”
In these the original nucleus of the ovum, the germinal vesicle, is lost. Part
of it is extruded, and part dissolved in the cell contents; only a very small
part of it is left to form the basis of a fresh nucleus, the pronucleus
femininus. It is the latter alone that combines in conception with the
invading nucleus of the fertilising spermatozoon (the pronucleus
masculinus).
The impregnation of the ovum commences with a decay of the germinal
vesicle, or the original nucleus of the ovum (Fig. 8). We have seen that this
is in most unripe ova a large, transparent, round vesicle. This germinal
vesicle contains a viscous fluid (the caryolymph). The firm nuclear frame
(caryobasis) is formed of the enveloping membrane and a mesh-work of
nuclear threads running across the interior, which is filled with the nuclear
sap. In a knot of the network is contained the dark, stiff, opaque nuclear
corpuscle or nucleolus. When the impregnation of the ovum sets in, the
greater part of the germinal vesicle is dissolved in the cell; the nuclear
membrane and mesh-work disappear; the nuclear sap is distributed in the
protoplasm; a small portion of the nuclear base is extruded; another small
portion is left, and is converted into the secondary nucleus, or the female
pro-nucleus (Fig. 24 e k).
The small portion of the nuclear base which is extruded from the
impregnated ovum is known as the “directive bodies” or “polar cells”; there
are many disputes as to their origin and significance, but we are as yet

imperfectly acquainted with them. As a rule, they are two small round
granules, of the same size and appearance as the remaining pro-nucleus.
They are detached cell-buds; their separation from the large mother-cell
takes place in the same way as in ordinary “indirect cell-division.” Hence,
the polar cells are probably to be conceived as “abortive ova,” or
“rudimentary ova,” which proceed from a simple original ovum by cleavage
in the same way that several sperm-cells arise from one “sperm-mother-
cell,” in reproduction from sperm. The male sperm-cells in the testicles
must undergo similar changes in view of the coming impregnation as the
ova in the female ovary. In this maturing of the sperm each of the original
seed-cells divides by double segmentation into four daughter-cells, each
furnished with a fourth of the original nuclear matter (the hereditary
chromatin); and each of these four descendant cells becomes a
spermatozoon, ready for impregnation. Thus is prevented the doubling of
the chromatin in the coalescence of the two nuclei at conception. As the two
polar cells are extruded and lost, and have no further part in the fertilisation
of the ovum, we need not discuss them any further. But we must give more
attention to the female pro-nucleus which alone remains after the extrusion
of the polar cells and the dissolving of the germinal vesicle (Fig. 23 e k).
This tiny round corpuscle of chromatin now acts as a centre of attraction for
the invading spermatozoon in the large ripe ovum, and coalesces with its
“head,” the male pro-nucleus. The product of this blending, which is the
most important part of the act of impregnation, is the stem-nucleus, or the
first segmentation nucleus (archicaryon)—that is to say, the nucleus of the
new-born embryonic stem-cell or “first segmentation cell.” This stem-cell is
the starting point of the subsequent embryonic processes.
Hertwig has shown that the tiny transparent ova of the echinoderms are
the most convenient for following the details of this important process of
impregnation. We can, in this case, easily and successfully accomplish
artificial impregnation, and follow the formation of the stem-cell step by
step within the space of ten minutes. If we put ripe ova of the star-fish or
sea-urchin in a watch glass with sea-water and add a drop of ripe sperm-
fluid, we find each ovum impregnated within five minutes. Thousands of
the fine, mobile ciliated cells, which we have described as “sperm-threads”
(Fig. 20), make their way to the ova, owing to a sort of chemical sensitive
action which may be called “smell.” But only one of these innumerable
spermatozoa is chosen—namely, the one that first reaches the ovum by the

serpentine motions of its tail, and touches the ovum with its head. At the
spot where the point of its head touches the surface of the ovum the
protoplasm of the latter is raised in the form of a small wart, the
“impregnation rise” (Fig. 25 A). The spermatozoon then bores its way into
this with its head, the tail outside wriggling about all the time (Fig. 25 B,
C). Presently the tail also disappears within the ovum. At the same time the
ovum secretes a thin external yelk-membrane (Fig. 25 C), starting from the
point of impregnation; and this prevents any more spermatozoa from
entering.

Fig. 24—An impregnated echinoderm ovum, with small homogeneous
nucleus (e k).
(From Hertwig.)

Inside the impregnated ovum we now see a rapid series of most
important changes. The pear-shaped head of the sperm-cell, or the “head of
the spermatozoon,” grows larger and rounder, and is converted into the
male pro-nucleus (Fig. 26 s k). This has an attractive influence on the fine
granules or particles which are distributed in the protoplasm of the ovum;
they arrange themselves in lines in the figure of a star. But the attraction or
the “affinity” between the two nuclei is even stronger. They move towards
each other inside the yelk with increasing speed, the male (Fig. 27 s k)
going more quickly than the female nucleus (e k). The tiny male nucleus
takes with it the radiating mantle which spreads like a star about it. At last
the two sexual nuclei touch (usually in the centre of the globular ovum), lie
close together, are flattened at the points of contact, and coalesce into a
common mass. The small central particle of nuclein which is formed from
this combination of the nuclei is the stem-nucleus, or the first segmentation
nucleus; the new-formed cell, the product of the impregnation, is our stem-
cell, or “first segmentation sphere” (Fig. 2).

Fig. 25—Impregnation of the ovum of a star-fish. (From Hertwig.)
Only a small part of the surface of the ovum is shown. One of the numerous
spermatozoa approaches the “impregnation rise” (A), touches it (B), and
then penetrates into the protoplasm of the ovum (C).

Hence the one essential point in the process of sexual reproduction or
impregnation is the formation of a new cell, the stem-cell, by the
combination of two originally different cells, the female ovum and the male
spermatozoon. This process is of the highest importance, and merits our
closest attention; all that happens in the later development of this first cell
and in the life of the organism that comes of it is determined from the first
by the chemical and morphological composition of the stem-cell, its nucleus
and its body. We must, therefore, make a very careful study of the rise and
structure of the stem-cell.
The first question that arises is as to the two different active elements, the
nucleus and the protoplasm, in the actual coalescence. It is obvious that the
nucleus plays the more important part in this. Hence Hertwig puts his
theory of conception in the principle: “Conception consists in the
copulation of two cell-nuclei, which come from a male and a female cell.”
And as the phenomenon of heredity is inseparably connected with the
reproductive process, we may further conclude that these two copulating
nuclei “convey the characteristics which are transmitted from parents to
offspring.” In this sense I had in 1866 (in the ninth chapter of the General
Morphology) ascribed to the reproductive nucleus the function of
generation and heredity, and to the nutritive protoplasm the duties of
nutrition and adaptation. As, moreover, there is a complete coalescence of
the mutually attracted nuclear substances in conception, and the new
nucleus formed (the stem-nucleus) is the real starting-point for the
development of the fresh organism, the further conclusion may be drawn
that the male nucleus conveys to the child the qualities of the father, and the

female nucleus the features of the mother. We must not forget, however, that
the protoplasmic bodies of the copulating cells also fuse together in the act
of impregnation; the cell-body of the invading spermatozoon (the trunk and
tail of the male ciliated cell) is dissolved in the yelk of the female ovum.
This coalescence is not so important as that of the nuclei, but it must not be
overlooked; and, though this process is not so well known to us, we see
clearly at least the formation of the star-like figure (the radial arrangement
of the particles in the plasma) in it (Figs. 26–27).
The older theories of impregnation generally went astray in regarding the
large ovum as the sole base of the new organism, and only ascribed to the
spermatozoon the work of stimulating and originating its development. The
stimulus which it gave to the ovum was sometimes thought to be purely
chemical, at other times rather physical (on the principle of transferred
movement), or again a mystic and transcendental process. This error was
partly due to the imperfect knowledge at that time of the facts of
impregnation, and partly to the striking difference in the sizes of the two
sexual cells. Most of the earlier observers thought that the spermatozoon
did not penetrate into the ovum. And even when this had been
demonstrated, the spermatozoon was believed to disappear in the ovum
without leaving a trace. However, the splendid research made in the last
three decades with the finer technical methods of our time has completely
exposed the error of this. It has been shown that the tiny sperm-cell is not
subordinated to, but coordinated with, the large ovum. The nuclei of the
two cells, as the vehicles of the hereditary features of the parents, are of
equal physiological importance. In some cases we have succeeded in
proving that the mass of the active nuclear substance which combines in the
copulation of the two sexual nuclei is originally the same for both.
These morphological facts are in perfect harmony with the familiar
physiological truth that the child inherits from both parents, and that on the
average they are equally distributed. I say “on the average,” because it is
well known that a child may have a greater likeness to the father or to the
mother; that goes without saying, as far as the primary sexual characters
(the sexual glands) are concerned. But it is also possible that the
determination of the latter—the weighty determination whether the child is
to be a boy or a girl—depends on a slight qualitative or quantitative
difference in the nuclein or the coloured nuclear matter which comes from
both parents in the act of conception.

Figs. 26 and 27.—Impregnation of the ovum of the sea-urchin. (From
Hertwig.) In Fig. 26 the little sperm-nucleus (sk) moves towards the larger
nucleus of the ovum (ek). In Fig. 27 they nearly touch, and are surrounded
by the radiating mantle of protoplasm.

The striking differences of the respective sexual cells in size and shape,
which occasioned the erroneous views of earlier scientists, are easily
explained on the principle of division of labour. The inert, motionless ovum
grows in size according to the quantity of provision it stores up in the form
of nutritive yelk for the development of the germ. The active swimming
sperm-cell is reduced in size in proportion to its need to seek the ovum and
bore its way into its yelk. These differences are very conspicuous in the
higher animals, but they are much less in the lower animals. In those
protists (unicellular plants and animals) which have the first rudiments of
sexual reproduction the two copulating cells are at first quite equal. In these
cases the act of impregnation is nothing more than a sudden growth, in
which the originally simple cell doubles its volume, and is thus prepared for
reproduction (cell-division). Afterwards slight differences are seen in the
size of the copulating cells; though the smaller ones still have the same
shape as the larger ones. It is only when the difference in size is very
pronounced that a notable difference in shape is found: the sprightly sperm-
cell changes more in shape and the ovum in size.
Quite in harmony with this new conception of the equivalence of the two
gonads, or the equal physiological importance of the male and female sex-
cells and their equal share in the process of heredity, is the important fact
established by Hertwig (1875), that in normal impregnation only one single
spermatozoon copulates with one ovum; the membrane which is raised on
the surface of the yelk immediately after one sperm-cell has penetrated
(Fig. 25 C) prevents any others from entering. All the rivals of the fortunate

penetrator are excluded, and die without. But if the ovum passes into a
morbid state, if it is made stiff by a lowering of its temperature or stupefied
with narcotics (chloroform, morphia, nicotine, etc.), two or more
spermatozoa may penetrate into its yelk-body. We then witness
polyspermism. The more Hertwig chloroformed the ovum, the more
spermatozoa were able to bore their way into its unconscious body.

Fig. 28—Stem-cell of a rabbit, magnified. In the centre of the granular
protoplasm of the fertilised ovum (d) is seen the little, bright stem-nucleus,
z is the ovolemma, with a mucous membrane (h). s are dead spermatozoa.

These remarkable facts of impregnation are also of the greatest interest in
psychology, especially as regards the theory of the cell-soul, which I
consider to be its chief foundation. The phenomena we have described can
only be understood and explained by ascribing a certain lower degree of
psychic activity to the sexual principles. They feel each other’s proximity,
and are drawn together by a sensitive impulse (probably related to smell);
they move towards each other, and do not rest until they fuse together.
Physiologists may say that it is only a question of a peculiar physico-
chemical phenomenon, and not a psychic action; but the two cannot be
separated. Even the psychic functions, in the strict sense of the word, are
only complex physical processes, or “psycho-physical” phenomena, which
are determined in all cases exclusively by the chemical composition of their
material substratum.
The monistic view of the matter becomes clear enough when we
remember the radical importance of impregnation as regards heredity. It is
well known that not only the most delicate bodily structures, but also the
subtlest traits of mind, are transmitted from the parents to the children. In

this the chromatic matter of the male nucleus is just as important a vehicle
as the large caryoplasmic substance of the female nucleus; the one transmits
the mental features of the father, and the other those of the mother. The
blending of the two parental nuclei determines the individual psychic
character of the child.
But there is another important psychological question—the most
important of all—that has been definitely answered by the recent
discoveries in connection with conception. This is the question of the
immortality of the soul. No fact throws more light on it and refutes it more
convincingly than the elementary process of conception that we have
described. For this copulation of the two sexual nuclei (Figs. 26 and 27)
indicates the precise moment at which the individual begins to exist. All the
bodily and mental features of the new-born child are the sum-total of the
hereditary qualities which it has received in reproduction from parents and
ancestors. All that man acquires afterwards in life by the exercise of his
organs, the influence of his environment, and education—in a word, by
adaptation—cannot obliterate that general outline of his being which he
inherited from his parents. But this hereditary disposition, the essence of
every human soul, is not “eternal,” but “temporal”; it comes into being only
at the moment when the sperm-nucleus of the father and the nucleus of the
maternal ovum meet and fuse together. It is clearly irrational to assume an
“eternal life without end” for an individual phenomenon, the
commencement of which we can indicate to a moment by direct visual
observation.
The great importance of the process of impregnation in answering such
questions is quite clear. It is true that conception has never been studied
microscopically in all its details in the human case—notwithstanding its
occurrence at every moment—for reasons that are obvious enough.
However, the two cells which need consideration, the female ovum and the
male spermatozoon, proceed in the case of man in just the same way as in
all the other mammals; the human fœtus or embryo which results from
copulation has the same form as with the other animals. Hence, no scientist
who is acquainted with the facts doubts that the processes of impregnation
are just the same in man as in the other animals.
The stem-cell which is produced, and with which every man begins his
career, cannot be distinguished in appearance from those of other mammals,

such as the rabbit (Fig. 28). In the case of man, also, this stem-cell differs
materially from the original ovum, both in regard to form
(morphologically), in regard to material composition (chemically), and in
regard to vital properties (physiologically). It comes partly from the father
and partly from the mother. Hence it is not surprising that the child who is
developed from it inherits from both parents. The vital movements of each
of these cells form a sum of mechanical processes which in the last analysis
are due to movements of the smallest vital parts, or the molecules, of the
living substance. If we agree to call this active substance plasson, and its
molecules plastidules, we may say that the individual physiological
character of each of these cells is due to its molecular plastidule-movement.
Hence, the plastidule-movement of the cytula is the resultant of the
combined plastidule-movements of the female ovum and the male sperm-
cell.[15]
[15] The plasson of the stem-cell or cytula may, from the anatomical point of
view, be regarded as homogeneous and structureless, like that of the monera.
This is not inconsistent with our hypothetical ascription to the plastidules (or
molecules of the plasson) of a complex molecular structure. The complexity of
this is the greater in proportion to the complexity of the organism that is
developed from it and the length of the chain of its ancestry, or to the multitude
of antecedent processes of heredity and adaptation.

Chapter VIII.
THE GASTRÆA THEORY

There is a substantial agreement throughout the animal world in the first
changes which follow the impregnation of the ovum and the formation of
the stem-cell; they begin in all cases with the segmentation of the ovum and
the formation of the germinal layers. The only exception is found in the
protozoa, the very lowest and simplest forms of animal life; these remain
unicellular throughout life. To this group belong the amœbae, gregarinæ,
rhizopods, infusoria, etc. As their whole organism consists of a single cell,
they can never form germinal layers, or definite strata of cells. But all the
other animals—all the tissue-forming animals, or metazoa, as we call them,
in contradistinction to the protozoa—construct real germinal layers by the
repeated cleavage of the impregnated ovum. This we find in the lower
cnidaria and worms, as well as in the more highly-developed molluscs,
echinoderms, articulates, and vertebrates.
In all these metazoa, or multicellular animals, the chief embryonic
processes are substantially alike, although they often seem to a superficial
observer to differ considerably. The stem-cell that proceeds from the
impregnated ovum always passes by repeated cleavage into a number of
simple cells. These cells are all direct descendants of the stem-cell, and are,
for reasons we shall see presently, called segmentation-cells. The repeated
cleavage of the stem-cell, which gives rise to these segmentation-spheres,
has long been known as “segmentation.” Sooner or later the segmentation-
cells join together to form a round (at first, globular) embryonic sphere
(blastula); they then form into two very different groups, and arrange
themselves in two separate strata—the two primary germinal layers. These
enclose a digestive cavity, the primitive gut, with an opening, the primitive
mouth. We give the name of the gastrula to the important embryonic form
that has these primitive organs, and the name of gastrulation to the

formation of it. This ontogenetic process has a very great significance, and
is the real starting-point of the construction of the multicellular animal
body.
The fundamental embryonic processes of the cleavage of the ovum and
the formation of the germinal layers have been very thoroughly studied in
the last thirty years, and their real significance has been appreciated. They
present a striking variety in the different groups, and it was no light task to
prove their essential identity in the whole animal world. But since I
formulated the gastræa theory in 1872, and afterwards (1875) reduced all
the various forms of segmentation and gastrulation to one fundamental type,
their identity may be said to have been established. We have thus mastered
the law of unity which governs the first embryonic processes in all the
animals.
Man is like all the other higher animals, especially the apes, in regard to
these earliest and most important processes. As the human embryo does not
essentially differ, even at a much later stage of development—when we
already perceive the cerebral vesicles, the eyes, ears, gill-arches, etc.—from
the similar forms of the other higher mammals, we may confidently assume
that they agree in the earliest embryonic processes, segmentation and the
formation of germinal layers. This has not yet, it is true, been established by
observation. We have never yet had occasion to dissect a woman
immediately after impregnation and examine the stem-cell or the
segmentation-cells in her oviduct. However, as the earliest human embryos
we have examined, and the later and more developed forms, agree with
those of the rabbit, dog, and other higher mammals, no reasonable man will
doubt but that the segmentation and formation of layers are the same in both
cases.
But the special form of segmentation and layer formation which we find
in the mammal is by no means the original, simple, palingenetic form. It has
been much modified and cenogenetically altered by a very complex
adaptation to embryonic conditions. We cannot, therefore, understand it
altogether in itself. In order to do this, we have to make a comparative
study of segmentation and layer-formation in the animal world; and we
have especially to seek the original, palingenetic form from which the
modified cenogenetic (see p. 4) form has gradually been developed.

This original unaltered form of segmentation and layer-formation is
found to-day in only one case in the vertebrate-stem to which man belongs
—the lowest and oldest member of the stem, the wonderful lancelet or
amphioxus (cf. Chapters XVI and XVII). But we find a precisely similar
palingenetic form of embryonic development in the case of many of the
invertebrate animals, as, for instance, the remarkable ascidia, the pond-snail
(Limnæus), and arrow-worm (Sagitta), and many of the echinoderms and
cnidaria, such as the common star-fish and sea-urchin, many of the medusæ
and corals, and the simpler sponges (Olynthus). We may take as an
illustration the palingenetic segmentation and germinal layer-formation in
an eight-fold insular coral, which I discovered in the Red Sea, and
described as Monoxenia Darwinii.
The impregnated ovum of this coral (Fig. 29 A, B) first splits into two
equal cells (C). First, the nucleus of the stem-cell and its central body divide
into two halves. These recede from and repel each other, and act as centres
of attraction on the surrounding protoplasm; in consequence of this, the
protoplasm is constricted by a circular furrow, and, in turn, divides into two
halves. Each of the two segmentation-cells thus produced splits in the same
way into two equal cells. The four segmentation-cells (grand-daughters of
the stem-cell) lie in one plane. Now, however, each of them subdivides into
two equal halves, the cleavage of the nucleus again preceding that of the
surrounding protoplasm. The eight cells which thus arise break into sixteen,
these into thirty-two, and then (each being constantly halved) into sixty-
four, 128, and so on.[16] The final result of this repeated cleavage is the
formation of a globular cluster of similar segmentation-cells, which we call
the mulberry-formation or morula. The cells are thickly pressed together
like the parts of a mulberry or blackberry, and this gives a lumpy
appearance to the surface of the sphere (Fig. E).[17]
[16] The number of segmentation-cells thus produced increases geometrically in
the original gastrulation, or the purest palingenetic form of cleavage. However,
in different animals the number reaches a different height, so that the morula,
and also the blastula, may consist sometimes of thirty-two, sometimes of sixty-
four, and sometimes of 128, or more, cells.

[17] The segmentation-cells which make up the morula after the close of the
palingenetic cleavage seem usually to be quite similar, and to present no
differences as to size, form, and composition. That, however, does not prevent
them from differentiating into animal and vegetative cells, even during the
cleavage.

Fig. 29—Gastrulation of a coral (Monoxenia Darwinii). A, B, stem-cell
(cytula) or impregnated ovum. In Figure A (immediately after
impregnation) the nucleus is invisible. In Figure B (a little later) it is quite
clear. C two segmentation-cells. D four segmentation-cells. E mulberry-
formation (morula). F blastosphere (blastula). G blastula (transverse
section). H depula, or hollowed blastula (transverse section). I gastrula
(longitudinal section). K gastrula, or cup-sphere, external appearance.)

When the cleavage is thus ended, the mulberry-like mass changes into a
hollow globular sphere. Watery fluid or jelly gathers inside the globule; the
segmentation-cells are loosened, and all rise to the surface. There they are
flattened by mutual pressure, and assume the shape of truncated pyramids,
and arrange themselves side by side in one regular layer (Figs. F, G). This
layer of cells is called the germinal membrane (or blastoderm); the
homogeneous cells which compose its simple structure are called

blastodermic cells; and the whole hollow sphere, the walls of which are
made of the preceding, is called the blastula or blastosphere.[18]
[18] The blastula of the lower animals must not be confused with the very
different blastula of the mammal, which is properly called the gastrocystis or
blastocystis. This cenogenetic gastrocystis and the palingenetic blastula are
sometimes very wrongly comprised under the common name of blastula or
vesicula blastodermica.

In the case of our coral, and of many other lower forms of animal life, the
young embryo begins at once to move independently and swim about in the
water. A fine, long, thread-like process, a sort of whip or lash, grows out of
each blastodermic cell, and this independently executes vibratory
movements, slow at first, but quicker after a time (Fig. F). In this way each
blastodermic cell becomes a ciliated cell. The combined force of all these
vibrating lashes causes the whole blastula to move about in a rotatory
fashion. In many other animals, especially those in which the embryo
develops within enclosed membranes, the ciliated cells are only formed at a
later stage, or even not formed at all. The blastosphere may grow and
expand by the blastodermic cells (at the surface of the sphere) dividing and
increasing, and more fluid is secreted in the internal cavity. There are still
to-day some organisms that remain throughout life at the structural stage of
the blastula—hollow vesicles that swim about by a ciliary movement in the
water, the wall of which is composed of a single layer of cells, such as the
volvox, the magosphæra, synura, etc. We shall speak further of the great
phylogenetic significance of this fact in Chapter XIX.
A very important and remarkable process now follows—namely, the
curving or invagination of the blastula (Fig. H). The vesicle with a single
layer of cells for wall is converted into a cup with a wall of two layers of
cells (cf. Figs. G, H, I). A certain spot at the surface of the sphere is
flattened, and then bent inward. This depression sinks deeper and deeper,
growing at the cost of the internal cavity. The latter decreases as the hollow
deepens. At last the internal cavity disappears altogether, the inner side of
the blastoderm (that which lines the depression) coming to lie close on the
outer side. At the same time, the cells of the two sections assume different
sizes and shapes; the inner cells are more round and the outer more oval
(Fig. I). In this way the embryo takes the form of a cup or jar-shaped body,
with a wall made up of two layers of cells, the inner cavity of which opens
to the outside at one end (the spot where the depression was originally

formed). We call this very important and interesting embryonic form the
“cup-embryo” or “cup-larva” (gastrula, Fig. 29, I longitudinal section, K
external view). I have in my Natural History of Creation given the name of
depula to the remarkable intermediate form which appears at the passage of
the blastula into the gastrula. In this intermediate stage there are two
cavities in the embryo—the original cavity (blastocœl) which is
disappearing, and the primitive gut-cavity (progaster) which is forming.
I regard the gastrula as the most important and significant embryonic
form in the animal world. In all real animals (that is, excluding the
unicellular protists) the segmentation of the ovum produces either a pure,
primitive, palingenetic gastrula (Fig. 29 I, K) or an equally instructive
cenogenetic form, which has been developed in time from the first, and can
be directly reduced to it. It is certainly a fact of the greatest interest and
instructiveness that animals of the most different stems—vertebrates and
tunicates, molluscs and articulates, echinoderms and annelids, cnidaria and
sponges—proceed from one and the same embryonic form. In illustration I
give a few pure gastrula forms from various groups of animals (Figs. 30–
35, explanation given below each).

Fig. 30 (A)—Gastrula of a very simple primitive-gut animal or
gastræad (gastrophysema). (Haeckel.)
Fig. 31 (B)—Gastrula of a worm (Sagitta). (From Kowalevsky.)
Fig. 32 (C)—Gastrula of an echinoderm (star-fish, Uraster), not
completely folded in (depula). (From Alexander Agassiz.)

Fig. 33 (D)—Gastrula of an arthropod (primitive crab, Nauplius) (as 32).
Fig. 34 (E)—Gastrula of a mollusc (pond-snail, Linnæus). (From Karl
Rabl.)
Fig. 35 (F)—Gastrula of a vertebrate (lancelet, Amphioxus). (From
Kowalevsky.) (Front view.)
In each figure d is the primitive-gut cavity, o primitive mouth,
s segmentation-cavity, i entoderm (gut-layer), e ectoderm (skin layer).

In view of this extraordinary significance of the gastrula, we must make a
very careful study of its original structure. As a rule, the typical gastrula is
very small, being invisible to the naked eye, or at the most only visible as a
fine point under very favourable conditions, and measuring generally 1/500
to 1/250 of an inch (less frequently 1/50 inch, or even more) in diameter. In
shape it is usually like a roundish drinking-cup. Sometimes it is rather oval,
at other times more ellipsoid or spindle-shaped; in some cases it is half
round, or even almost round, and in others lengthened out, or almost
cylindrical.
I give the name of primitive gut (progaster) and primitive mouth
(prostoma) to the internal cavity of the gastrula-body and its opening;
because this cavity is the first rudiment of the digestive cavity of the
organism, and the opening originally served to take food into it. Naturally,
the primitive gut and mouth change very considerably afterwards in the
various classes of animals. In most of the cnidaria and many of the annelids
(worm-like animals) they remain unchanged throughout life. But in most of
the higher animals, and so in the vertebrates, only the larger central part of
the later alimentary canal develops from the primitive gut; the later mouth
is a fresh development, the primitive mouth disappearing or changing into
the anus. We must therefore distinguish carefully between the primitive gut
and mouth of the gastrula and the later alimentary canal and mouth of the
fully developed vertebrate.[19]
[19] My distinction (1872) between the primitive gut and mouth and the later
permanent stomach (metagaster) and mouth (metastoma) has been much
criticised; but it is as much justified as the distinction between the primitive
kidneys and the permanent kidneys. Professor E. Ray-Lankester suggested three
years afterwards (1875) the name archenteron for the primitive gut, and
blastoporus for the primitive mouth.

Fig. 36—Gastrula of a lower sponge (lynthus). A external view, B
longitudinal section through the axis, g primitive-gut cavity, a primitive
mouth-aperture, i inner cell-layer (entoderm, endoblast, gut-layer), e
external cell-layer (outer germinal layer, ectoderm, ectoblast, or skin-layer).

The two layers of cells which line the gut-cavity and compose its wall are
of extreme importance. These two layers, which are the sole builders of the
whole organism, are no other than the two primary germinal layers, or the
primitive germ-layers. I have spoken in the introductory section (Chapter
III) of their radical importance. The outer stratum is the skin-layer, or
ectoderm (Figs. 30–35e); the inner stratum is the gut-layer, or entoderm (i).
The former is often also called the ectoblast, or epiblast, and the latter the
endoblast, or hypoblast. From these two primary germinal layers alone is
developed the entire organism of all the metazoa or multicellular animals.
The skin-layer forms the external skin, the gut-layer forms the internal skin
or lining of the body. Between these two germinal layers are afterwards
developed the middle germinal layer (mesoderma) and the body-cavity
(cœloma) filled with blood or lymph.
The two primary germinal layers were first distinguished by Pander in
1817 in the incubated chick. Twenty years later (1849) Huxley pointed out
that in many of the lower zoophytes, especially the medusæ, the whole
body consists throughout life of these two primary germinal layers. Soon
afterwards (1853) Allman introduced the names which have come into
general use; he called the outer layer the ectoderm (“outer-skin”), and the
inner the entoderm (“inner-skin”). But in 1867 it was shown, particularly by
Kowalevsky, from comparative observation, that even in invertebrates, also,
of the most different classes—annelids, molluscs, echinoderms, and
articulates—the body is developed out of the same two primary layers.

Finally, I discovered them (1872) in the lowest tissue-forming animals, the
sponges, and proved in my gastræa theory that these two layers must be
regarded as identical throughout the animal world, from the sponges and
corals to the insects and vertebrates, including man. This fundamental
“homology [identity] of the primary germinal layers and the primitive gut”
has been confirmed during the last thirty years by the careful research of
many able observers, and is now pretty generally admitted for the whole of
the metazoa.
As a rule, the cells which compose the two primary germinal layers show
appreciable differences even in the gastrula stage. Generally (if not always)
the cells of the skin-layer or ectoderm (Figs. 36 c and 37 e) are the smaller,
more numerous, and clearer; while the cells of the gut-layer, or entoderm
(i), are larger, less numerous, and darker. The protoplasm of the ectodermic
(outer) cells is clearer and firmer than the thicker and softer cell-matter of
the entodermic (inner) cells; the latter are, as a rule, much richer in yelk-
granules (albumen and fatty particles) than the former. Also the cells of the
gut-layer have, as a rule, a stronger affinity for colouring matter, and take
on a tinge in a solution of carmine, aniline, etc., more quickly and
appreciably than the cells of the skin-layer. The nuclei of the entoderm-cells
are usually roundish, while those of the ectoderm-cells are oval.
When the doubling-process is complete, very striking histological
differences between the cells of the two layers are found (Fig. 37). The tiny,
light ectoderm-cells (e) are sharply distinguished from the larger and darker
entoderm-cells (i). Frequently this differentiation of the cell-forms sets in at
a very early stage, during the segmentation-process, and is already very
appreciable in the blastula.
We have, up to the present, only considered that form of segmentation
and gastrulation which, for many and weighty reasons, we may regard as
the original, primordial, or palingenetic form. We might call it “equal” or
homogeneous segmentation, because the divided cells retain a resemblance
to each other at first (and often until the formation of the blastoderm). We
give the name of the “bell-gastrula,” or archigastrula, to the gastrula that
succeeds it. In just the same form as in the coral we considered (Monoxenia,
Fig. 29), we find it in the lowest zoophyta (the gastrophysema, Fig. 30), and
the simplest sponges (olynthus, Fig. 36); also in many of the medusæ and
hydrapolyps, lower types of worms of various classes (brachiopod, arrow-

worm, Fig. 31), tunicates (ascidia), many of the echinoderms (Fig. 32),
lower articulates (Fig. 33), and molluscs (Fig. 34), and, finally, in a slightly
modified form, in the lowest vertebrate (the amphioxus, Fig. 35).

Fig. 37—Cells from the two primary germinal layers of the mammal
(from both layers of the blastoderm). i larger and darker cells of the inner
stratum, the vegetal layer or entoderm. e smaller and clearer cells from the
outer stratum, the animal layer or ectoderm.

The gastrulation of the amphioxus is especially interesting because this
lowest and oldest of all the vertebrates is of the highest significance in
connection with the evolution of the vertebrate stem, and therefore with that
of man (compare Chapters XVI and XVII). Just as the comparative
anatomist traces the most elaborate features in the structures of the various
classes of vertebrates to divergent development from this simple primitive
vertebrate, so comparative embryology traces the various secondary forms
of vertebrate gastrulation to the simple, primary formation of the germinal
layers in the amphioxus. Although this formation, as distinguished from the
cenogenetic modifications of the vertebrate, may on the whole be regarded
as palingenetic, it is nevertheless different in some features from the quite
primitive gastrulation such as we have, for instance, in the Monoxenia (Fig.
29) and the Sagitta. Hatschek rightly observes that the segmentation of the
ovum in the amphioxus is not strictly equal, but almost equal, and
approaches the unequal. The difference in size between the two groups of
cells continues to be very noticeable in the further course of the
segmentation; the smaller animal cells of the upper hemisphere divide more
quickly than the larger vegetal cells of the lower (Fig. 38 A, B). Hence the
blastoderm, which forms the single-layer wall of the globular blastula at the
end of the cleavage-process, does not consist of homogeneous cells of equal
size, as in the Sagitta and the Monoxenia; the cells of the upper half of the
blastoderm (the mother-cells of the ectoderm) are more numerous and

smaller, and the cells of the lower half (the mother-cells of the entoderm)
less numerous and larger. Moreover, the segmentation-cavity of the blastula
(Fig. 38 C, h) is not quite globular, but forms a flattened spheroid with
unequal poles of its vertical axis. While the blastula is being folded into a
cup at the vegetal pole of its axis, the difference in the size of the
blastodermic cells increases (Fig. 38 D, E); it is most conspicuous when the
invagination is complete and the segmentation-cavity has disappeared (Fig.
38 F). The larger vegetal cells of the entoderm are richer in granules, and so
darker than the smaller and lighter animal cells of the ectoderm.

Fig. 38—Gastrulation of the amphioxus, from Hatschek (vertical
section through the axis of the ovum). A, B, C three stages in the formation
of the blastula; D, E curving of the blastula; F complete gastrula. h
segmentation-cavity. g primitive gut-cavity.

But the unequal gastrulation of the amphioxus diverges from the typical
equal cleavage of the Sagitta, the Monoxenia (Fig. 29), and the Olynthus
(Fig. 36), in another important particular. The pure archigastrula of the latter
forms is uni-axial, and it is round in its whole length in transverse section.
The vegetal pole of the vertical axis is just in the centre of the primitive
mouth. This is not the case in the gastrula of the amphioxus. During the
folding of the blastula the ideal axis is already bent on one side, the growth
of the blastoderm (or the increase of its cells) being brisker on one side than
on the other; the side that grows more quickly, and so is more curved (Fig.
39 v), will be the anterior or belly-side, the opposite, flatter side will form
the back (d). The primitive mouth, which at first, in the typical
archigastrula, lay at the vegetal pole of the main axis, is forced away to the

dorsal side; and whereas its two lips lay at first in a plane at right angles to
the chief axis, they are now so far thrust aside that their plane cuts the axis
at a sharp angle. The dorsal lip is therefore the upper and more forward, the
ventral lip the lower and hinder. In the latter, at the ventral passage of the
entoderm into the ectoderm, there lie side by side a pair of very large cells,
one to the right and one to the left (Fig. 39 p): these are the important polar
cells of the primitive mouth, or “the primitive cells of the mesoderm.” In
consequence of these considerable variations arising in the course of the
gastrulation, the primitive uni-axial form of the archigastrula in the
amphioxus has already become tri-axial, and thus the two-sidedness, or
bilateral symmetry, of the vertebrate body has already been determined.
This has been transmitted from the amphioxus to all the other modified
gastrula-forms of the vertebrate stem.
Apart from this bilateral structure, the gastrula of the amphioxus
resembles the typical archigastrula of the lower animals (Figs. 30–36) in
developing the two primary germinal layers from a single layer of cells.
This is clearly the oldest and original form of the metazoic embryo.
Although the animals I have mentioned belong to the most diverse classes,
they nevertheless agree with each other, and many more animal forms, in
having retained to the present day, by a conservative heredity, this
palingenetic form of gastrulation which they have from their earliest
common ancestors. But this is not the case with the great majority of the
animals. With these the original embryonic process has been gradually more
or less altered in the course of millions of years by adaptation to new
conditions of development. Both the segmentation of the ovum and the
subsequent gastrulation have in this way been considerably changed. In
fact, these variations have become so great in the course of time that the
segmentation was not rightly understood in most animals, and the gastrula
was unrecognised. It was not until I had made an extensive comparative
study, lasting a considerable time (in the years 1866–75), in animals of the
most diverse classes, that I succeeded in showing the same common typical
process in these apparently very different forms of gastrulation, and tracing
them all to one original form. I regard all those that diverge from the
primary palingenetic gastrulation as secondary, modified, and cenogenetic.
The more or less divergent form of gastrula that is produced may be called a
secondary, modified gastrula, or a metagastrula. The reader will find a

scheme of these different kinds of segmentation and gastrulation at the
close of this chapter.
By far the most important process that determines the various
cenogenetic forms of gastrulation is the change in the nutrition of the ovum
and the accumulation in it of nutritive yelk. By this we understand various
chemical substances (chiefly granules of albumin and fat-particles) which
serve exclusively as reserve-matter or food for the embryo. As the metazoic
embryo in its earlier stages of development is not yet able to obtain its food
and so build up the frame, the necessary material has to be stored up in the
ovum. Hence we distinguish in the ova two chief elements—the active
formative yelk (protoplasm) and the passive food-yelk (deutoplasm,
wrongly spoken of as “the yelk”). In the little palingenetic ova, the
segmentation of which we have already considered, the yelk-granules are so
small and so regularly distributed in the protoplasm of the ovum that the
even and repeated cleavage is not affected by them. But in the great
majority of the animal ova the food-yelk is more or less considerable, and is
stored in a certain part of the ovum, so that even in the unfertilised ovum
the “granary” can clearly be distinguished from the formative plasm. As a
rule, the formative-yelk (with the germinal vesicle) then usually gathers at
one pole and the food-yelk at the other. The first is the animal, and the
second the vegetal, pole of the vertical axis of the ovum.

Fig. 39—Gastrula of the amphioxus, seen from left side (diagrammatic
median section). (From Hatschek.) g primitive gut, u primitive mouth, p
peristomal pole-cells, i entoderm, e ectoderm, d dorsal side, v ventral side.

In these “telolecithal” ova, or ova with the yelk at one end (for instance,
in the cyclostoma and amphibia), the gastrulation then usually takes place
in such a way that in the cleavage of the impregnated ovum the animal
(usually the upper) half splits up more quickly than the vegetal (lower). The

contractions of the active protoplasm, which effect this continual cleavage
of the cells, meet a greater resistance in the lower vegetal half from the
passive deutoplasm than in the upper animal half. Hence we find in the
latter more but smaller, and in the former fewer but larger, cells. The animal
cells produce the external, and the vegetal cells the internal, germinal layer.
Although this unequal segmentation of the cyclostoma, ganoids, and
amphibia seems at first sight to differ from the original equal segmentation
(for instance, in the monoxenia, Fig. 29), they both have this in common,
that the cleavage process throughout affects the whole cell; hence Remak
called it total segmentation, and the ova in question holoblastic, or “whole-
cleaving.” It is otherwise with the second chief group of ova, which he
distinguished from these as meroblastic, or “partially-cleaving ”: to this
class belong the familiar large eggs of birds and reptiles, and of most fishes.
The inert mass of the passive food-yelk is so large in these cases that the
protoplasmic contractions of the active yelk cannot effect any further
cleavage. In consequence, there is only a partial segmentation. While the
protoplasm in the animal section of the ovum continues briskly to divide,
multiplying the nuclei, the deutoplasm in the vegetal section remains more
or less undivided; it is merely consumed as food by the forming cells. The
larger the accumulation of food, the more restricted is the process of
segmentation. It may, however, continue for some time (even after the
gastrulation is more or less complete) in the sense that the vegetal cell-
nuclei distributed in the deutoplasm slowly increase by cleavage; as each of
them is surrounded by a small quantity of protoplasm, it may afterwards
appropriate a portion of the food-yelk, and thus form a real “yelk-cell”
(merocyte). When this vegetal cell-formation continues for a long time,
after the two primary germinal layers have been formed, it takes the name
of the “after-segmentation.”
The meroblastic ova are only found in the larger and more highly
developed animals, and only in those whose embryo needs a longer time
and richer nourishment within the fœtal membranes. According as the yelk-
food accumulates at the centre or at the side of the ovum, we distinguish
two groups of dividing ova, periblastic and discoblastic. In the periblastic
the food-yelk is in the centre, enclosed inside the ovum (hence they are also
called “centrolecithal” ova): the formative yelk surrounds the food-yelk,
and so suffers itself a superficial cleavage. This is found among the
articulates (crabs, spiders, insects, etc.). In the discoblastic ova the food-

yelk gathers at one side, at the vegetal or lower pole of the vertical axis,
while the nucleus of the ovum and the great bulk of the formative yelk lie at
the upper or animal pole (hence these ova are also called “telolecithal”). In
these cases the cleavage of the ovum begins at the upper pole, and leads to
the formation of a dorsal discoid embryo. This is the case with all
meroblastic vertebrates, most fishes, the reptiles and birds, and the
oviparous mammals (the monotremes).
The gastrulation of the discoblastic ova, which chiefly concerns us, offers
serious difficulties to microscopic investigation and philosophic
consideration. These, however, have been mastered by the comparative
embryological research which has been conducted by a number of
distinguished observers during the last few decades—especially the
brothers Hertwig, Rabl, Kupffer, Selenka, Rückert, Goette, Rauber, etc.
These thorough and careful studies, aided by the most perfect modern
improvements in technical method (in tinting and dissection), have given a
very welcome support to the views which I put forward in my work, On the
Gastrula and the Segmentation of the Animal Ovum [not translated], in
1875. As it is very important to understand these views and their
phylogenetic foundation clearly, not only as regards evolution in general,
but particularly in connection with the genesis of man, I will give here a
brief statement of them as far as they concern the vertebrate-stem:—
1. All the vertebrates, including man, are phylogenetically (or
genealogically) related—that is, are members of one single natural stem.
2. Consequently, the embryonic features in their individual development
must also have a genetic connection.
3. As the gastrulation of the amphioxus shows the original palingenetic
form in its simplest features, that of the other vertebrates must have been
derived from it.
4. The cenogenetic modifications of the latter are more appreciable the
more food-yelk is stored up in the ovum.
5. Although the mass of the food-yelk may be very large in the ova of the
discoblastic vertebrates, nevertheless in every case a blastula is developed
from the morula, as in the holoblastic ova.
6. Also, in every case, the gastrula develops from the blastula by curving
or invagination.

7. The cavity which is produced in the fœtus by this curving is, in each
case, the primitive gut (progaster), and its opening the primitive mouth
(prostoma).
8. The food-yelk, whether large or small, is always stored in the ventral
wall of the primitive gut; the cells (called “merocytes”) which may be
formed in it subsequently (by “after-segmentation”) also belong to the inner
germinal layer, like the cells which immediately enclose the primitive gut-
cavity.
9. The primitive mouth, which at first lies below at the lower pole of the
vertical axis, is forced, by the growth of the yelk, backwards and then
upwards, towards the dorsal side of the embryo; the vertical axis of the
primitive gut is thus gradually converted into horizontal.
10. The primitive mouth is closed sooner or later in all the vertebrates,
and does not evolve into the permanent mouth-aperture; it rather
corresponds to the “properistoma,” or region of the anus. From this
important point the formation of the middle germinal layer proceeds,
between the two primary layers.
The wide comparative studies of the scientists I have named have further
shown that in the case of the discoblastic higher vertebrates (the three
classes of amniotes) the primitive mouth of the embryonic disc, which was
long looked for in vain, is found always, and is nothing else than the
familiar “primitive groove.” Of this we shall see more as we proceed.
Meantime we realise that gastrulation may be reduced to one and the same
process in all the vertebrates. Moreover, the various forms it takes in the
invertebrates can always be reduced to one of the four types of
segmentation described above. In relation to the distinction between total
and partial segmentation, the grouping of the various forms is as follows:—
I. Palingenetic
1. Equal segmentation A. Total
(primitive)
(bell-gastrula). segmentation
segmentation.
(without independent
II. Cenogenetic 2. Unequal segmentation
food-yelk).
segmentation (hooded gastrula).
(modified by 3. Discoid segmentation B. Partial
adaptation). (discoid gastrula). segmentation
4. Superficial (with independent
segmentation food-yelk).

(spherical gastrula).
The lowest metazoa we know—namely, the lower zoophyta (sponges,
simple polyps, etc.)—remain throughout life at a stage of development
which differs little from the gastrula; their whole body consists of two
layers of cells. This is a fact of extreme importance. We see that man, and
also other vertebrates, pass quickly through a stage of development in
which they consist of two layers, just as these lower zoophyta do
throughout life. If we apply our biogenetic law to the matter, we at once
reach this important conclusion. “Man and all the other animals which pass
through the two-layer stage, or gastrula-form, in the course of their
embryonic development, must descend from a primitive simple stem-form,
the whole body of which consisted throughout life (as is the case with the
lower zoophyta to-day) merely of two cell-strata or germinal layers.” We
will call this primitive stem-form, with which we shall deal more fully later
on, the gastræa—that is to say, “primitive-gut animal.”
According to this gastræa-theory there was originally in all the
multicellular animals one organ with the same structure and function. This
was the primitive gut; and the two primary germinal layers which form its
wall must also be regarded as identical in all. This important homology or
identity of the primary germinal layers is proved, on the one hand, from the
fact that the gastrula was originally formed in the same way in all cases—
namely, by the curving of the blastula; and, on the other hand, by the fact
that in every case the same fundamental organs arise from the germinal
layers. The outer or animal layer, or ectoderm, always forms the chief
organs of animal life—the skin, nervous system, sense-organs, etc.; the
inner or vegetal layer, or entoderm, gives rise to the chief organs of
vegetative life—the organs of nourishment, digestion, blood-formation, etc.
In the lower zoophyta, whose body remains at the two-layer stage
throughout life, the gastræads, the simplest sponges (Olynthus), and polyps
(Hydra), these two groups of functions, animal and vegetative, are strictly
divided between the two simple primary layers. Throughout life the outer or
animal layer acts simply as a covering for the body, and accomplishes its
movement and sensation. The inner or vegetative layer of cells acts
throughout life as a gut-lining, or nutritive layer of enteric cells, and often
also yields the reproductive cells.

The best known of these “gastræads,” or “gastrula-like animals,” is the
common fresh-water polyp (Hydra). This simplest of all the cnidaria has, it
is true, a crown of tentacles round its mouth. Also its outer germinal layer
has certain special modifications. But these are secondary additions, and the
inner germinal layer is a simple stratum of cells. On the whole, the hydra
has preserved to our day by heredity the simple structure of our primitive
ancestor, the gastræa (cf. Chapter XIX).
In all other animals, particularly the vertebrates, the gastrula is merely a
brief transitional stage. Here the two-layer stage of the embryonic
development is quickly succeeded by a three-layer, and then a four-layer,
stage. With the appearance of the four superimposed germinal layers we
reach again a firm and steady standing-ground, from which we may follow
the further, and much more difficult and complicated, course of embryonic
development.

SUMMARY OF THE CHIEF DIFFERENCES IN THE OVUM-
SEGMENTATION AND GASTRULATION OF ANIMALS.
The animal stems are indicated by the letters a–g: a Zoophyta. b Annelida.
c Mollusca. d Echinoderma. e Articulata. f Tunicata. g Vertebrata.

I. a. Many lower zoophyta (sponges,
Total hydrapolyps, medusæ, simpler
Segmentation. corals).
Holoblastic ova. b. Many lower annelids (sagitta,
phoronis,
I. Primitive many nematoda, etc.,
Segmentation. terebratula, argiope,
Archiblastic ova. pisidium).
c. Some lower molluscs.
Bell-gastrula d. Many echinoderms.
Gastrula without (archigastrula.) e. A few lower articulata (some
separate brachiopods,
food-yelk. copepods: Tardigrades,
Hologastrula. pteromalina).
f. Many tunicata.
g. The acrania (amphioxus).

a. Many zoophyta (sponges,
medusæ,
corals, siphonophoræ,
ctenophora).
b. Most worms.
II. Unequal c. Most molluscs.
Segmentation. d. Many echinoderms (viviparous
Amphiblastic ova. species and
some others).
Hooded-gastrula e. Some of the lower articulata
(amphigastrula). (both crustacea
and tracheata).
f. Many tunicata.
g. Cyclostoma, the oldest fishes,
amphibia,
mammals (not including man).
c. Cephalopods or cuttlefish.
III. Discoid
II. e. Many articulata, wood-lice,
Segmentation.
Partial scorpions, etc.
Discoblastic ova.
Segmentation. g. Primitive fishes, bony fishes,
Meroblastic ova. reptiles, birds,
Discoid gastrula.
monotremes.
Gastrula with IV. Superficial
separate e. The great majority of the
Segmentation.
food-yelk. articulata
Periblastic ova.
Merogastrula. (crustaceans, myriapods,
Spherical-
arachnids, insects).
gastrula.

Chapter IX.
[20]
THE GASTRULATION OF THE VERTEBRATE
[20] Cf. Balfour’s Manual of Comparative Embryology, vol. ii; Theodore
Morgan’s The Development of the Frog’s Egg.

The remarkable processes of gastrulation, ovum-segmentation, and
formation of germinal layers present a most conspicuous variety. There is
to-day only the lowest of the vertebrates, the amphioxus, that exhibits the
original form of those processes, or the palingenetic gastrulation which we
have considered in the preceding chapter, and which culminates in the
formation of the archigastrula (Fig. 38). In all other extant vertebrates these
fundamental processes have been more or less modified by adaptation to the
conditions of embryonic development (especially by changes in the food-
yelk); they exhibit various cenogenetic types of the formation of germinal
layers. However, the different classes vary considerably from each other. In
order to grasp the unity that underlies the manifold differences in these
phenomena and their historical connection, it is necessary to bear in mind
always the unity of the vertebrate-stem. This “phylogenetic unity,” which I
developed in my General Morphology in 1866, is now generally admitted.
All impartial zoologists agree to-day that all the vertebrates, from the
amphioxus and the fishes to the ape and man, descend from a common
ancestor, “the primitive vertebrate.” Hence the embryonic processes, by
which each individual vertebrate is developed, must also be capable of
being reduced to one common type of embryonic development; and this
primitive type is most certainly exhibited to-day by the amphioxus.
It must, therefore, be our next task to make a comparative study of the
various forms of vertebrate gastrulation, and trace them backwards to that
of the lancelet. Broadly speaking, they fall first into two groups: the older
cyclostoma, the earliest fishes, most of the amphibia, and the viviparous
mammals, have holoblastic ova—that is to say, ova with total, unequal

segmentation; while the younger cyclostoma, most of the fishes, the
cephalopods, reptiles, birds, and monotremes, have meroblastic ova, or ova
with partial discoid segmentation. A closer study of them shows, however,
that these two groups do not present a natural unity, and that the historical
relations between their several divisions are very complicated. In order to
understand them properly, we must first consider the various modifications
of gastrulation in these classes. We may begin with that of the amphibia.
The most suitable and most available objects of study in this class are the
eggs of our indigenous amphibia, the tailless frogs and toads, and the tailed
salamander. In spring they are to be found in clusters in every pond, and
careful examination of the ova with a lens is sufficient to show at least the
external features of the segmentation. In order to understand the whole
process rightly and follow the formation of the germinal layers and the
gastrula, the ova of the frog and salamander must be carefully hardened;
then the thinnest possible sections must be made of the hardened ova with
the microtome, and the tinted sections must be very closely compared under
a powerful microscope.
The ova of the frog or toad are globular in shape, about the twelfth of an
inch in diameter, and are clustered in jelly-like masses, which are lumped
together in the case of the frog, but form long strings in the case of the toad.
When we examine the opaque, grey, brown, or blackish ova closely, we find
that the upper half is darker than the lower. The middle of the upper half is
in many species black, while the middle of the lower half is white.[21] In this
way we get a definite axis of the ovum with two poles. To give a clear idea
of the segmentation of this ovum, it is best to compare it with a globe, on
the surface of which are marked the various parallels of longitude and
latitude. The superficial dividing lines between the different cells, which
come from the repeated segmentation of the ovum, look like deep furrows
on the surface, and hence the whole process has been given the name of
furcation. In reality, however, this “furcation,” which was formerly regarded
as a very mysterious process, is nothing but the familiar, repeated cell-
segmentation. Hence also the segmentation-cells which result from it are
real cells.
[21] The colouring of the eggs of the amphibia is caused by the accumulation of
dark-colouring matter at the animal pole of the ovum. In consequence of this, the
animal cells of the ectoderm are darker than the vegetal cells of the entoderm.

We find the reverse of this in the case of most animals, the protoplasm of the
entoderm cells being usually darker and coarser-grained.

Fig. 40—The cleavage of the frog’s ovum (magnified). A stem-cell. B
the first two segmentation-cells. C four cells. D eight cells (4 animal and 4
vegetative). E twelve cells (8 animal and 4 vegetative). F sixteen cells (8
animal and 8 vegetative). G twenty-four cells (16 animal and 8 vegetative).
H thirty-two cells. I forty-eight cells. K sixty-four cells. L ninety-six cells.
M 160 cells (128 animal and 32 vegetative).

The unequal segmentation which we observe in the ovum of the
amphibia has the special feature of beginning at the upper and darker pole
(the north pole of the terrestrial globe in our illustration), and slowly
advancing towards the lower and brighter pole (the south pole). Also the
upper and darker hemisphere remains in this position throughout the course
of the segmentation, and its cells multiply much more briskly. Hence the
cells of the lower hemisphere are found to be larger and less numerous. The
cleavage of the stem-cell (Fig. 40 A) begins with the formation of a
complete furrow, which starts from the north pole and reaches to the south
(B). An hour later a second furrow arises in the same way, and this cuts the
first at a right angle (Fig. 40 C). The ovum is thus divided into four equal
parts. Each of these four “segmentation cells” has an upper and darker and a
lower, brighter half. A few hours later a third furrow appears, vertically to
the first two (Fig. 40 D). The globular germ now consists of eight cells, four
smaller ones above (northern) and four larger ones below (southern). Next,

each of the four upper ones divides into two halves by a cleavage beginning
from the north pole, so that we now have eight above and four below (Fig.
40 E). Later, the four new longitudinal divisions extend gradually to the
lower cells, and the number rises from twelve to sixteen (F). Then a second
circular furrow appears, parallel to the first, and nearer to the north pole, so
that we may compare it to the north polar circle. In this way we get twenty-
four segmentation-cells—sixteen upper, smaller, and darker ones, and eight
smaller and brighter ones below (G). Soon, however, the latter also sub-
divide into sixteen, a third or “meridian of latitude” appearing, this time in
the southern hemisphere: this makes thirty-two cells altogether (H). Then
eight new longitudinal lines are formed at the north pole, and these proceed
to divide, first the darker cells above and afterwards the lighter southern
cells, and finally reach the south pole. In this way we get in succession
forty, forty-eight, fifty-six, and at last sixty-four cells (I, K). In the
meantime, the two hemispheres differ more and more from each other.
Whereas the sluggish lower hemisphere long remains at thirty-two cells, the
lively northern hemisphere briskly sub-divides twice, producing first sixty-
four and then 128 cells (L, M). Thus we reach a stage in which we count on
the surface of the ovum 128 small cells in the upper half and thirty-two
large ones in the lower half, or 160 altogether. The dissimilarity of the two
halves increases: while the northern breaks up into a great number of small
cells, the southern consists of a much smaller number of larger cells.
Finally, the dark cells of the upper half grow almost over the surface of the
ovum, leaving only a small circular spot
at the south pole, where the large and clear cells of the lower half are
visible. This white region at the south pole corresponds, as we shall see
afterwards, to the primitive mouth of the gastrula. The whole mass of the
inner and larger and clearer cells (including the white polar region) belongs
to the entoderm or ventral layer. The outer envelope of dark smaller cells
forms the ectoderm or skin-layer.

Figs. 41–44—Four vertical sections of the fertilised ovum of the toad,
in four successive stages of development. The letters have the same
meaning throughout: F segmentation-cavity. D covering of same (D dorsal
half of the embryo, P ventral half). P yelk-stopper (white round field at the
lower pole). Z yelk-cells of the entoderm (Remak’s “glandular embryo”). N
primitive gut cavity (progaster or Rusconian alimentary cavity). The
primitive mouth (prostoma) is closed by the yelk-stopper, P. s partition
between the primitive gut cavity (N) and the segmentation cavity (F). k k′,
section of the large circular lip-border of the primitive mouth (the
Rusconian anus). The line of dots between k and k′ indicates the earlier
connection of the yelk-stopper (P) with the central mass of the yelk-cells
(Z). In Fig. 44 the ovum has turned 90°, so that the back of the embryo is
uppermost and the ventral side down. (From Stricker.).

Fig. 45—Blastula of the water-salamander (Triton). fh segmentation-
cavity, dz yelk-cells, rz border-zone. (From Hertwig.)

In the meantime, a large cavity, full of fluid, has been formed within the
globular body—the segmentation-cavity or embryonic cavity (blastocœl,
Figs. 41–44 F). It extends considerably as the cleavage proceeds, and
afterwards assumes an almost semi-circular form (Fig. 41 F). The frog-
embryo now represents a modified embryonic vesicle or blastula, with
hollow animal half and solid vegetal half.
Now a second, narrower but longer, cavity arises by a process of folding
at the lower pole, and by the falling away from each other of the white
entoderm-cells (Figs. 41–44 N). This is the primitive gut-cavity or the
gastric cavity of the gastrula, progaster or archenteron. It was first observed
in the ovum of the amphibia by Rusconi, and so called the Rusconian
cavity. The reason of its peculiar narrowness here is that it is, for the most
part, full of yelk-cells of the entoderm. These also stop up the whole of the
wide opening of the primitive mouth, and form what is known as the “yelk-
stopper,” which is seen freely at the white round spot at the south pole (P).
Around it the ectoderm is much thicker, and forms the border of the
primitive mouth, the most important part of the embryo (Fig. 44 k, k′). Soon
the primitive gut-cavity stretches further and further at the expense of the
segmentation-cavity (F), until at last the latter disappears altogether. The
two cavities are only separated by a thin partition (Fig. 43 s). With the
formation of the primitive gut our frog-embryo has reached the gastrula
stage, though it is clear that this cenogenetic amphibian gastrula is very
different from the real palingenetic gastrula we have considered (Figs. 30–
36).
In the growth of this hooded gastrula we cannot sharply mark off the
various stages which we distinguish successively in the bell-gastrula as
morula and gastrula. Nevertheless, it is not difficult to reduce the whole
cenogenetic or disturbed development of this amphigastrula to the true
palingenetic formation of the archigastrula of the amphioxus.

Fig. 46—Embryonic vesicle of triton (blastula), outer view, with the
transverse fold of the primitive mouth (u). (From Hertwig.)

This reduction becomes easier if, after considering the gastrulation of the
tailless amphibia (frogs and toads), we glance for a moment at that of the
tailed amphibia, the salamanders. In some of the latter, that have only
recently been carefully studied, and that are phylogenetically older, the
process is much simpler and clearer than is the case with the former and
longer known. Our common water-salamander (Triton taeniatus) is a
particularly good subject for observation. Its nutritive yelk is much smaller
and its formative yelk less obscured with black pigment-cells than in the
case of the frog; and its gastrulation has better retained the original
palingenetic character. It was first described by Scott and Osborn (1879),
and Oscar Hertwig especially made a careful study of it (1881), and rightly
pointed out its great importance in helping us to understand the vertebrate
development. Its globular blastula (Fig. 45) consists of loosely-aggregated,
yelk-filled entodermic cells or yelk-cells (dz) in the lower vegetal half; the
upper, animal half encloses the hemispherical segmentation-cavity (fh), the
curved roof of which is formed of two or three strata of small ectodermic
cells. At the point where the latter pass into the former (at the equator of the
globular vesicle) we have the border zone (rz). The folding which leads to
the formation of the gastrula takes place at a spot in this border zone, the
primitive mouth (Fig. 46 u).

Fig. 47—Sagittal section of a hooded-embryo (depula) of triton
(blastula at the commencement of gastrulation). ak outer germinal layer, ik
inner germinal layer, fh segmentation-cavity, ud primitive gut, u primitive
mouth, dl and vl dorsal and ventral lips of the mouth, dz yelk-cells. (From
Hertwig.)

Unequal segmentation takes place in some of the cyclostoma and in the
oldest fishes in just the same way as in most of the amphibia. Among the
cyclostoma (“round-mouthed”) the familiar lampreys are particularly
interesting. In respect of organisation and development they are half-way
between the acrania (lancelet) and the lowest real fishes (Selachii); hence I
divided the group of the cyclostoma in 1886 from the real fishes with which
they were formerly associated, and formed of them a special class of
vertebrates. The ovum-segmentation in our common river-lamprey
(Petromyzon fluviatilis) was described by Max Schultze in 1856, and
afterwards by Scott (1882) and Goette (1890).
Unequal total segmentation follows the same lines in the oldest fishes,
the selachii and ganoids, which are directly descended from the cyclostoma.
The primitive fishes (Selachii), which we must regard as the ancestral group
of the true fishes, were generally considered, until a short time ago, to be
discoblastic. It was not until the beginning of the twentieth century that
Bashford Dean made the important discovery in Japan that one of the oldest
living fishes of the shark type (Cestracion japonicus) has the same total
unequal segmentation as the amphiblastic plated fishes (ganoides).[22] This
is particularly interesting in connection with our subject, because the few
remaining survivors of this division, which was so numerous in paleozoic
times, exhibit three different types of gastrulation.
[22] Bashford Dean, Holoblastic Cleavage in the Egg of a Shark, Cestracion
japonicus Macleay. Annotationes zoologicae japonenses, vol. iv, Tokio, 1901.

Fig. 48—Sagittal section of the gastrula of the water-salamander
(Triton). (From Hertwig.) Letters as in Fig. 47; except—p yelk-stopper, mk
beginning of the middle germinal layer.)

The oldest and most conservative forms of the modern ganoids are the
scaly sturgeons (Sturiones), plated fishes of great evolutionary importance,
the eggs of which are eaten as caviar; their cleavage is not essentially
different from that of the lampreys and the amphibia. On the other hand, the
most modern of the plated fishes, the beautifully scaled bony pike of the
North American rivers (Lepidosteus), approaches the osseous fishes, and is
discoblastic like them. A third genus (Amia) is midway between the
sturgeons and the latter.
The group of the lung-fishes (Dipneusta or Dipnoi) is closely connected
with the older ganoids. In respect of their whole organisation they are
midway between the gill-breathing fishes and the lung-breathing amphibia;
they share with the former the shape of the body and limbs, and with the
latter the form of the heart and lungs. Of the older dipnoi (Paladipneusta)
we have now only one specimen, the remarkable Ceratodus of East
Australia; its amphiblastic gastrulation has been recently explained by
Richard Semon (cf. Chapter XXI). That of the two modern dipneusta, of
which Protopterus is found in Africa and Lepidosiren in America, is not
materially different. (Cf. Fig. 51.)

Fig. 49—Ovum-segmentation of the lamprey (Petromyzon fluviatalis),
in four successive stages. The small cells of the upper (animal) hemisphere
divide much more quickly than the cells of the lower (vegetal) hemisphere.

Fig. 50—Gastrulation of the lamprey (Petromyzon fluviatilis). A
blastula, with wide embryonic cavity (blastocoel, bl), g incipient
invagination. B depula, with advanced invagination, from the primitive
mouth (g). C gastrula, with complete primitive gut: the embryonic cavity
has almost disappeared in consequence of invagination.

All these amphiblastic vertebrates, Petromyzon and Cestracion,
Accipenser and Ceratodus, and also the salamanders and batrachia, belong
to the old, conservative groups of our stem. Their unequal ovum-
segmentation and gastrulation have many peculiarities in detail, but can
always be reduced with comparative ease to the original cleavage and
gastrulation of the lowest vertebrate, the amphioxus; and this is little
removed, as we have seen, from the very simple archigastrula of the Sagitta
and Monoxenia (see Fig. 29–36). All these and many other classes of
animals generally agree in the circumstance that in segmentation their ovum
divides into a large number of cells by repeated cleavage. All such ova have
been called, after Remak, “whole-cleaving” (holoblasta), because their
division into cells is complete or total.

Fig. 51—Gastrulation of ceratodus (from Semon). A and C stage with
four cells, B and D with sixteen cells. A and B are seen from above, C and
D sideways. E stage with thirty-two cells; F blastula; G gastrula in
longitudinal section. fh segmentation-cavity. gh primitive gut or gastric
cavity.

In a great many other classes of animals this is not the case, as we find
(in the vertebrate stem) among the birds, reptiles, and most of the fishes;
among the insects and most of the spiders and crabs (of the articulates); and
the cephalopods (of the molluscs). In all these animals the mature ovum,
and the stem-cell that arises from it in fertilisation, consist of two different
and separate parts, which we have called formative yelk and nutritive yelk.
The formative yelk alone consists of living protoplasm, and is the active,
evolutionary, and nucleated part of the ovum; this alone divides in
segmentation, and produces the numerous cells which make up the embryo.
On the other hand, the nutritive yelk is merely a passive part of the contents
of the ovum, a subordinate element which contains nutritive material
(albumin, fat, etc.), and so represents in a sense the provision-store of the
developing embryo. The latter takes a quantity of food out of this store, and
finally consumes it all. Hence the nutritive yelk is of great indirect
importance in embryonic development, though it has no direct share in it. It
either does not divide at all, or only later on, and does not generally consist

of cells. It is sometimes large and sometimes small, but generally many
times larger than the formative yelk; and hence it is that it was formerly
thought the more important of the two. As the respective significance of
these two parts of the ovum is often wrongly described, it must be borne in
mind that the nutritive yelk is only a secondary addition to the primary cell,
it is an inner enclosure, not an external appendage. All ova that have this
independent nutritive yelk are called, after Remak, “partially-cleaving”
(meroblasta). Their segmentation is incomplete or partial.

Fig. 52—Ovum of a deep-sea bony fish. b protoplasm of the stem-cell,
k nucleus of same, d clear globule of albumin, the nutritive yelk, f fat-
globule of same, c outer membrane of the ovum, or ovolemma.)

There are many difficulties in the way of understanding this partial
segmentation and the gastrula that arises from it. We have only recently
succeeded, by means of comparative research, in overcoming these
difficulties, and reducing this cenogenetic form of gastrulation to the
original palingenetic type. This is comparatively easy in the small
meroblastic ova which contain little nutritive yelk—for instance, in the
marine ova of a bony fish, the development of which I observed in 1875 at
Ajaccio in Corsica. I found them joined together in lumps of jelly, floating
on the surface of the sea; and, as the little ovula were completely
transparent, I could easily follow the development of the germ step by step.
These ovula are glossy and colourless globules of little more than the 50th
of an inch. Inside a structureless, thin, but firm membrane (ovolemma, Fig.
52 c) we find a large, quite clear, and transparent globule of albumin (d). At
both poles of its axis this globule has a pit-like depression. In the pit at the
upper, animal pole (which is turned downwards in the floating ovum) there
is a bi-convex lens composed of protoplasm, and this encloses the nucleus
(k); this is the formative yelk of the stem-cell, or the germinal disk (b). The
small fat-globule (f) and the large albumin-globule (d) together form the

nutritive yelk. Only the formative yelk undergoes cleavage, the nutritive
yelk not dividing at all at first.
The segmentation of the lens-shaped formative yelk (b) proceeds quite
independently of the nutritive yelk, and in perfect geometrical order.
When the mulberry-like cluster of cells has been formed, the border-cells
of the lens separate from the rest and travel into the yelk and the border-
layer. From this the blastula is developed; the regular bi-convex lens being
converted into a disk, like a watch-glass, with thick borders. This lies on the
upper and less curved polar surface of the nutritive yelk like the watch glass
on the yelk. Fluid gathers between the outer layer and the border, and the
segmentation-cavity is formed. The gastrula is then formed by invagination,
or a kind of turning-up of the edge of the blastoderm. In this process the
segmentation-cavity disappears.
The space underneath the entoderm corresponds to the primitive gut-
cavity, and is filled with the decreasing food-yelk (n). Thus the formation of
the gastrula of our fish is complete. In contrast to the two chief forms of
gastrula we considered previously, we give the name of discoid gastrula
(discogastrula, Fig. 54) to this third principal type.
Very similar to the discoid gastrulation of the bony fishes is that of the
hags or myxinoida, the remarkable cyclostomes that live parasitically in the
body-cavity of fishes, and are distinguished by several notable peculiarities
from their nearest relatives, the lampreys. While the amphiblastic ova of the
latter are small and develop like those of the amphibia, the cucumber-
shaped ova of the hag are about an inch long, and form a discoid gastrula.
Up to the present it has only been observed in one species (Bdellostoma
Stouti), by Dean and Doflein (1898).
It is clear that the important features which distinguish the discoid
gastrula from the other chief forms we have considered are determined by
the large food-yelk. This takes no direct part in the building of the germinal
layers, and completely fills the primitive gut-cavity of the gastrula, even
protruding at the mouth-opening. If we imagine the original bell-gastrula
(Figs. 30–36) trying to swallow a ball of food which is much bigger than
itself, it would spread out round it in discoid shape in the attempt, just as we
find to be the case here (Fig. 54). Hence we may derive the discoid gastrula
from the original bell-gastrula, through the intermediate stage of the hooded
gastrula. It has arisen through the accumulation of a store of food-stuff at

the vegetal pole, a “nutritive yelk” being thus formed in contrast to the
“formative yelk.” Nevertheless, the gastrula is formed here, as in the
previous cases, by the folding or invagination of the blastula. We can,
therefore, reduce this cenogenetic form of the discoid segmentation to the
palingenetic form of the primitive cleavage.

Fig. 53—Ovum-segmentation of a bony fish. A first cleavage of the
stem-cell (cytula), B division of same into four segmentation-cells (only
two visible), C the germinal disk divides into the blastoderm (b) and the
periblast (p). d nutritive yelk, f fat-globule, c ovolemma, z space between
the ovolemma and the ovum, filled with a clear fluid.)

This reduction is tolerably easy and confident in the case of the small
ovum of our deep-sea bony fish, but it becomes difficult and uncertain in
the case of the large ova that we find in the majority of the other fishes and
in all the reptiles and birds. In these cases the food-yelk is, in the first place,
comparatively colossal, the formative yelk being almost invisible beside it;
and, in the second place, the food-yelk contains a quantity of different
elements, which are known as “yelk-granules, yelk-globules, yelk-plates,
yelk-flakes, yelk-vesicles,” and so on. Frequently these definite elements in
the yelk have been described as real cells, and it has been wrongly stated
that a portion of the embryonic body is built up from these cells. This is by
no means the case. In every case, however large it is—and even when cell-
nuclei travel into it during the cleavage of the border—the nutritive yelk
remains a dead accumulation of food, which is taken into the gut during
embryonic development and consumed by the embryo. The latter develops
solely from the living formative yelk of the stem-cell. This is equally true of
the ova of our small bony fishes and of the colossal ova of the primitive
fishes, reptiles, and birds.

The gastrulation of the primitive fishes or selachii (sharks and rays) has
been carefully studied of late years by Ruckert, Rabl, and H.E. Ziegler in
particular, and is very important in the sense that this group is the oldest
among living fishes, and their gastrulation can be derived directly from that
of the cyclostoma by the accumulation of a large quantity of food-yelk. The
oldest sharks (Cestracion) still have the unequal segmentation inherited
from the cyclostoma. But while in this case, as in the case of the amphibia,
the small ovum completely divides into cells in segmentation, this is no
longer so in the great majority of the selachii (or Elasmobranchii). In these
the contractility of the active protoplasm no longer suffices to break up the
huge mass of the passive deutoplasm completely into cells; this is only
possible in the upper or dorsal part, but not in the lower or ventral section.
Hence we find in the primitive fishes a blastula with a small eccentric
segmentation-cavity (Fig. 55 b), the wall of which varies greatly in
composition. The circular border of the germinal disk which connects the
roof and floor of the segmentation-cavity corresponds to the border-zone at
the equator of the amphibian ovum. In the middle of its hinder border we
have the beginning of the invagination of the primitive gut (Fig. 56 ud); it
extends gradually from this spot (which corresponds to the Rusconian anus
of the amphibia) forward and around, so that the primitive mouth becomes
first crescent-shaped and then circular, and, as it opens wider, surrounds the
ball of the larger food-yelk.

Fig. 54—Discoid gastrula (discogastrula) of a bony fish. e ectoderm, i
entoderm, w border-swelling or primitive mouth, n albuminous globule of
the nutritive yelk, f fat-globule of same, c external membrane (ovolemma),
d partition between entoderm and ectoderm (earlier the segmentation-
cavity.)

Essentially different from the wide-mouthed discoid gastrula of most of
the selachii is the narrow-mouthed discoid gastrula (or epigastrula) of the

amniotes, the reptiles, birds, and monotremes; between the two—as an
intermediate stage—we have the amphigastrula of the amphibia. The latter
has developed from the amphigastrula of the ganoids and dipneusts,
whereas the discoid amniote gastrula has been evolved from the amphibian
gastrula by the addition of food-yelk. This change of gastrulation is still
found in the remarkable ophidia (Gymnophiona, Cœcilia, or Peromela),
serpent-like amphibia that live in moist soil in the tropics, and in many
respects represent the transition from the gill-breathing amphibia to the
lung-breathing reptiles. Their embryonic development has been explained
by the fine studies of the brothers Sarasin of Ichthyophis glutinosa at
Ceylon (1887), and those of August Brauer of the Hypogeophis rostrata in
the Seychelles (1897). It is only by the historical and comparative study of
these that we can understand the difficult and obscure gastrulation of the
amniotes.

Fig. 55—Longitudinal section through the blastula of a shark
(Pristiuris). (From Ruckert.) (Looked at from the left; to the right is the
hinder end, H, to the left the fore end, V.) B segmentation-cavity, kz cells of
the germinal membrane, dk yelk-nuclei.

The bird’s egg is particularly important for our purpose, because most of
the chief studies of the development of the vertebrates are based on
observations of the hen’s egg during hatching. The mammal ovum is much
more difficult to obtain and study, and for this practical and obvious reason
very rarely thoroughly investigated. But we can get hens’ eggs in any
quantity at any time, and, by means of artificial incubation, follow the
development of the embryo step by step. The bird’s egg differs considerably
from the tiny mammal ovum in size, a large quantity of food-yelk
accumulating within the original yelk or the protoplasm of the ovum. This
is the yellow ball which we commonly call the yolk of the egg. In order to
understand the bird’s egg aright—for it is very often quite wrongly
explained—we must examine it in its original condition, and follow it from

the very beginning of its development in the bird’s ovary. We then see that
the original ovum is a quite small, naked, and simple cell with a nucleus,
not differing in either size or shape from the original ovum of the mammals
and other animals (cf. Fig. 13 E). As in the case of all the craniota (animals
with a skull), the original or primitive ovum (protovum) is covered with a
continuous layer of small cells. This membrane is the follicle, from which
the ovum afterwards issues. Immediately underneath it the structureless
yelk-membrane is secreted from the yelk.
The small primitive ovum of the bird begins very early to take up into
itself a quantity of food-stuff through the yelk-membrane, and work it up
into the “yellow yelk.” In this way the ovum enters on its second stage (the
metovum), which is many times larger than the first, but still only a single
enlarged cell. Through the accumulation of the store of yellow yelk within
the ball of protoplasm the nucleus it contains (the germinal vesicle) is
forced to the surface of the ball. Here it is surrounded by a small quantity of
protoplasm, and with this forms the lens-shaped formative yelk (Fig. 15 b).
This is seen on the yellow yelk-ball, at a certain point of the surface, as a
small round white spot—the “tread” (cicatricula). From this point a thread-
like column of white nutritive yelk (d), which contains no yellow yelk-
granules, and is softer than the yellow food-yelk, proceeds to the middle of
the yellow yelk-ball, and forms there a small central globule of white yelk
(Fig. 15 d). The whole of this white yelk is not sharply separated from the
yellow yelk, which shows a slight trace of concentric layers in the hard-
boiled egg (Fig. 15 c). We also find in the hen’s egg, when we break the
shell and take out the yelk, a round small white disk at its surface which
corresponds to the tread. But this small white “germinal disk” is now
further developed, and is really the gastrula of the chick. The body of the
chick is formed from it alone. The whole white and yellow yelk-mass is
without any significance for the formation of the embryo, it being merely
used as food by the developing chick. The clear, glarous mass of albumin
that surrounds the yellow yelk of the bird’s egg, and also the hard chalky
shell, are only formed within the oviduct round the impregnated ovum.

Fig. 56—Longitudinal section of the blastula of a shark (Pristiurus) at
the beginning of gastrulation. (From Ruckert.) (Seen from the left.) V fore
end, H hind end, B segmentation-cavity, ud first trace of the primitive gut,
dk yelk-nuclei, fd fine-grained yelk, gd coarse-grained yelk.

When the fertilisation of the bird’s ovum has taken place within the
mother’s body, we find in the lens-shaped stem-cell the progress of flat,
discoid segmentation (Fig. 57). First two equal segmentation-cells (A) are
formed from the ovum. These divide into four (B), then into eight, sixteen
(C), thirty-two, sixty-four, and so on. The cleavage of the cells is always
preceded by a division of their nuclei. The cleavage surfaces between the
segmentation-cells appear at the free surface of the tread as clefts. The first
two divisions are vertical to each other, in the form of a cross (B). Then
there are two more divisions, which cut the former at an angle of forty-five
degrees. The tread, which thus becomes the germinal disk, now has the
appearance of an eight-rayed star. A circular cleavage next taking place
round the middle, the eight triangular cells divide into sixteen, of which
eight are in the middle and eight distributed around (C). Afterwards circular
clefts and radial clefts, directed towards the centre, alternate more or less
irregularly (D, E). In most of the amniotes the formation of concentric and
radial clefts is irregular from the very first; and so also in the hen’s egg. But
the final outcome of the cleavage-process is once more the formation of a
large number of small cells of a similar nature. As in the case of the fish-
ovum, these segmentation-cells form a round, lens-shaped disk, which
corresponds to the morula, and is embedded in a small depression of the
white yelk. Between the lens-shaped disk of the morula-cells and the
underlying white yelk a small cavity is now formed by the accumulation of
fluid, as in the fishes. Thus we get the peculiar and not easily recognisable
blastula of the bird (Fig. 58). The small segmentation-cavity (fh) is very flat
and much compressed. The upper or dorsal wall (dw) is formed of a single
layer of clear, distinctly separated cells; this corresponds to the upper or
animal hemisphere of the triton-blastula (Fig. 45). The lower or ventral wall

of the flat dividing space (vw) is made up of larger and darker
segmentation-cells; it corresponds to the lower or vegetal hemisphere of the
blastula of the water-salamander (Fig. 45 dz). The nuclei of the yelk-cells,
which are in this case especially numerous at the edge of the lens-shaped
blastula, travel into the white yelk, increase by cleavage, and contribute
even to the further growth of the germinal disk by furnishing it with food-
stuff.

Fig. 57—Diagram of discoid segmentation in the bird’s ovum
(magnified). Only the formative yelk (the tread) is shown in these six
figures (A to F), because cleavage only takes place in this. The much larger
food-yelk, which does not share in the cleavage, is left out and merely
indicated by the dark ring without.

The invagination or the folding inwards of the bird-blastula takes place in
this case also at the hinder pole of the subsequent chief axis, in the middle
of the hind border of the round germinal disk (Fig. 59 s). At this spot we
have the most brisk cleavage of the cells; hence the cells are more
numerous and smaller here than in the fore-half of the germinal disk. The
border-swelling or thick edge of the disk is less clear but whiter behind, and
is more sharply separated from contiguous parts. In the middle of its hind
border there is a white, crescent-shaped groove—Koller’s sickle-groove
(Fig. 59 s); a small projecting process in the centre of it is called the sickle-
knob (sk). This important cleft is the primitive mouth, which was described
for a long time as the “primitive groove.” If we make a vertical section
through this part, we see that a flat and broad cleft stretches under the
germinal disk forwards from the primitive mouth; this is the primitive gut
(Fig. 60 ud). Its roof or dorsal wall is formed by the folded upper part of the

blastula, and its floor or ventral wall by the white yelk (wd), in which a
number of yelk-nuclei (dk) are distributed. There is a brisk multiplication of
these at the edge of the germinal disk, especially in the neighbourhood of
the sickle-shaped primitive mouth.
We learn from sections through later stages of this discoid bird-gastrula
that the primitive gut-cavity, extending forward from the primitive mouth as
a flat pouch, undermines the whole region of the round flat lens-shaped
blastula (Fig. 61 ud). At the same time, the segmentation-cavity gradually
disappears altogether, the folded inner germinal layer (ik) placing itself
from underneath on the overlying outer germinal layer (ak). The typical
process of invagination, though greatly disguised, can thus be clearly seen
in this case, as Goette and Rauber, and more recently Duval (Fig. 61), have
shown.
The older embryologists (Pander, Baer, Remak), and, in recent times
especially, His, Kölliker, and others, said that the two primary germinal
layers of the hen’s ovum—the oldest and most frequent subject of
observation!—arose by horizontal cleavage of a simple germinal disk. In
opposition to this accepted view, I affirmed in my Gastræa Theory (1873)
that the discoid bird-gastrula, like that of all other vertebrates, is formed by
folding (or invagination), and that this typical process is merely altered in a
peculiar way and disguised by the immense accumulation of food-yelk and
the flat spreading of the discoid blastula at one part of its surface. I
endeavoured to establish this view by the derivation of the vertebrates from
one source, and especially by proving that the birds descend from the
reptiles, and these from the amphibia. If this is correct, the discoid gastrula
of the amniotes must have been formed by the folding-in of a hollow
blastula, as has been shown by Remak and Rusconi of the discoid gastrula
of the amphibia, their direct ancestors. The accurate and extremely careful
observations of the authors I have mentioned (Goette, Rauber, and Duval)
have decisively proved this recently for the birds; and the same has been
done for the reptiles by the fine studies of Kupffer, Beneke, Wenkebach,
and others. In the shield-shaped germinal disk of the lizard (Fig. 62), the
crocodile, the tortoise, and other reptiles, we find in the middle of the hind
border (at the same spot as the sickle groove in the bird) a transverse furrow
(u), which leads into a flat, pouch-like, blind sac, the primitive gut. The fore
(dorsal) and hind (ventral) lips of the transverse furrow correspond exactly
to the lips of the primitive mouth (or sickle-groove) in the birds.

Fig. 58—Vertical section of the blastula of a hen (discoblastula). fh
segmentation-cavity, dw dorsal wall of same, vw ventral wall, passing
directly into the white yelk (wd). (From Duval.)
Fig. 59—The germinal disk of the hen’s ovum at the beginning of
gastrulation; A before incubation, B in the first hour of incubation. (From
Koller.) ks germinal-disk, V its fore and H its hind border; es embryonic
shield, s sickle-groove, sk sickle knob, d yelk.
Fig. 60—Longitudinal section of the germinal disk of a siskin
(discogastrula). (From Duval.) ud primitive gut, vl, hl fore and hind lips of
the primitive mouth (or sickle-edge); ak outer germinal layer, ik inner
germinal layer, dk yelk-nuclei, wd white yelk.

Fig. 61—Longitudinal section of the discoid gastrula of the
nightingale. (From Duval.) ud primitive gut, vl, hl fore and hind lips of the
primitive mouth; ak, ik outer and inner germinal layers; vr fore-border of
the discogastrula.

Fig. 62—Germinal disk of the lizard (Lacerta agilis). (From Kupffer.) u
primitive mouth, s sickle, es embryonic shield, hf and df light and dark
germinative area.

The gastrulation of the mammals must be derived from this special
embryonic development of the reptiles and birds. This latest and most
advanced class of the vertebrates has, as we shall see afterwards, evolved at
a comparatively recent date from an older group of reptiles; and all these
amniotes must have come originally from a common stem-form. Hence the
distinctive embryonic process of the mammal must have arisen by
cenogenetic modifications from the older form of gastrulation of the reptiles
and birds. Until we admit this thesis we cannot understand the formation of
the germinal layers in the mammal, and therefore in man.
I first advanced this fundamental principle in my essay On the
Gastrulation of Mammals (1877), and sought to show in this way that I
assumed a gradual degeneration of the food-yelk and the yelk-sac on the
way from the proreptiles to the mammals. “The cenogenetic process of
adaptation,” I said, “which has occasioned the atrophy of the rudimentary
yelk-sac of the mammal, is perfectly clear. It is due to the fact that the
young of the mammal, whose ancestors were certainly oviparous, now
remain a long time in the womb. As the great store of food-yelk, which the
oviparous ancestors gave to the egg, became superfluous in their
descendants owing to the long carrying in the womb, and the maternal
blood in the wall of the uterus made itself the chief source of nourishment,
the now useless yelk-sac was bound to atrophy by embryonic adaptation.”
My opinion met with little approval at the time; it was vehemently
attacked by Kölliker, Hensen, and His in particular. However, it has been

gradually accepted, and has recently been firmly established by a large
number of excellent studies of mammal gastrulation, especially by Edward
Van Beneden’s studies of the rabbit and bat, Selenka’s on the marsupials
and rodents, Heape’s and Lieberkühn’s on the mole, Kupffer and Keibel’s
on the rodents, Bonnet’s on the ruminants, etc. From the general
comparative point of view, Carl Rabl in his theory of the mesoderm, Oscar
Hertwig in the latest edition of his Manual (1902), and Hubrecht in his
Studies in Mammalian Embryology (1891), have supported the opinion, and
sought to derive the peculiarly modified gastrulation of the mammal from
that of the reptile.
In the meantime (1884) the studies of Wilhelm Haacke and Caldwell
provided a proof of the long-suspected and very interesting fact, that the
lowest mammals, the monotremes, lay eggs, like the birds and reptiles, and
are not viviparous like the other mammals. Although the gastrulation of the
monotremes was not really known until studied by Richard Semon in 1894,
there could be little doubt, in view of the great size of their food-yelk, that
their ovum-segmentation was discoid, and led to the formation of a sickle-
mouthed discogastrula, as in the case of the reptiles and birds. Hence I had,
in 1875 (in my essay on The Gastrula and Ovum-segmentation of Animals),
counted the monotremes among the discoblastic vertebrates. This
hypothesis was established as a fact nineteen years afterwards by the careful
observations of Semon; he gave in the second volume of his great work,
Zoological Journeys in Australia (1894), the first description and correct
explanation of the discoid gastrulation of the monotremes. The fertilised
ova of the two living monotremes (Echidna and Ornithorhynchus) are balls
of one-fifth of an inch in diameter, enclosed in a stiff shell; but they grow
considerably during development, so that when laid the egg is three times as
large. The structure of the plentiful yelk, and especially the relation of the
yellow and the white yelk, are just the same as in the reptiles and birds. As
with these, partial cleavage takes place at a spot on the surface at which the
small formative yelk and the nucleus it encloses are found. First is formed a
lens-shaped circular germinal disk. This is made up of several strata of
cells, but it spreads over the yelk-ball, and thus becomes a one-layered
blastula.

Fig. 63—Ovum of the opossum (Didelphys) divided into four. (From
Selenka.) b the four segmentation-cells, r directive body, c unnucleated
coagulated matter, p, albumin-membrane.

If we then imagine the yelk it contains to be dissolved and replaced by a
clear liquid, we have the characteristic blastula of the higher mammals. In
these the gastrulation proceeds in two phases, as Semon rightly observes:
firstly, formation of the entoderm by cleavage at the centre and further
growth at the edge; secondly, invagination. In the monotremes more
primitive conditions have been retained better than in the reptiles and birds.
In the latter, before the commencement of the gastrula-folding, we have, at
least at the periphery, a two-layered embryo forming from the cleavage. But
in the monotremes the formation of the cenogenetic entoderm does not
precede the invagination; hence in this case the construction of the germinal
layers is less modified than in the other amniota.

Fig. 64—Blastula of the opossum (Didelphys). (From Selenka.) a
animal pole of the blastula, v vegetal pole, en mother-cell of the entoderm,
ex ectodermic cells, s spermia, ib unnucleated yelk-balls (remainder of the
food-yelk), p albumin membrane.

The marsupials, a second sub-class, come next to the oviparous
monotremes, the oldest of the mammals. But as in their case the food-yelk
is already atrophied, and the little ovum develops within the mother’s body,
the partial cleavage has been reconverted into total. One section of the
marsupials still show points of agreement with the monotremes, while
another section of them, according to the splendid investigations of
Selenka, form a connecting-link between these and the placentals.
The fertilised ovum of the opossum (Didelphys) divides, according to
Selenka, first into two, then four, then eight equal cells; hence the
segmentation is at first equal or homogeneous. But in the course of the
cleavage a larger cell, distinguished by its less clear plasm and its
containing more yelk-granules (the mother cell of the entoderm, Fig. 64 en),
separates from the others; the latter multiply more rapidly than the former.
As, further, a quantity of fluid gathers in the morula, we get a round
blastula, the wall of which is of varying thickness, like that of the
amphioxus (Fig. 38 E) and the amphibia (Fig. 45). The upper or animal
hemisphere is formed of a large number of small cells; the lower or vegetal
hemisphere of a small number of large cells. One of the latter, distinguished
by its size (Fig. 64 en), lies at the vegetal pole of the blastula-axis, at the
point where the primitive mouth afterwards appears. This is the mother-cell
of the entoderm; it now begins to multiply by cleavage, and the daughter-
cells (Fig. 65 i) spread out from this spot over the inner surface of the
blastula, though at first only over the vegetal hemisphere. The less clear
entodermic cells (i) are distinguished at first by their rounder shape and
darker nuclei from the higher, clearer, and longer entodermic cells (e),
afterwards both are greatly flattened, the inner blastodermic cells more than
the outer.

Fig. 65—Blastula of the opossum (Didelphys) at the beginning of
gastrulation. (From Selenka.) e ectoderm, i entoderm; a animal pole, u
primitive mouth at the vegetal pole, f segmentation-cavity, d unnucleated
yelk-balls (relics of the reduced food-yelk), c nucleated curd (without yelk-
granules)
Fig. 66—Oval gastrula of the opossum (Didelphys), about eight hours old.
(From Selenka) (external view).)

The unnucleated yelk-balls and curd (Fig. 65 d) that we find in the fluid
of the blastula in these marsupials are very remarkable; they are the relics of
the atrophied food-yelk, which was developed in their ancestors, the
monotremes, and in the reptiles.
In the further course of the gastrulation of the opossum the oval shape of
the gastrula (Fig. 66) gradually changes into globular, a larger quantity of
fluid accumulating in the vesicle. At the same time, the entoderm spreads
further and further over the inner surface of the ectoderm (e). A globular
vesicle is formed, the wall of which consists of two thin simple strata of
cells; the cells of the outer germinal layer are rounder, and those of the inner
layer flatter. In the region of the primitive mouth (p) the cells are less
flattened, and multiply briskly. From this point—from the hind (ventral) lip
of the primitive mouth, which extends in a central cleft, the primitive
groove—the construction of the mesoderm proceeds.
Gastrulation is still more modified and curtailed cenogenetically in the
placentals than in the marsupials. It was first accurately known to us by the
distinguished investigations of Edward Van Beneden in 1875, the first
object of study being the ovum of the rabbit. But as man also belongs to this

sub-class, and as his as yet unstudied gastrulation cannot be materially
different from that of the other placentals, it merits the closest attention. We
have, in the first place, the peculiar feature that the two first segmentation-
cells that proceed from the cleavage of the fertilised ovum (Fig. 68) are of
different sizes and natures; the difference is sometimes greater, sometimes
less (Fig. 69). One of these first daughter-cells of the ovum is a little larger,
clearer, and more transparent than the other. Further, the smaller cell takes a
colour in carmine, osmium, etc., more strongly than the larger. By repeated
cleavage of it a morula is formed, and from this a blastula, which changes
in a very characteristic way into the greatly modified gastrula. When the
number of the segmentation-cells in the mammal embryo has reached
ninety-six (in the rabbit, about seventy hours after impregnation) the fœtus
assumes a form very like the archigastrula (Fig. 72). The spherical embryo
consists of a central mass of thirty-two soft, round cells with dark nuclei,
which are flattened into polygonal shape by mutual pressure, and colour
dark-brown with osmic acid (Fig. 72 i). This dark central group of cells is
surrounded by a lighter spherical membrane, consisting of sixty-four cube-
shaped, small, and fine-grained cells which lie close together in a single
stratum, and only colour slightly in osmic acid (Fig. 72 e). The authors who
regard this embryonic form as the primary gastrula of the placental
conceive the outer layer as the ectoderm and the inner as the entoderm. The
entodermic membrane is only interrupted at one spot, one, two, or three of
the ectodermic cells being loose there. These form the yelk-stopper, and fill
up the mouth of the gastrula (a). The central primitive gut-cavity (d) is full
of entodermic cells. The uni-axial type of the mammal gastrula is
accentuated in this way. However, opinions still differ considerably as to the
real nature of this “provisional gastrula” of the placental and its relation to
the blastula into which it is converted.
As the gastrulation proceeds a large spherical blastula is formed from this
peculiar solid amphigastrula of the placental, as we saw in the case of the
marsupial. The accumulation of fluid in the solid gastrula (Fig. 73 A) leads
to the formation of an eccentric cavity, the group of the darker entodermic
cells (hy) remaining directly attached at one spot with the round enveloping
stratum of the lighter ectodermic cells (ep). This spot corresponds to the
original primitive mouth (prostoma or blastoporus). From this important
spot the inner germinal layer spreads all round on the inner surface of the

outer layer, the cell-stratum of which forms the wall of the hollow sphere;
the extension proceeds from the vegetal towards the animal pole.

Fig. 67—Longitudinal section through the oval gastrula of the
opossum (Fig. 69). (From Selenka.) p primitive mouth, e ectoderm, i
entoderm, d yelk remains in the primitive gut-cavity (u).

The cenogenetic gastrulation of the placental has been greatly modified
by secondary adaptation in the various groups of this most advanced and
youngest sub-class of the mammals. Thus, for instance, we find in many of
the rodents (guinea-pigs, mice, etc.) apparently a temporary inversion of the
two germinal layers. This is due to a folding of the blastodermic wall by
what is called the “girder,” a plug-shaped growth of Rauber’s “roof-layer.”
It is a thin layer of flat epithelial cells, that is freed from the surface of the
blastoderm in some of the rodents; it has no more significance in
connection with the general course of placental gastrulation than the
conspicuous departure from the usual globular shape in the blastula of some
of the ungulates. In some pigs and ruminants it grows into a thread-like,
long and thin tube.
Thus the gastrulation of the placentals, which diverges most from that of
the amphioxus, the primitive form, is reduced to the original type, the
invagination of a modified blastula. Its chief peculiarity is that the folded
part of the blastoderm does not form a completely closed (only open at the
primitive mouth) blind sac, as is usual; but this blind sac has a wide opening
at the ventral curve (opposite to the dorsal mouth); and through this opening
the primitive gut communicates from the first with the embryonic cavity of
the blastula. The folded crest-shaped entoderm grows with a free circular
border on the inner surface of the entoderm towards the vegetal pole; when

it has reached this, and the inner surface of the blastula is completely grown
over, the primitive gut is closed. This remarkable direct transition of the
primitive gut-cavity into the segmentation-cavity is explained simply by the
assumption that in most of the mammals the yelk-mass, which is still
possessed by the oldest forms of the class (the monotremes) and their
ancestors (the reptiles), is atrophied. This proves the essential unity of
gastrulation in all the vertebrates, in spite of the striking differences in the
various classes.

Fig. 68—Stem-cell of the mammal ovum (from the rabbit).
k stem-nucleus, n nuclear corpuscle, p protoplasm of the stem-cell, z
modified zona pellucida, h outer albuminous membrane, s dead sperm-cells.

Fig. 69—Incipient cleavage of the mammal ovum (from the rabbit).
The stem-cell has divided into two unequal cells, one lighter (e) and one
darker (i). z zona pellucida, h outer albuminous membrane, s dead sperm-
cell.

Fig. 70—The first four segmentation-cells of the mammal ovum (from
the rabbit).
e the two larger (and lighter) cells, i the two smaller (and darker) cells, z
zona pellucida, h outer albuminous membrane.

Fig. 71—Mammal ovum with eight segmentation-cells (from the
rabbit). e four larger and lighter cells, i four smaller and darker cells, z zona
pellucida, h outer albuminous membrane.

In order to complete our consideration of the important processes of
segmentation and gastrulation, we will, in conclusion, cast a brief glance at
the fourth chief type—superficial segmentation. In the vertebrates this form
is not found at all. But it plays the chief part in the large stem of the
articulates—the insects, spiders, myriapods, and crabs. The distinctive form
of gastrula that comes of it is the “vesicular gastrula” (Perigastrula).
In the ova which undergo this superficial cleavage the formative yelk is
sharply divided from the nutritive yelk, as in the preceding cases of the ova
of birds, reptiles, fishes, etc.; the formative yelk alone undergoes cleavage.
But while in the ova with discoid gastrulation the formative yelk is not in
the centre, but at one pole of the uni-axial ovum, and the food-yelk gathered
at the other pole, in the ova with superficial cleavage we find the formative

yelk spread over the whole surface of the ovum; it encloses spherically the
food-yelk, which is accumulated in the middle of the ova. As the
segmentation only affects the former and not the latter, it is bound to be
entirely “superficial”; the store of food in the middle is quite untouched by
it. As a rule, it proceeds in regular geometrical progression. In the end the
whole of the formative yelk divides into a number of small and
homogeneous cells, which lie close together in a single stratum on the entire
surface of the ovum, and form a superficial blastoderm. This blastoderm is a
simple, completely closed vesicle, the internal cavity of which is entirely
full of food-yelk. This real blastula only differs from that of the primitive
ova in its chemical composition. In the latter the content is water or a
watery jelly; in the former it is a thick mixture, rich in food-yelk, of
albuminous and fatty substances. As this quantity of food-yelk fills the
centre of the ovum before cleavage begins, there is no difference in this
respect between the morula and the blastula. The two stages rather agree in
this.
When the blastula is fully formed, we have again in this case the
important folding or invagination that determines gastrulation. The space
between the skin-layer and the gut-layer (the remainder of the
segmentation-cavity) remains full of food-yelk, which is gradually used up.
This is the only material difference between our vesicular gastrula
(perigastrula) and the original form of the bell-gastrula (archigastrula).
Clearly the one has been developed from the other in the course of time,
owing to the accumulation of food-yelk in the centre of the ovum.[23]
[23] On the reduction of all forms of gastrulation to the original palingenetic
form see especially the lucid treatment of the subject in Arnold Lang’s Manual of
Comparative Anatomy (1888), Part I.

We must count it an important advance that we are thus in a position to
reduce all the various embryonic phenomena in the different groups of
animals to these four principal forms of segmentation and gastrulation. Of
these four forms we must regard one only as the original palingenetic, and
the other three as cenogenetic and derivative. The unequal, the discoid, and
the superficial segmentation have all clearly arisen by secondary adaptation
from the primary segmentation; and the chief cause of their development
has been the gradual formation of the food-yelk, and the increasing

antithesis between animal and vegetal halves of the ovum, or between
ectoderm (skin-layer) and entoderm (gut-layer).

Fig. 72—Gastrula of the placental mammal (epigastrula from the
rabbit), longitudinal section through the axis. e ectodermic cells (sixty-four,
lighter and smaller), i entodermic cells (thirty-two, darker and larger), d
central entodermic cell, filling the primitive gut-cavity, o peripheral
entodermic cell, stopping up the opening of the primitive mouth (yelk-
stopper in the Rusconian anus).

The numbers of careful studies of animal gastrulation that have been
made in the last few decades have completely established the views I have
expounded, and which I first advanced in the years 1872–76. For a time
they were greatly disputed by many embryologists. Some said that the
original embryonic form of the metazoa was not the gastrula, but the
“planula”—a double-walled vesicle with closed cavity and without mouth-
aperture; the latter was supposed to pierce through gradually. It was
afterwards shown that this planula (found in several sponges, etc.) was a
later evolution from the gastrula.

Fig. 73—Gastrula of the rabbit. A as a solid, spherical cluster of cells,
B changing into the embryonic vesicle, bp primitive mouth, ep ectoderm, hy
entoderm.

It was also shown that what is called delamination—the rise of the two
primary germinal layers by the folding of the surface of the blastoderm (for
instance, in the Geryonidæ and other medusæ)—was a secondary
formation, due to cenogenetic variations from the original invagination of
the blastula. The same may be said of what is called “immigration,” in
which certain cells or groups of cells are detached from the simple layer of
the blastoderm, and travel into the interior of the blastula; they attach
themselves to the inner wall of the blastula, and form a second internal
epithelial layer—that is to say, the entoderm. In these and many other
controversies of modern embryology the first requisite for clear and natural
explanation is a careful and discriminative distinction between palingenetic
(hereditary) and cenogenetic (adaptive) processes. If this is properly
attended to, we find evidence everywhere of the biogenetic law.

Chapter X.
THE CŒLOM THEORY

The two “primary germinal layers” which the gastræa theory has shown
to be the first foundation in the construction of the body are found in this
simplest form throughout life only in animals of the lowest grade—in the
gastræads, olynthus (the stem-form of the sponges), hydra, and similar very
simple animals. In all the other animals new strata of cells are formed
subsequently between these two primary body-layers, and these are
generally comprehended under the title of the middle layer, or mesoderm.
As a rule, the various products of this middle layer afterwards constitute the
great bulk of the animal frame, while the original entoderm, or internal
germinal layer, is restricted to the clothing of the alimentary canal and its
glandular appendages; and, on the other hand, the ectoderm, or external
germinal layer, furnishes the outer clothing of the body, the skin and
nervous system.
In some large groups of the lower animals, such as the sponges, corals,
and flat-worms, the middle germinal layer remains a single connected mass,
and most of the body is developed from it; these have been called the three-
layered metazoa, in opposition to the two-layered animals described. Like
the two-layered animals, they have no body-cavity—that is to say, no cavity
distinct from the alimentary system. On the other hand, all the higher
animals have this real body-cavity (cœloma), and so are called cœlomaria.
In all these we can distinguish four secondary germinal layers, which
develop from the two primary layers. To the same class belong all true
vermalia (excepting the platodes), and also the higher typical animal stems
that have been evolved from them—molluscs, echinoderms, articulates,
tunicates, and vertebrates.

Figs. 74 and 75—Diagram of the four secondary germinal layers,
transverse section through the metazoic embryo: Fig. 74 of an annelid, Fig.
75 of a vermalian. a primitive gut, dd ventral glandular layer, df ventral
fibre-layer, hm skin-fibre-layer, hs skin-sense-layer, u beginning of the
rudimentary kidneys, n beginning of the nerve-plates.

The body-cavity (cœloma) is therefore a new acquisition of the animal
body, much younger than the alimentary system, and of great importance. I
first pointed out this fundamental significance of the cœlom in my
Monograph on the Sponges (1872), in the section which draws a distinction
between the body-cavity and the gut-cavity, and which follows immediately
on the germ-layer theory and the ancestral tree of the animal kingdom (the
first sketch of the gastræa theory). Up to that time these two principal
cavities of the animal body had been confused, or very imperfectly
distinguished; chiefly because Leuckart, the founder of the cœlenterata
group (1848), has attributed a body-cavity, but not a gut-cavity, to these
lowest metazoa. In reality, the truth is just the other way about.
The ventral cavity, the original organ of nutrition in the multicellular
animal-body, is the oldest and most important organ of all the metazoa, and,
together with the primitive mouth, is formed in every case in the gastrula as
the primitive gut; it is only at a much later stage that the body-cavity, which
is entirely wanting in the cœlenterata, is developed in some of the metazoa
between the ventral and the body wall. The two cavities are entirely
different in content and purport. The alimentary cavity (enteron) serves the
purpose of digestion; it contains water and food taken from without, as well
as the pulp (chymus) formed from this by digestion. On the other hand, the
body-cavity, quite distinct from the gut and closed externally, has nothing to
do with digestion; it encloses the gut itself and its glandular appendages,

and also contains the sexual products and a certain amount of blood or
lymph, a fluid that is transuded through the ventral wall.
As soon as the body-cavity appears, the ventral wall is found to be
separated from the enclosing body-wall, but the two continue to be directly
connected at various points. We can also then always distinguish a number
of different layers of tissue in both walls—at least two in each. These
tissue-layers are formed originally from four different simple cell-layers,
which are the much-discussed four secondary germinal layers. The
outermost of these, the skin-sense-layer (Figs. 74, 75 hs), and the
innermost, the gut-gland-layer (dd), remain at first simple epithelia or
covering-layers. The one covers the outer surface of the body, the other the
inner surface of the ventral wall; hence they are called confining or limiting
layers. Between them are the two middle-layers, or mesoblasts, which
enclose the body-cavity.

Fig. 76—Cœlomula of sagitta (gastrula with a couple of cœlom-
pouches. (From Kowalevsky.) bl.p primitive mouth, al primitive gut, pv
cœlom-folds, m permanent mouth.

The four secondary germinal layers are so distributed in the structure of
the body in all the cœlomaria (or all metazoa that have a body-cavity) that
the outer two, joined fast together, constitute the body-wall, and the inner
two the ventral wall; the two walls are separated by the cavity of the cœlom.
Each of the walls is made up of a limiting layer and a middle layer. The two
limiting layers chiefly give rise to epithelia, or covering-tissues, and glands
and nerves, while the middle layers form the great bulk of the fibrous tissue,
muscles, and connective matter. Hence the latter have also been called
fibrous or muscular layers. The outer middle layer, which lies on the inner
side of the skin-sense-layer, is the skin fibre-layer; the inner middle layer,
which attaches from without to the ventral glandular layer, is the ventral
fibre layer. The former is usually called briefly the parietal, and the latter

the visceral layer or mesoderm. Of the many different names that have been
given to the four secondary germinal layers, the following are those most in
use to-day:—
1. Skin-sense-layer I. Neural layer The two secondary germinal
(outer limiting layer). (neuroblast). layers of the body-wall:
2. Skin-fibre-layer II. Parietal layer I. Epithelial.
(outer middle layer). (myoblast). II. Fibrous.
3. Gut-fibre-layer III. Visceral layer The two secondary germinal
(inner middle layer). (genoblast). layers of the gut-wall:
4. Gut-gland-layer IV. Enteral layer III. Fibrous.
(inner limiting layer). (enteroblast) IV. Epithelial.
The first scientist to recognise and clearly distinguish the four secondary
germinal layers was Baer. It is true that he was not quite clear as to their
origin and further significance, and made several mistakes in detail in
explaining them. But, on the whole, their great importance did not escape
him. However, in later years his view had to be given up in consequence of
more accurate observations. Remak then propounded a three-layer theory,
which was generally accepted. These theories of cleavage, however, began
to give way thirty years ago, when Kowalevsky (1871) showed that in the
case of Sagitta (a very clear and typical subject of gastrulation) the two
middle germinal layers and the two limiting layers arise not by cleavage,
but by folding—by a secondary invagination of the primary inner germ-
layer. This invagination or folding proceeds from the primitive mouth, at
the two sides of which (right and left) a couple of pouches are formed. As
these cœlom-pouches or cœlom-sacs detach themselves from the primitive
gut, a double body-cavity is formed (Figs. 74–76).

Fig. 77—Cœlomula of sagitta, in section. (From Hertwig.) D dorsal
side, V ventral side, ik inner germinal layer, mv visceral mesoblast, lh body-
cavity, mp parietal mesoblast, ak outer germinal layer.

The same kind of cœlom-formation as in sagitta was afterwards found by
Kowalevsky in brachiopods and other invertebrates, and in the lowest
vertebrate—the amphioxus. Further instances were discovered by two
English embryologists, to whom we owe very considerable advance in
ontogeny—E. Ray-Lankester and F. Balfour. On the strength of these and
other studies, as well as most extensive research of their own, the brothers
Oscar and Richard Hertwig constructed in 1881 the Cœlom Theory. In order
to appreciate fully the great merit of this illuminating and helpful theory,
one must remember what a chaos of contradictory views was then
represented by the “problem of the mesoderm,” or the much-disputed
“question of the origin of the middle germinal layer.” The cœlom theory
brought some light and order into this infinite confusion by establishing the
following points: 1. The body-cavity originates in the great majority of
animals (especially in all the vertebrates) in the same way as in sagitta: a
couple of pouches or sacs are formed by folding inwards at the primitive
mouth, between the two primary germinal layers; as these pouches detach
from the primitive gut, a pair of cœlom-sacs (right and left) are formed; the
coalescence of these produces a simple body-cavity. 2. When these cœlom-
embryos develop, not as a pair of hollow pouches, but as solid layers of
cells (in the shape of a pair of mesodermal streaks)—as happens in the
higher vertebrates—we have a secondary (cenogenetic) modification of the
primary (palingenetic) structure; the two walls of the pouches, inner and
outer, have been pressed together by the expansion of the large food-yelk. 3.
Hence the mesoderm consists from the first of two genetically distinct
layers, which do not originate by the cleavage of a primary simple middle
layer (as Remak supposed). 4. These two middle layers have, in all
vertebrates, and the great majority of the invertebrates, the same radical
significance for the construction of the animal body; the inner middle layer,
or the visceral mesoderm, (gut-fibre layer), attaches itself to the original
entoderm, and forms the fibrous, muscular, and connective part of the
visceral wall; the outer middle layer, or the parietal mesoderm (skin-fibre-
layer), attaches itself to the original ectoderm and forms the fibrous,
muscular, and connective part of the body-wall. 5. It is only at the point of

origination, the primitive mouth and its vicinity, that the four secondary
germinal layers are directly connected; from this point the two middle
layers advance forward separately between the two primary germinal
layers, to which they severally attach themselves. 6. The further separation
or differentiation of the four secondary germinal layers and their division
into the various tissues and organs take place especially in the later fore-part
or head of the embryo, and extend backwards from there towards the
primitive mouth.

Fig. 78—Section of a young sagitta. (From Hertwig.) dh visceral cavity,
ik and ak inner and outer limiting layers, mv and mp inner and outer middle
layers, lk body-cavity, dm and vm dorsal and visceral mesentery.

All animals in which the body-cavity demonstrably arises in this way
from the primitive gut (vertebrates, tunicates, echinoderms, articulates, and
a part of the vermalia) were comprised by the Hertwigs under the title of
enterocœla, and were contrasted with the other groups of the pseudocœla
(with false body-cavity) and the cœlenterata (with no body-cavity).
However, this radical distinction and the views as to classification which it
occasioned have been shown to be untenable. Further, the absolute
differences in tissue-formation which the Hertwigs set up between the
enterocœla and pseudocœla cannot be sustained in this connection. For
these and other reasons their cœlom-theory has been much criticised and
partly abandoned. Nevertheless, it has rendered a great and lasting service
in the solution of the difficult problem of the mesoderm, and a material part
of it will certainly be retained. I consider it an especial merit of the theory
that it has established the identity of the development of the two middle
layers in all the vertebrates, and has traced them as cenogenetic
modifications back to the original palingenetic form of development that we
still find in the amphioxus. Carl Rabl comes to the same conclusion in his
able Theory of the Mesoderm, and so do Ray-Lankester, Rauber, Kupffer,
Ruckert, Selenka, Hatschek, and others. There is a general agreement in

these and many other recent writers that all the different forms of cœlom-
construction, like those of gastrulation, follow one and the same strict
hereditary law in the vast vertebrate stem; in spite of their apparent
differences, they are all only cenogenetic modifications of one palingenetic
type, and this original type has been preserved for us down to the present
day by the invaluable amphioxus.
But before we go into the regular cœlomation of the amphioxus, we will
glance at that of the arrow-worm (Sagitta), a remarkable deep-sea worm
that is interesting in many ways for comparative anatomy and ontogeny. On
the one hand, the transparency of the body and the embryo, and, on the
other hand, the typical simplicity of its embryonic development, make the
sagitta a most instructive object in connection with various problems. The
class of the chætogatha, which is only represented by the cognate genera of
Sagitta and Spadella, is in another respect also a most remarkable branch of
the extensive vermalia stem. It was therefore very gratifying that Oscar
Hertwig (1880) fully explained the anatomy, classification, and evolution of
the chætognatha in his careful monograph.

Figs. 79 and 80.—Transverse section of amphioxus-larvæ. (From
Hatschek.) Fig. 79 at the commencement of cœlom formation (still without
segments), Fig. 80 at the stage with four primitive segments. ak, ik, mk
outer, inner, and middle germinal layer, hp horn plate, mp medullary plate,
ch chorda, * and * disposition of the cœlom-pouches, lh body-cavity.)

The spherical blastula that arises from the impregnated ovum of the
sagitta is converted by a folding at one pole into a typical archigastrula,
entirely similar to that of the Monoxenia which I described (Chapter VIII,
Fig. 29). This oval, uni-axial cup-larva (circular in section) becomes
bilateral (or tri-axial) by the growth of a couple of cœlom-pouches from the

primitive gut (Figs. 76, 77). To the right and left a sac-shaped fold appears
towards the top pole (where the permanent mouth, m, afterwards arises).
The two sacs are at first separated by a couple of folds of the entoderm (Fig.
76 pv), and are still connected with the primitive gut by wide apertures;
they also communicate for a short time with the dorsal side (Fig. 77 d).
Soon, however, the cœlom-pouches completely separate from each other
and from the primitive gut; at the same time they enlarge so much that they
close round the primitive gut (Fig. 78). But in the middle line of the dorsal
and ventral sides the pouches remain separated, their approaching walls
joining here to form a thin vertical partition, the mesentery (dm and vm).
Thus Sagitta has throughout life a double body-cavity (Fig. 78 lk), and the
gut is fastened to the body-wall both above and below by a mesentery—
below by the ventral mesentery (vm), and above by the dorsal mesentery
(dm). The inner layer of the two cœlom-pouches (mv) attaches itself to the
entoderm (ik), and forms with it the visceral wall. The outer layer (mp)
attaches itself to the ectoderm (ak), and forms with it the outer body-wall.
Thus we have in Sagitta a perfectly clear and simple illustration of the
original cœlomation of the enterocœla. This palingenetic fact is the more
important, as the greater part of the two body-cavities in Sagitta changes
afterwards into sexual glands—the fore or female part into a pair of ovaries,
and the hind or male part into a pair of testicles.
Cœlomation takes place with equal clearness and transparency in the case
of the amphioxus, the lowest vertebrate, and its nearest relatives, the
invertebrate tunicates, the sea-squirts. However, in these two stems, which
we class together as Chordonia, this important process is more complex, as
two other processes are associated with it—the development of the chorda
from the entoderm and the separation of the medullary plate or nervous
centre from the ectoderm. Here again the skulless amphioxus has preserved
to our own time by tenacious heredity the chief phenomena in their original
form, while it has been more or less modified by embryonic adaptation in
all the other vertebrates (with skulls). Hence we must once more thoroughly
understand the palingenetic embryonic features of the lancelet before we go
on to consider the cenogenetic forms of the craniota.

Figs. 81 and 82.—Transverse section of amphioxus embryo. Fig. 81 at
the stage with five somites, Fig. 82 at the stage with eleven somites. (From
Hatschek.) ak outer germinal layer, mp medullary plate, n nerve-tube, ik
inner germinal layer, dh visceral cavity, lh body-cavity, mk middle germinal
layer (mk1 parietal, mk2 visceral), us primitive segment, ch chorda.

The cœlomation of the amphioxus, which was first observed by
Kowalevsky in 1867, has been very carefully studied since by Hatschek
(1881). According to him, there are first formed on the bilateral gastrula we
have already considered (Figs. 36, 37) three parallel longitudinal folds—
one single ectodermal fold in the central line of the dorsal surface, and a
pair of entodermic folds at the two sides of the former. The broad
ectodermal fold that first appears in the middle line of the flattened dorsal
surface, and forms a shallow longitudinal groove, is the beginning of the
central nervous system, the medullary tube. Thus the primary outer
germinal layer divides into two parts, the middle medullary plate (Fig. 81
mp) and the horny-plate (ak), the beginning of the outer skin or epidermis.
As the parallel borders of the concave medullary plate fold towards each
other and grow underneath the horny-plate, a cylindrical tube is formed, the
medullary tube (Fig. 82 n); this quickly detaches itself altogether from the
horny-plate. At each side of the medullary tube, between it and the
alimentary tube (Figs. 79–82 dh), the two parallel longitudinal folds grow
out of the dorsal wall of the alimentary tube, and these form the two cœlom-
pouches (Figs. 80, 81 lh). This part of the entoderm, which thus represents
the first structure of the middle germinal layer, is shown darker than the rest
of the inner germinal layer in Figs. 79–82. The edges of the folds meet, and
thus form closed tubes (Fig. 81 in section).
During this interesting process the outline of a third very important
organ, the chorda or axial rod, is being formed between the two cœlom-

pouches. This first foundation of the skeleton, a solid cylindrical
cartilaginous rod, is formed in the middle line of the dorsal primitive gut-
wall, from the entodermal cell-streak that remains here between the two
cœlom-pouches (Figs. 79–82 ch). The chorda appears at first in the shape of
a flat longitudinal fold or a shallow groove (Figs. 80, 81); it does not
become a solid cylindrical cord until after separation from the primitive gut
(Fig. 82). Hence we might say that the dorsal wall of the primitive gut
forms three parallel longitudinal folds at this important period—one single
fold and a pair of folds. The single middle fold becomes the chorda, and lies
immediately below the groove of the ectoderm, which becomes the
medullary tube; the pair of folds to the right and left lie at the sides between
the former and the latter, and form the cœlom-pouches. The part of the
primitive gut that remains after the cutting off of these three dorsal
primitive organs is the permanent gut; its entoderm is the gut-gland-layer or
enteric layer.

Figs. 83 and 84—Chordula of the amphioxus. Fig. 83 median
longitudinal section (seen from the left). Fig. 84 transverse section. (From
Hatschek.) In Fig. 83 the cœlom-pouches are omitted, in order to show the
chordula more clearly. Fig. 84 is rather diagrammatic. h horny-plate, m
medullary tube, n wall of same (n′ dorsal, n″ ventral), ch chorda, np
neuroporus, ne canalis neurentericus, d gut-cavity, r gut dorsal wall, b gut
ventral wall, z yelk-cells in the latter, u primitive mouth, o mouth-pit, p
promesoblasts (primitive or polar cells of the mesoderm), w parietal layer, v
visceral layer of the mesoderm, c cœlom, f rest of the segmentation-cavity.

Figs. 85 and 86—Chordula of the amphibia (the ringed adder). (From
Goette.) Fig. 85 median longitudinal section (seen from the left), Fig. 86
transverse section (slightly diagrammatic). Lettering as in Figs. 83 and 84.

I give the name of chordula or chorda-larva to the embryonic stage of
the vertebrate organism which is represented by the amphioxus larva at this
period (Figs. 83, 84, in the third period of development according to
Hatschek). (Strabo and Plinius give the name of cordula or cordyla to
young fish larvæ.) I ascribe the utmost phylogenetic significance to it, as it
is found in all the chorda-animals (tunicates as well as vertebrates) in
essentially the same form. Although the accumulation of food-yelk greatly
modifies the form of the chordula in the higher vertebrates, it remains the
same in its main features throughout. In all cases the nerve-tube (m) lies on
the dorsal side of the bilateral, worm-like body, the gut-tube (d) on the
ventral side, the chorda (ch) between the two, on the long axis, and the
cœlom pouches (c) at each side. In every case these primitive organs
develop in the same way from the germinal layers, and the same organs
always arise from them in the mature chorda-animal. Hence we may
conclude, according to the laws of the theory of descent, that all these
chordonia or chordata (tunicates and vertebrates) descend from an ancient
common ancestral form, which we may call Chordæa. We should regard
this long-extinct Chordæa, if it were still in existence, as a special class of
unarticulated worm (chordaria). It is especially noteworthy that neither the
dorsal nerve-tube nor the ventral gut-tube, nor even the chorda that lies
between them, shows any trace of articulation or segmentation; even the
two cœlom-sacs are not segmented at first (though in the amphioxus they
quickly divide into a series of parts by transverse folding). These

ontogenetic facts are of the greatest importance for the purpose of learning
those ancestral forms of the vertebrates which we have to seek in the group
of the unarticulated vermalia. The cœlom-pouches were originally sexual
glands in these ancient chordonia.

Figs. 87 and 88—Diagrammatic vertical section of cœlomula-embryos
of vertebrates. (From Hertwig.) Fig. 87, vertical section through the
primitive mouth, Fig. 88, vertical section before the primitive mouth. u
primitive mouth, ud primitive gut. d yelk, dk yelk-nuclei, dh gut-cavity, lh
body-cavity, mp medullary plate, ch chorda plate, ak and ik outer and inner
germinal layers, pb parietal and vb visceral mesoblast.

Figs. 89 and 90—Transverse section of cœlomula embryos of triton.
(From Hertwig.) Fig. 89, section through the primitive mouth. Fig. 90,
section in front of the primitive mouth, u primitive mouth. dh gut-cavity, dz
yelk-cells, dp yelk-stopper, ak outer and ik inner germinal layer, pb parietal
and vb visceral middle layer, m medullary plate, ch chorda.

Fig. 91. A, B, C.—Vertical section of the dorsal part of three triton-
embryos. (From Hertwig.) In Fig. A the medullary swellings (the parallel
borders of the medullary plate) begin to rise; in Fig. B they grow towards
each other; in Fig. C they join and form the medullary tube. mp medullary
plate, mf medullary folds, n nerve-tube, ch chorda, lh body-cavity, mk1 and
mk2 parietal and visceral mesoblasts, uv primitive-segment cavities, ak
ectoderm, ik entoderm, dz yelk-cells, dh gut-cavity.

From the evolutionary point of view the cœlom-pouches are, in any case,
older than the chorda; since they also develop in the same way as in the
chordonia in a number of invertebrates which have no chorda (for instance,
Sagitta, Figs. 76–78). Moreover, in the amphioxus the first outline of the
chorda appears later than that of the cœlom-sacs. Hence we must, according
to the biogenetic law, postulate a special intermediate form between the
gastrula and the chordula, which we will call cœlomula, an unarticulated,
worm-like body with primitive gut, primitive mouth, and a double body-

cavity, but no chorda. This embryonic form, the bilateral cœlomula (Fig.
81), may in turn be regarded as the ontogenetic reproduction (maintained by
heredity) of an ancient ancestral form of the cœlomaria, the Cœlomæa (cf.
Chapter XX).
In Sagitta and other worm-like animals the two cœlom-pouches
(presumably gonads or sex-glands) are separated by a complete median
partition, the dorsal and ventral mesentery (Fig. 78 dm, vm); but in the
vertebrates only the upper part of this vertical partition is maintained, and
forms the dorsal mesentery. This mesentery afterwards takes the form of a
thin membrane, which fastens the visceral tube to the chorda (or the
vertebral column). At the under side of the visceral tube the cœlom-sacs
blend together, their inner or median walls breaking down and disappearing.
The body-cavity then forms a single simple hollow, in which the gut is quite
free, or only attached to the dorsal wall by means of the mesentery.
The development of the body-cavity and the formation of the chordula in
the higher vertebrates is, like that of the gastrula, chiefly modified by the
pressure of the food-yelk on the embryonic structures, which forces its
hinder part into a discoid expansion. These cenogenetic modifications seem
to be so great that until twenty years ago these important processes were
totally misunderstood. It was generally believed that the body-cavity in man
and the higher vertebrates was due to the division of a simple middle layer,
and that the latter arose by cleavage from one or both of the primary
germinal layers. The truth was brought to light at last by the comparative
embryological research of the Hertwigs. They showed in their Cœlom
Theory (1881) that all vertebrates are true enterocœla, and that in every case
a pair of cœlom-pouches are developed from the primitive gut by folding.
The cenogenetic chordula-forms of the craniotes must therefore be derived
from the palingenetic embryology of the amphioxus in the same way as I
had previously proved for their gastrula-forms.
The chief difference between the cœlomation of the acrania (amphioxus)
and the other vertebrates (with skulls—craniotes) is that the two cœlom-
folds of the primitive gut in the former are from the first hollow vesicles,
filled with fluid, but in the latter are empty pouches, the layers of which
(inner and outer) close with each other. In common parlance we still call a
pouch or pocket by that name, whether it is full or empty. It is different in
ontogeny; in some of our embryological literature ordinary logic does not

count for very much. In many of the manuals and large treatises on this
science it is proved that vesicles, pouches, or sacs deserve that name only
when they are inflated and filled with a clear fluid. When they are not so
filled (for instance, when the primitive gut of the gastrula is filled with yelk,
or when the walls of the empty cœlom-pouches are pressed together), these
vesicles must not be cavities any longer, but “solid structures.”
The accumulation of food-yelk in the ventral wall of the primitive gut
(Figs. 85, 86) is the simple cause that converts the sac-shaped cœlom-
pouches of the acrania into the leaf-shaped cœlom-streaks of the craniotes.
To convince ourselves of this we need only compare, with Hertwig, the
palingenetic cœlomula of the amphioxus (Figs. 80, 81) with the
corresponding cenogenetic form of the amphibia (Figs. 89–90), and
construct the simple diagram that connects the two (Figs. 87, 88). If we
imagine the ventral half of the primitive gut-wall in the amphioxus embryo
(Figs. 79–84) distended with food-yelk, the vesicular cœlom-pouches (lh)
must be pressed together by this, and forced to extend in the shape of a thin
double plate between the gut-wall and body-wall (Figs. 86, 87). This
expansion follows a downward and forward direction. They are not directly
connected with these two walls. The real unbroken connection between the
two middle layers and the primary germ-layers is found right at the back, in
the region of the primitive mouth (Fig. 87 u). At this important spot we
have the source of embryonic development (blastocrene), or “zone of
growth,” from which the cœlomation (and also the gastrulation) originally
proceeds.

Fig. 92—Transverse section of the chordula-embryo of a bird (from a
hen’s egg at the close of the first day of incubation). (From Kölliker.) h
horn-plate (ectoderm), m medullary plate, Rf dorsal folds of same, Pv
medullary furrow, ch chorda, uwp median (inner) part of the middle layer
(median wall of the cœlom-pouches), sp lateral (outer) part of same, or
lateral plates, uwh structure of the body-cavity, dd gut-gland-layer.

Hertwig even succeeded in showing, in the cœlomula-embryo of the
water salamander (Triton), between the first structures of the two middle
layers, the relic of the body-cavity, which is represented in the
diagrammatic transitional form (Figs. 87, 88). In sections both through the
primitive mouth itself (Fig. 89) and in front of it (Fig. 90) the two middle
layers (pb and vb) diverge from each other, and disclose the two body-
cavities as narrow clefts. At the primitive-mouth itself (Fig. 90 u) we can
penetrate into them from without. It is only here at the border of the
primitive mouth that we can show the direct transition of the two middle
layers into the two limiting layers or primary germinal layers.
The structure of the chorda also shows the same features in these
cœlomula-embryos of the amphibia (Fig. 91) as in the amphioxus (Figs. 79–
82). It arises from the entodermic cell-streak, which forms the middle
dorsal-line of the primitive gut, and occupies the space between the flat
cœlom-pouches (Fig. 91 A). While the nervous centre is formed here in the
middle line of the back and separated from the ectoderm as “medullary
tube,” there takes place at the same time, directly underneath, the severance
of the chorda from the entoderm (Fig. 91 A, B, C). Under the chorda is
formed (out of the ventral entodermic half of the gastrula) the permanent
gut or visceral cavity (enteron) (Fig. 91 B, dh). This is done by the
coalescence, under the chorda in the median line, of the two dorsal side-
borders of the gut-gland-layer (ik), which were previously separated by the
chorda-plate (Fig. 91 A, ch); these now alone form the clothing of the
visceral cavity (dh) (enteroderm, Fig. 91 C). All these important
modifications take place at first in the fore or head-part of the embryo, and
spread backwards from there; here at the hinder end, the region of the
primitive mouth, the important border of the mouth (or properistoma)
remains for a long time the source of development or the zone of fresh
construction, in the further building-up of the organism. One has only to
compare carefully the illustrations given (Figs. 85–91) to see that, as a fact,
the cenogenetic cœlomation of the amphibia can be deduced directly from
the palingenetic form of the acrania (Figs. 79–84).

Fig. 93—Transverse section of the vertebrate-embryo of a bird (from
a hen’s egg on the second day of incubation). (From Kölliker.) h horn-plate,
mr medullary tube, ch chorda, uw primitive segments, uwh primitive-
segment cavity (median relic of the cœlom), sp lateral cœlom-cleft, hpl
skin-fibre-layer, df gut-fibre-layer, ung primitive-kidney passage, ao
primitive aorta, dd gut-gland-layer.

The same principle holds good for the amniotes, the reptiles, birds, and
mammals, although in this case the processes of cœlomation are more
modified and more difficult to identify on account of the colossal
accumulation of food-yelk and the corresponding notable flattening of the
germinal disk. However, as the whole group of the amniotes has been
developed at a comparatively late date from the class of the amphibia, their
cœlomation must also be directly traceable to that of the latter. This is really
possible as a matter of fact; even the older illustrations showed an essential
identity of features. Thus forty years ago Kölliker gave, in the first edition
of his Human Embryology (1861), some sections of the chicken-embryo,
the features of which could at once be reduced to those already described
and explained in the sense of Hertwig’s cœlom-theory. A section through
the embryo in the hatched hen’s egg towards the close of the first day of
incubation shows in the middle of the dorsal surface a broad ectodermic
medullary groove (Fig. 92 Rf), and underneath the middle of the chorda (ch)
and at each side of it a couple of broad mesodermic layers (sp). These
enclose a narrow space or cleft (uwh), which is nothing else than the
structure of the body-cavity. The two layers that enclose it—the upper
parietal layer (hpl) and the lower visceral layer (df)—are pressed together
from without, but clearly distinguishable. This is even clearer a little later,
when the medullary furrow is closed into the nerve-tube (Fig. 93 mr).
Special importance attaches to the fact that here again the four secondary
germinal layers are already sharply distinct, and easily separated from each
other. There is only one very restricted area in which they are connected,
and actually pass into each other; this is the region of the primitive mouth,
which is contracted in the amniotes into a dorsal longitudinal cleft, the
primitive groove. Its two lateral lip-borders form the primitive streak, which
has long been recognised as the most important embryonic source and
starting-point of further processes. Sections through this primitive streak
(Figs. 94 and 95) show that the two primary germinal layers grow at an

early stage (in the discoid gastrula of the chick, a few hours after
incubation) into the primitive streak (x), and that the two middle layers
extend outward from this thickened axial plate (y) to the right and left
between the former. The plates of the cœlom-layers, the parietal skin-fibre-
layer (m) and the visceral gut-fibre-layer (f), are seen to be still pressed
close together, and only diverge later to form the body-cavity. Between the
inner borders of the two flat cœlom-pouches lies the chorda (Fig. 95 x),
which here again develops from the middle line of the dorsal wall of the
primitive gut.

Figs. 94 and 95—Transverse section of the primitive-streak (primitive
mouth) of the chick. Fig. 94 a few hours after the commencement of
incubation, Fig. 95 a little later. (From Waldeyer.) h horn-plate, n nerve-
plate, m skin-fibre-layer, f gut-fibre-layer, d gut-gland-layer, y primitive
streak or axial plate, in which all four germinal layers meet, x structure of
the chorda, u region of the later primitive kidneys.

Cœlomation takes place in the vertebrates in just the same way as in the
birds and reptiles. This was to be expected, as the characteristic gastrulation
of the mammal has descended from that of the reptiles. In both cases a
discoid gastrula with primitive streak arises from the segmented ovum, a
two-layered germinal disk with long and small hinder primitive mouth.
Here again the two primary germinal layers are only directly connected
(Fig. 96 pr) along the primitive streak (at the folding-point of the blastula),
and from this spot (the border of the primitive mouth) the middle germinal
layers (mk) grow out to right and left between the preceding. In the fine
illustration of the cœlomula of the rabbit which Van Beneden has given us

(Fig. 96) one can clearly see that each of the four secondary germinal layers
consists of a single stratum of cells.
Finally, we must point out, as a fact of the utmost importance for our
anthropogeny and of great general interest, that the four-layered cœlomula
of man has just the same construction as that of the rabbit (Fig. 96). A
vertical section that Count Spee made through the primitive mouth or streak
of a very young human germinal disk (Fig. 97) clearly shows that here
again the four secondary germ-layers are inseparably connected only at the
primitive streak, and that here also the two flattened cœlom-pouches (mk)
extend outwards to right and left from the primitive mouth between the
outer and inner germinal layers. In this case, too, the middle germinal layer
consists from the first of two separate strata of cells, the parietal (mp) and
visceral (mv) mesoblasts.
These concordant results of the best recent investigations (which have
been confirmed by the observations of a number of scientists I have not
enumerated) prove the unity of the vertebrate-stem in point of cœlomation,
no less than of gastrulation. In both respects the invaluable amphioxus—the
sole survivor of the acrania—is found to be the original model that has
preserved for us in palingenetic form by a tenacious heredity these most
important embryonic processes. From this primary model of construction
we can cenogenetically deduce all the embryonic forms of the other
vertebrates, the craniota, by secondary modifications. My thesis of the
universal formation of the gastrula by folding of the blastula has now been
clearly proved for all the vertebrates; so also has been Hertwig’s thesis of
the origin of the middle germinal layers by the folding of a couple of
cœlom-pouches which appear at the border of the primitive mouth. Just as
the gastræa-theory explains the origin and identity of the two primary
layers, so the cœlom-theory explains those of the four secondary layers. The
point of origin is always the properistoma, the border of the original
primitive mouth of the gastrula, at which the two primary layers pass
directly into each other.

Fig. 96—Transverse section of the primitive groove (or primitive
mouth) of a rabbit. (From Van Beneden.) pr primitive mouth, ul lips of
same (primitive lips), ak and ik outer and inner germinal layers, mk middle
germinal layer, mp parietal layer, mv visceral layer of the mesoderm.

Fig. 97—Transverse section of the primitive mouth (or groove) of a
human embryo (at the cœlomula stage). (From Count Spee.) pr primitive
mouth, ul lips of same (primitive folds), ak and ik outer and inner germinal
layers, mk middle layer, mp parietal layer, mv visceral layer of the
mesoblasts.

Moreover, the cœlomula is important as the immediate source of the
chordula, the embryonic reproduction of the ancient, typical, unarticulated,
worm-like form, which has an axial chorda between the dorsal nerve-tube
and the ventral gut-tube. This instructive chordula (Figs. 83–86) provides a
valuable support of our phylogeny; it indicates the important moment in our
stem-history at which the stem of the chordonia (tunicates and vertebrates)
parted for ever from the divergent stems of the other metazoa (articulates,
echinoderms, and molluscs).
I may express here my opinion, in the form of a chordæa-theory, that the
characteristic chordula-larva of the chordonia has in reality this great
significance—it is the typical reproduction (preserved by heredity) of the

ancient common stem-form of all the vertebrates and tunicates, the long-
extinct Chordæa. We will return in Chapter XX to these worm-like
ancestors, which stand out as luminous points in the obscure stem-history of
the invertebrate ancestors of our race.

Chapter XI.
THE VERTEBRATE CHARACTER OF MAN

We have now secured a number of firm standing-places in the
labyrinthian course of our individual development by our study of the
important embryonic forms which we have called the cytula, morula,
blastula, gastrula, cœlomula, and chordula. But we have still in front of us
the difficult task of deriving the complicated frame of the human body, with
all its different parts, organs, members, etc., from the simple form of the
chordula. We have previously considered the origin of this four-layered
embryonic form from the two-layered gastrula. The two primary germinal
layers, which form the entire body of the gastrula, and the two middle
layers of the cœlomula that develop between them, are the four simple cell-
strata, or epithelia, which alone go to the formation of the complex body of
man and the higher animals. It is so difficult to understand this construction
that we will first seek a companion who may help us out of many
difficulties.
This helpful associate is the science of comparative anatomy. Its task is,
by comparing the fully-developed bodily forms in the various groups of
animals, to learn the general laws of organisation according to which the
body is constructed; at the same time, it has to determine the affinities of
the various groups by critical appreciation of the degrees of difference
between them. Formerly, this work was conceived in a teleological sense,
and it was sought to find traces of the plan of the Creator in the actual
purposive organisation of animals. But comparative anatomy has gone
much deeper since the establishment of the theory of descent; its
philosophic aim now is to explain the variety of organic forms by
adaptation, and their similarity by heredity. At the same time, it has to
recognise in the shades of difference in form the degree of blood-
relationship, and make an effort to construct the ancestral tree of the animal

world. In this way, comparative anatomy enters into the closest relations
with comparative embryology on the one hand, and with the science of
classification on the other.
Now, when we ask what position man occupies among the other
organisms according to the latest teaching of comparative anatomy and
classification, and how man’s place in the zoological system is determined
by comparison of the mature bodily forms, we get a very definite and
significant reply; and this reply gives us extremely important conclusions
that enable us to understand the embryonic development and its
evolutionary purport. Since Cuvier and Baer, since the immense progress
that was effected in the early decades of the nineteenth century by these two
great zoologists, the opinion has generally prevailed that the whole animal
kingdom may be distributed in a small number of great divisions or types.
They are called types because a certain typical or characteristic structure is
constantly preserved within each of these large sections. Since we applied
the theory of descent to this doctrine of types, we have learned that this
common type is an outcome of heredity; all the animals of one type are
blood-relatives, or members of one stem, and can be traced to a common
ancestral form. Cuvier and Baer set up four of these types: the vertebrates,
articulates, molluscs, and radiates. The first three of these are still retained,
and may be conceived as natural phylogenetic unities, as stems or phyla in
the sense of the theory of descent. It is quite otherwise with the fourth type
—the radiata. These animals, little known as yet at the beginning of the
nineteenth century, were made to form a sort of lumber-room, into which
were cast all the lower animals that did not belong to the other three types.
As we obtained a closer acquaintance with them in the course of the last
sixty years, it was found that we must distinguish among them from four to
eight different types. In this way the total number of animal stems or phyla
has been raised to eight or twelve (cf. Chapter XX).
These twelve stems of the animal kingdom are, however, by no means
co-ordinate and independent types, but have definite relations, partly of
subordination, to each other, and a very different phylogenetic meaning.
Hence they must not be arranged simply in a row one after the other, as was
generally done until thirty years ago, and is still done in some manuals. We
must distribute them in three subordinate principal groups of very different
value, and arrange the various stems phylogenetically on the principles
which I laid down in my Monograph on the Sponges, and developed in the

Study of the Gastræa Theory. We have first to distinguish the unicellular
animals (protozoa) from the multicellular tissue-forming (metazoa). Only
the latter exhibit the important processes of segmentation and gastrulation;
and they alone have a primitive gut, and form germinal layers and tissues.
The metazoa, the tissue-animals or gut-animals, then sub-divide into two
main sections, according as a body-cavity is or is not developed between
the primary germinal layers. We may call these the cœlenteria and
cœlomaria, the former are often also called zoophytes or cœlenterata, and
the latter bilaterals. This division is the more important as the cœlenteria
(without cœlom) have no blood and blood-vessels, nor an anus. The
cœlomaria (with body-cavity) have generally an anus, and blood and blood-
vessels. There are four stems belonging to the cœlenteria: the gastræads
(“primitive-gut animals”), sponges, cnidaria, and platodes. Of the
cœlomaria we can distinguish six stems: the vermalia at the bottom
represent the common stem-group (derived from the platodes) of these, the
other five typical stems of the cœlomaria—the molluscs, echinoderms,
articulates, tunicates, and vertebrates—being evolved from them.
Man is, in his whole structure, a true vertebrate, and develops from an
impregnated ovum in just the same characteristic way as the other
vertebrates. There can no longer be the slightest doubt about this
fundamental fact, nor of the fact that all the vertebrates form a natural
phylogenetic unity, a single stem. The whole of the members of this stem,
from the amphioxus and the cyclostoma to the apes and man, have the same
characteristic disposition, connection, and development of the central
organs, and arise in the same way from the common embryonic form of the
chordula. Without going into the difficult question of the origin of this stem,
we must emphasise the fact that the vertebrate stem has no direct affinity
whatever to five of the other ten stems; these five isolated phyla are the
sponges, cnidaria, molluscs, articulates, and echinoderms. On the other
hand, there are important and, to an extent, close phylogenetic relations to
the other five stems—the protozoa (through the amœbæ), the gastræads
(through the blastula and gastrula), the platodes and vermalia (through the
cœlomula), and the tunicates (through the chordula).
How we are to explain these phylogenetic relations in the present state of
our knowledge, and what place is assigned to the vertebrates in the animal
ancestral tree, will be considered later (Chapter XX). For the present our

task is to make plainer the vertebrate character of man, and especially to
point out the chief peculiarities of organisation by which the vertebrate stem
is profoundly separated from the other eleven stems of the animal kingdom.
Only after these comparative-anatomical considerations shall we be in a
position to attack the difficult question of our embryology. The
development of even the simplest and lowest vertebrate from the simple
chordula (Figs. 83–86) is so complicated and difficult to follow that it is
necessary to understand the organic features of the fully-formed vertebrate
in order to grasp the course of its embryonic evolution. But it is equally
necessary to confine our attention, in this general anatomic description of
the vertebrate-body, to the essential facts, and pass by all the unessential.
Hence, in giving now an ideal anatomic description of the chief features of
the vertebrate and its internal organisation, I omit all the subordinate points,
and restrict myself to the most important characteristics.
Much, of course, will seem to the reader to be essential that is only of
subordinate and secondary interest, or even not essential at all, in the light
of comparative anatomy and embryology. For instance, the skull and
vertebral column and the extremities are non-essential in this sense. It is
true that these parts are very important physiologically; but for the
morphological conception of the vertebrate they are not essential, because
they are only found in the higher, not the lower, vertebrates. The lowest
vertebrates have neither skull nor vertebræ, and no extremities or limbs.
Even the human embryo passes through a stage in which it has no skull or
vertebræ; the trunk is quite simple, and there is yet no trace of arms and
legs. At this stage of development man, like every other higher vertebrate, is
essentially similar to the simplest vertebrate form, which we now find in
only one living specimen. This one lowest vertebrate that merits the closest
study—undoubtedly the most interesting of all the vertebrates after man—is
the famous lancelet or amphioxus, to which we have already often referred.
As we are going to study it more closely later on (Chapters XVI and XVII),
I will only make one or two passing observations on it here.
The amphioxus lives buried in the sand of the sea, is about one or two
inches in length, and has, when fully developed, the shape of a very simple,
longish, lancet-like leaf; hence its name of the lancelet. The narrow body is
compressed on both sides, almost equally pointed at the fore and hind ends,
without any trace of external appendages or articulation of the body into
head, neck, breast, abdomen, etc. Its whole shape is so simple that its first

discoverer thought it was a naked snail. It was not until much later—half a
century ago—that the tiny creature was studied more carefully, and was
found to be a true vertebrate. More recent investigations have shown that it
is of the greatest importance in connection with the comparative anatomy
and ontogeny of the vertebrates, and therefore with human phylogeny. The
amphioxus reveals the great secret of the origin of the vertebrates from the
invertebrate vermalia, and in its development and structure connects
directly with certain lower tunicates, the ascidia.
When we make a number of sections of the body of the amphioxus,
firstly vertical longitudinal sections through the whole body from end to
end, and secondly transverse sections from right to left, we get anatomic
pictures of the utmost instructiveness (cf. Figs. 98–102). In the main they
correspond to the ideal which we form, with the aid of comparative
anatomy and ontogeny, of the primitive type or build of the vertebrate—the
long-extinct form to which the whole stem owes its origin. As we take the
phylogenetic unity of the vertebrate stem to be beyond dispute, and assume
a common origin from a primitive stem-form for all the vertebrates, from
amphioxus to man, we are justified in forming a definite morphological
idea of this primitive vertebrate (Prospondylus or Vertebræa). We need only
imagine a few slight and unessential changes in the real sections of the
amphioxus in order to have this ideal anatomic figure or diagram of the
primitive vertebrate form, as we see in Figs. 98–102. The amphioxus
departs so little from this primitive form that we may, in a certain sense,
describe it as a modified “primitive vertebrate.”[24]
[24] The ideal figure of the vertebrate as given in Figs. 98–102 is a hypothetical
scheme or diagram, that has been chiefly constructed on the lines of the
amphioxus, but with a certain attention to the comparative anatomy and
ontogeny of the ascidia and appendicularia on the one hand, and of the
cyclostoma and selachii on the other. This diagram has no pretension whatever to
be an “exact picture,” but merely an attempt to reconstruct hypothetically the
unknown and long extinct vertebrate stem-form, an ideal “archetype.”

The outer form of our hypothetical primitive vertebrate was at all events
very simple, and probably more or less similar to that of the lancelet. The
bilateral or bilateral-symmetrical body is stretched out lengthways and
compressed at the sides (Figs. 98–100), oval in section (Figs. 101, 102).
There are no external articulation and no external appendages, in the shape
of limbs, legs, or fins. On the other hand, the division of the body into two

sections, head and trunk, was probably clearer in Prospondylus than it is in
its little-changed ancestor, the amphioxus. In both animals the fore or head-
half of the body contains different organs from the trunk, and different on
the dorsal from on the ventral side. As this important division is found even
in the sea-squirt, the remarkable invertebrate stem-relative of the
vertebrates, we may assume that it was also found in the prochordonia, the
common ancestors of both stems. It is also very pronounced in the young
larvæ of the cyclostoma; this fact is particularly interesting, as this
palingenetic larva-form is in other respects also an important connecting-
link between the higher vertebrates and the acrania.
The head of the acrania, or the anterior half of the body (both of the real
amphioxus and the ideal prospondylus), contains the branchial (gill) gut and
heart in the ventral section and the brain and sense-organs in the dorsal
section. The trunk, or posterior half of the body, contains the hepatic (liver)
gut and sexual-glands in the ventral part, and the spinal marrow and most of
the muscles in the dorsal part.

Figs. 98–102.—The ideal primitive vertebrate (prospondylus).
Diagram. Fig. 98 side-view (from the left). Fig. 99 back-view. Fig. 100
front view. Fig. 101 transverse section through the head (to the left through
the gill-pouches, to the right through the gill-clefts). Fig. 102 transverse
section of the trunk (to the right a pro-renal canal is affected). a aorta, af
anus, au eye, b lateral furrow (primitive renal process), c cœloma (body-
cavity), d small intestine, e parietal eye (epiphysis), f fin border of the skin,
g auditory vesicle, gh brain, h heart, i muscular cavity (dorsal cœlom-
pouch), k gill-gut, ka gill-artery, kg gill-arch, ks gill-folds, l liver, ma
stomach, md mouth, ms muscles, na nose (smell pit), n renal canals, u

apertures of same, o outer skin, p gullet, r spinal marrow, a sexual glands
(gonads), t corium, u kidney-openings (pores of the lateral furrow), v
visceral vein (chief vein). x chorda, y hypophysis (urinary appendage), z
gullet-groove or gill-groove (hypobranchial groove).

In the longitudinal section of the ideal vertebrate (Fig. 98) we have in the
middle of the body a thin and flexible, but stiff, cylindrical rod, pointed at
both ends (ch). It goes the whole length through the middle of the body, and
forms, as the central skeletal axis, the original structure of the later vertebral
column. This is the axial rod, or chorda dorsalis, also called chorda
vertebralis, vertebral cord, axial cord, dorsal cord, notochorda, or, briefly,
chorda. This solid, but flexible and elastic, axial rod consists of a
cartilaginous mass of cells, and forms the inner axial skeleton or central
frame of the body; it is only found in vertebrates and tunicates, not in any
other animals. As the first structure of the spinal column it has the same
radical significance in all vertebrates, from the amphioxus to man. But it is
only in the amphioxus and the cyclostoma that the axial rod retains its
simplest form throughout life. In man and all the higher vertebrates it is
found only in the earlier embryonic period, and is afterwards replaced by
the articulated vertebral column.
The axial rod or chorda is the real solid chief axis of the vertebrate body,
and at the same time corresponds to the ideal long-axis, and serves to direct
us with some confidence in the orientation of the principal organs. We
therefore take the vertebrate-body in its original, natural disposition, in
which the long-axis lies horizontally, the dorsal side upward and the ventral
side downward (Fig. 98). When we make a vertical section through the
whole length of this long axis, the body divides into two equal and
symmetrical halves, right and left. In each half we have originally the same
organs in the same disposition and connection; only their disposal in
relation to the vertical plane of section, or median plane, is exactly
reversed: the left half is the reflection of the right. We call the two halves
antimera (opposed-parts). In the vertical plane of section that divides the
two halves the sagittal (“arrow”) axis, or “dorsoventral axis,” goes from the
back to the belly, corresponding to the sagittal seam of the skull. But when
we make a horizontal longitudinal section through the chorda, the whole
body divides into a dorsal and a ventral half. The line of section that passes
through the body from right to left is the transverse, frontal, or lateral axis.

The two halves of the vertebrate body that are separated by this
horizontal transverse axis and by the chorda have quite different characters.
The dorsal half is mainly the animal part of the body, and contains the
greater part of what are called the animal organs, the nervous system,
muscular system, osseous system, etc.—the instruments of movement and
sensation. The ventral half is essentially the vegetative half of the body, and
contains the greater part of the vertebrate’s vegetal organs, the visceral and
vascular systems, sexual system, etc.—the instruments of nutrition and
reproduction. Hence in the construction of the dorsal half it is chiefly the
outer, and in the construction of the ventral half chiefly the inner, germinal
layer that is engaged. Each of the two halves develops in the shape of a
tube, and encloses a cavity in which another tube is found. The dorsal half
contains the narrow spinal-column cavity or vertebral canal above the
chorda, in which lies the tube-shaped central nervous system, the medullary
tube. The ventral half contains the much more spacious visceral cavity or
body-cavity underneath the chorda, in which we find the alimentary canal
and all its appendages.
The medullary tube, as the central nervous system or psychic organ of the
vertebrate is called in its first stage, consists, in man and all the higher
vertebrates, of two different parts: the large brain, contained in the skull,
and the long spinal cord which stretches from there over the whole dorsal
part of the trunk. Even in the primitive vertebrate this composition is plainly
indicated. The fore half of the body, which corresponds to the head,
encloses a knob-shaped vesicle, the brain (gh); this is prolonged backwards
into the thin cylindrical tube of the spinal marrow (r). Hence we find here
this very important psychic organ, which accomplishes sensation, will, and
thought, in the vertebrates, in its simplest form. The thick wall of the nerve-
tube, which runs through the long axis of the body immediately over the
axial rod, encloses a narrow central canal filled with fluid (Figs. 98–102 r).
We still find the medullary tube in this very simple form for a time in the
embryo of all the vertebrates, and it retains this form in the amphioxus
throughout life; only in the latter case the cylindrical medullary tube barely
indicates the separation of brain and spinal cord. The lancelet’s medullary
tube runs nearly the whole length of the body, above the chorda, in the
shape of a long thin tube of almost equal diameter throughout, and there is
only a slight swelling of it right at the front to represent the rudiment of a
cerebral lobe. It is probable that this peculiarity of the amphioxus is

connected with the partial atrophy of its head, as the ascidian larvæ on the
one hand and the young cyclostoma on the other clearly show a division of
the vesicular brain, or head marrow, from the thinner, tubular spinal
marrow.
Probably we must trace to the same phylogenetic cause the defective
nature of the sense organs of the amphioxus, which we will describe later
(Chapter XVI). Prospondylus, on the other hand, probably had three pairs
of sense-organs, though of a simple character, a pair of, or a single olfactory
depression, right in front (Figs. 98, 99, na), a pair of eyes (au) in the lateral
walls of the brain, and a pair of simple auscultory vesicles (g) behind. There
was also, perhaps, a single parietal or “pineal” eye at the top of the skull
(epiphysis, e).
In the vertical median plane (or middle plane, dividing the bilateral body
into right and left halves) we have in the acrania, underneath the chorda, the
mesentery and visceral tube, and above it the medullary tube; and above the
latter a membranous partition of the two halves of the body. With this
partition is connected the mass of connective tissue which acts as a sheath
both for the medullary tube and the underlying chorda, and is, therefore,
called the chord-sheath (perichorda); it originates from the dorsal and
median part of the cœlom-pouches, which we shall call the skeleton plate or
“sclerotom” in the craniote embryo. In the latter the chief part of the
skeleton—the vertebral column and skull—develops from this chord-
sheath; in the acrania it retains its simple form as a soft connective matter,
from which are formed the membranous partitions between the various
muscular plates or myotomes (Figs. 98, 99 ms).
To the right and left of the cord-sheath, at each side of the medullary tube
and the underlying axial rod, we find in all the vertebrates the large masses
of muscle that constitute the musculature of the trunk and effect its
movements. Although these are very elaborately differentiated and
connected in the developed vertebrate (corresponding to the various parts of
the bony skeleton), in our ideal primitive vertebrate we can distinguish only
two pairs of these principal muscles, which run the whole length of the
body parallel to the chorda. These are the upper (dorsal) and lower (ventral)
lateral muscles of the trunk. The upper (dorsal) muscles, or the original
dorsal muscles (Fig. 102 ms), form the thick mass of flesh on the back. The
lower (ventral) muscles, or the original muscles of the belly, form the fleshy

wall of the abdomen. Both sets are segmented, and consist of a double row
of muscular plates (Figs. 98, 99 ms); the number of these myotomes
determines the number of joints in the trunk, or metamera. The myotomes
are also developed from the thick wall of the cœlom-pouches (Fig. 102 i).
Outside this muscular tube we have the external envelope of the
vertebrate body, which is known as the corium or cutis. This strong and
thick envelope consists, in its deeper strata, chiefly of fat and loose
connective tissue, and in its upper layers of cutaneous muscles and firmer
connective tissue. It covers the whole surface of the fleshy body, and is of
considerable thickness in all the craniota. But in the acrania the corium is
merely a thin plate of connective tissue, an insignificant “corium-plate”
(lamella corii, Figs. 98–102 t).
Immediately above the corium is the outer skin (epidermis, o), the
general covering of the whole outer surface. In the higher vertebrates the
hairs, nails, feathers, claws, scales, etc., grow out of this epidermis. It
consists, with all its appendages and products, of simple cells, and has no
blood-vessels. Its cells are connected with the terminations of the sensory
nerves. Originally, the outer skin is a perfectly simple covering of the outer
surface of the body, composed only of homogeneous cells—a permanent
horn-plate. In this simplest form, as a one-layered epithelium, we find it, at
first, in all the vertebrates, and throughout life in the acrania. It afterwards
grows thicker in the higher vertebrates, and divides into two strata—an
outer, firmer corneous (horn) layer and an inner, softer mucus-layer; also a
number of external and internal appendages grow out of it: outwardly, the
hairs, nails, claws, etc., and inwardly, the sweat-glands, fat-glands, etc.
It is probable that in our primitive vertebrate the skin was raised in the
middle line of the body in the shape of a vertical fin border (f). A similar
fringe, going round the greater part of the body, is found to-day in the
amphioxus and the cyclostoma; we also find one in the tail of fish-larvæ
and tadpoles.
Now that we have considered the external parts of the vertebrate and the
animal organs, which mainly lie in the dorsal half, above the chorda, we
turn to the vegetal organs, which lie for the most part in the ventral half,
below the axial rod. Here we find a large body-cavity or visceral cavity in
all the craniota. The spacious cavity that encloses the greater part of the
viscera corresponds to only a part of the original cœloma, which we

considered in Chapter X; hence it nay be called the metacœloma. As a rule,
it is still briefly called the cœloma; formerly it was known in anatomy as
the pleuroperitoneal cavity. In man and the other mammals (but only in
these) this cœloma divides, when fully developed, into two different
cavities, which are separated by a transverse partition—the muscular
diaphragm. The fore or pectoral cavity (pleura-cavity) contains the
œsophagus (gullet), heart, and lungs; the hind or peritoneal or abdominal
cavity contains the stomach, small and large intestines, liver, pancreas,
kidneys, etc. But in the vertebrate embryo, before the diaphragm is
developed, the two cavities form a single continuous body-cavity, and we
find it thus in all the lower vertebrates throughout life. This body-cavity is
clothed with a delicate layer of cells, the cœlom-epithelium. In the acrania
the cœlom is segmented both dorsally and ventrally, as their muscular
pouches and primitive genital organs plainly show (Fig. 102).
The chief of the viscera in the body-cavity is the alimentary canal, the
organ that represents the whole body in the gastrula. In all the vertebrates it
is a long tube, enclosed in the body-cavity and more or less differentiated in
length, and has two apertures—a mouth for taking in food (Figs. 98, 100
md) and an anus for the ejection of unusable matter or excrements (af).
With the alimentary canal a number of glands are connected which are of
great importance for the vertebrate body, and which all grow out of the
canal. Glands of this kind are the salivary glands, the lungs, the liver, and
many smaller glands. Nearly all these glands are wanting in the acrania;
probably there were merely a couple of simple hepatic tubes (Figs. 98, 100
l) in the vertebrate stem-form. The wall of the alimentary canal and all its
appendages consists of two different layers; the inner, cellular clothing is
the gut-gland-layer, and the outer, fibrous envelope consists of the gut-fibre-
layer; it is mainly composed of muscular fibres which accomplish the
digestive movements of the canal, and of connective-tissue fibres that form
a firm envelope. We have a continuation of it in the mesentery, a thin,
bandage-like layer, by means of which the alimentary canal is fastened to
the ventral side of the chorda, originally the dorsal partition of the two
cœlom-pouches. The alimentary canal is variously modified in the
vertebrates both as a whole and in its several sections, though the original
structure is always the same, and is very simple. As a rule, it is longer (often
several times longer) than the body, and therefore folded and winding
within the body-cavity, especially at the lower end. In man and the higher

vertebrates it is divided into several sections, often separated by valves—
the mouth, pharynx, œsophagus, stomach, small and large intestine, and
rectum. All these parts develop from a very simple structure, which
originally (throughout life in the amphioxus) runs from end to end under the
chorda in the shape of a straight cylindrical canal.
As the alimentary canal may be regarded morphologically as the oldest
and most important organ in the body, it is interesting to understand its
essential features in the vertebrate more fully, and distinguish them from
unessential features. In this connection we must particularly note that the
alimentary canal of every vertebrate shows a very characteristic division
into two sections—a fore and a hind chamber. The fore chamber is the
head-gut or branchial gut (Figs. 98–100 p, k), and is chiefly occupied with
respiration. The hind section is the trunk-gut or hepatic gut, which
accomplishes digestion (ma, d). In all vertebrates there are formed, at an
early stage, to the right and left in the fore-part of the head-gut, certain
special clefts that have an intimate connection with the original respiratory
apparatus of the vertebrate—the branchial (gill) clefts (ks). All the lower
vertebrates, the lancelets, lampreys, and fishes, are constantly taking in
water at the mouth, and letting it out again by the lateral clefts of the gullet.
This water serves for breathing. The oxygen contained in it is inspired by
the blood-canals, which spread out on the parts between the gill-clefts, the
gill-arches (kg). These very characteristic branchial clefts and arches are
found in the embryo of man and all the higher vertebrates at an early stage
of development, just as we find them throughout life in the lower
vertebrates. However, these clefts and arches never act as respiratory organs
in the mammals, birds, and reptiles, but gradually develop into quite
different parts. Still, the fact that they are found at first in the same form as
in the fishes is one of the most interesting proofs of the descent of these
three higher classes from the fishes.
Not less interesting and important is an organ that develops from the
ventral wall in all vertebrates—the gill-groove or hypobranchial groove. In
the acrania and the ascidiæ it consists throughout life of a glandular ciliated
groove, which runs down from the mouth in the ventral middle line of the
gill-gut, and takes small particles of food to the stomach (Fig. 101 z). But in
the craniota the thyroid gland (thyreoidea) is developed from it, the gland
that lies in front of the larynx, and which, when pathologically enlarged,
forms goitre (struma).

From the head-gut we get not only the gills, the organs of water-
breathing in the lower vertebrates, but also the lungs, the organs of
atmospheric breathing in the five higher classes. In these cases a vesicular
fold appears in the gullet of the embryo at an early stage, and gradually
takes the shape of two spacious sacs, which are afterwards filled with air.
These sacs are the two air-breathing lungs, which take the place of the
water-breathing gills. But the vesicular invagination, from which the lungs
arise, is merely the familiar air-filled vesicle, which we call the floating-
bladder of the fish, and which alters its specific weight, acting as
hydrostatic organ or floating apparatus. This structure is not found in the
lowest vertebrate classes—the acrania and cyclostoma. We shall see more
of it in Volume II.
The second chief section of the vertebrate-gut, the trunk or liver-gut,
which accomplishes digestion, is of very simple construction in the acrania.
It consists of two different chambers. The first chamber, immediately
behind the gill-gut, is the expanded stomach (ma); the second, narrower and
longer chamber, is the straight small intestine (d): it issues behind on the
ventral side by the anus (af). Near the limit of the two chambers in the
visceral cavity we find the liver, in the shape of a simple tube or blind sac
(l); in the amphioxus it is single; in the prospondylus it was probably double
(Figs. 98, 100 l).
Closely related morphologically and physiologically to the alimentary
canal is the vascular system of the vertebrate, the chief sections of which
develop from the fibrous gut-layer. It consists of two different but directly
connected parts, the system of blood-vessels and that of lymph-vessels. In
the passages of the one we find red blood, and in the other colourless
lymph. To the lymphatic system belong, first of all, the lymphatic canals
proper or absorbent veins, which are distributed among all the organs, and
absorb the used-up juices from the tissues, and conduct them into the
venous blood; but besides these there are the chyle-vessels, which absorb
the white chyle, the milky fluid prepared by the alimentary canal from the
food, and conduct this also to the blood.
The blood-vessel system of the vertebrate has a very elaborate
construction, but seems to have had a very simple form in the primitive
vertebrate, as we find it to-day permanently in the annelids (for instance,
earth-worms) and the amphioxus. We accordingly distinguish first of all as

essential, original parts of it two large single blood-canals, which lie in the
fibrous wall of the gut, and run along the alimentary canal in the median
plane of the body, one above and the other underneath the canal. These
principal canals give out numerous branches to all parts of the body, and
pass into each other by arches before and behind; we will call them the
primitive artery and the primitive vein. The first corresponds to the dorsal
vessel, the second to the ventral vessel, of the worms. The primitive or
principal artery, usually called the aorta (Fig. 98 a), lies above the gut in the
middle line of its dorsal side, and conducts oxidised or arterial blood from
the gills to the body. The primitive or principal vein (Fig. 100 v) lies below
the gut, in the middle line of its ventral side, and is therefore also called the
vena subintestinalis; it conducts carbonised or venous blood back from the
body to the gills. At the branchial section of the gut in front the two canals
are connected by a number of branches, which rise in arches between the
gill-clefts. These “branchial vascular arches” (kg) run along the gill-arches,
and have a direct share in the work of respiration. The anterior continuation
of the principal vein which runs on the ventral wall of the gill-gut, and gives
off these vascular arches upwards, is the branchial artery (ka). At the border
of the two sections of the ventral vessel it enlarges into a contractile
spindle-shaped tube (Figs. 98, 100 h). This is the first outline of the heart,
which afterwards becomes a four-chambered pump in the higher vertebrates
and man. There is no heart in the amphioxus, probably owing to
degeneration. In prospondylus the ventral gill-heart probably had the simple
form in which we still find it in the ascidia and the embryos of the craniota
(Figs. 98, 100 h).
The kidneys, which act as organs of excretion or urinary organs in all
vertebrates, have a very different and elaborate construction in the various
sections of this stem; we will consider them further in Chapter 2.29. Here I
need only mention that in our hypothetical primitive vertebrate they
probably had the same form as in the actual amphioxus—the primitive
kidneys (protonephra). These are originally made up of a double row of
little canals, which directly convey the used-up juices or the urine out of the
body-cavity (Fig. 102 n). The inner aperture of these pronephridial canals
opens with a ciliated funnel into the body-cavity; the external aperture
opens in lateral grooves of the epidermis, a couple of longitudinal grooves
in the lateral surface of the outer skin (Fig. 102 b). The pronephridial duct is
formed by the closing of this groove to the right and left at the sides. In all

the craniota it develops at an early stage in the horny plate; in the
amphioxus it seems to be converted into a wide cavity, the atrium, or
peribranchial space.
Next to the kidneys we have the sexual organs of the vertebrate. In most
of the members of this stem the two are united in a single urogenital
system; it is only in a few groups that the urinary and sexual organs are
separated (in the amphioxus, the cyclostoma, and some sections of the fish-
class). In man and all the higher vertebrates the sexual apparatus is made up
of various parts, which we will consider in Chapter XXIX. But in the two
lowest classes of our stem, the acrania and cyclostoma, they consist merely
of simple sexual glands or gonads, the ovaries of the female sex and the
testicles (spermaria) of the male; the former provide the ova, the latter the
sperm. In the craniota we always find only one pair of gonads; in the
amphioxus several pairs, arranged in succession. They must have had the
same form in our hypothetical prospondylus (Figs. 98, 100 s). These
segmental pairs of gonads are the original ventral halves of the cœlom-
pouches.
The organs which we have now enumerated in this general survey, and of
which we have noted the characteristic disposition, are those parts of the
organism that are found in all vertebrates without exception in the same
relation to each other, however much they may be modified. We have
chiefly had in view the transverse section of the body (Figs. 101, 102),
because in this we see most clearly the distinctive arrangement of them. But
to complete our picture we must also consider the segmentation or
metamera-formation of them, which has yet been hardly noticed, and which
is seen best in the longitudinal section. In man and all the more advanced
vertebrates the body is made up of a series or chain of similar members,
which succeed each other in the long axis of the body—the segments or
metamera of the organism. In man these homogeneous parts number thirty-
three in the trunk, but they run to several hundred in many of the vertebrates
(such as serpents or eels). As this internal articulation or metamerism is
mainly found in the vertebral column and the surrounding muscles, the
sections or metamera were formerly called pro-vertebræ. As a fact, the
articulation is by no means chiefly determined and caused by the skeleton,
but by the muscular system and the segmental arrangement of the kidneys
and gonads. However, the composition from these pro-vertebræ or internal
metamera is usually, and rightly, put forward as a prominent character of the

vertebrate, and the manifold division or differentiation of them is of great
importance in the various groups of the vertebrates. But as far as our
present task—the derivation of the simple body of the primitive vertebrate
from the chordula—is concerned, the articulate parts or metamera are of
secondary interest, and we need not go into them just now.

Fig. 103 A, B, C, D.—Instances of redundant mammary glands and
nipples (hypermastism). A a pair of small redundant breasts (with two
nipples on the left) above the large normal ones; from a 45-year-old Berlin
woman, who had had children 17 times (twins twice). (From Hansemann.)
B the highest number: ten nipples (all giving milk), three pairs above, one
pair below, the large normal breasts; from a 22-year-old servant at
Warschau. (From Neugebaur.) C three pairs of nipples: two pairs on the
normal glands and one pair above; from a 19-year-old Japanese girl. D four
pairs of nipples: one pair above the normal and two pairs of small accessory
nipples underneath; from a 22-year-old Bavarian soldier. (From
Wiedersheim.)

The characteristic composition of the vertebrate body develops from the
embryonic structure in the same way in man as in all the other vertebrates.
As all competent experts now admit the monophyletic origin of the
vertebrates on the strength of this significant agreement, and this “common

descent of all the vertebrates from one original stem-form” is admitted as an
historical fact, we have found the answer to “the question of questions.” We
may, moreover, point out that this answer is just as certain and precise in the
case of the origin of man from the mammals. This advanced vertebrate class
is also monophyletic, or has evolved from one common stem-group of
lower vertebrates (reptiles, and, earlier still, amphibia). This follows from
the fact that the mammals are clearly distinguished from the other classes of
the stem, not merely in one striking particular, but in a whole group of
distinctive characters.
It is only in the mammals that we find the skin covered with hair, the
breast-cavity separated from the abdominal cavity by a complete
diaphragm, and the larynx provided with an epiglottis. The mammals alone
have three small auscultory bones in the tympanic cavity—a feature that is
connected with the characteristic modification of their maxillary joint. Their
red blood-cells have no nucleus, whereas this is retained in all other
vertebrates. Finally, it is only in the mammals that we find the remarkable
function of the breast structure which has given its name to the whole class
—the feeding of the young by the mother’s milk. The mammary glands
which serve this purpose are interesting in so many ways that we may
devote a few lines to them here.
As is well known, the lower mammals, especially those which beget a
number of young at a time, have several mammary glands at the breast.
Hedgehogs and sows have five pairs, mice four or five pairs, dogs and
squirrels four pairs, cats and bears three pairs, most of the ruminants and
many of the rodents two pairs, each provided with a teat or nipple (mastos).
In the various genera of the half-apes (lemurs) the number varies a good
deal. On the other hand, the bats and apes, which only beget one young at a
time as a rule, have only one pair of mammary glands, and these are found
at the breast, as in man.
These variations in the number or structure of the mammary apparatus
(mammarium) have become doubly interesting in the light of recent
research in comparative anatomy. It has been shown that in man and the
apes we often find redundant mammary glands (hyper-mastism) and
corresponding teats (hyper-thelism) in both sexes. Fig. 103 shows four cases
of this kind—A, B, and C of three women, and D of a man. They prove that
all the above-mentioned numbers may be found occasionally in man. Fig.

103 A shows the breast of a Berlin woman who had had children seventeen
times, and who has a pair of small accessory breasts (with two nipples on
the left one) above the two normal breasts; this is a common occurrence,
and the small soft pad above the breast is not infrequently represented in
ancient statues of Venus. In Fig. 103 C we have the same phenomenon in a
Japanese girl of nineteen, who has two nipples on each breast besides (three
pairs altogether). Fig. 103 D is a man of twenty-two with four pairs of
nipples (as in the dog), a small pair above and two small pairs beneath the
large normal teats. The maximum number of five pairs (as in the sow and
hedgehog) was found in a Polish servant of twenty-two who had had
several children; milk was given by each nipple; there were three pairs of
redundant nipples above and one pair underneath the normal and very large
breasts (Fig. 103 B).
A number of recent investigations (especially among recruits) have
shown that these things are not uncommon in the male as well as the female
sex. They can only be explained by evolution, which attributes them to
atavism and latent heredity. The earlier ancestors of all the primates
(including man) were lower placentals, which had, like the hedgehog (one
of the oldest forms of the living placentals), several mammary glands (five
or more pairs) in the abdominal skin. In the apes and man only a couple of
them are normally developed, but from time to time we get a development
of the atrophied structures. Special notice should be taken of the
arrangement of these accessory mammæ; they form, as is clearly seen in
Fig. 103 B and D, two long rows, which diverge forward (towards the arm-
pit), and converge behind in the middle line (towards the loins). The milk-
glands of the polymastic lower placentals are arranged in similar lines.
The phylogenetic explanation of polymastism, as given in comparative
anatomy, has lately found considerable support in ontogeny. Hans Strahl, E.
Schmitt, and others, have found that there are always in the human embryo
at the sixth week (when it is three-fifths of an inch long) the microscopic
traces of five pairs of mammary glands, and that they are arranged at
regular distances in two lateral and divergent lines, which correspond to the
mammary lines. Only one pair of them—the central pair—are normally
developed, the others atrophying. Hence there is for a time in the human
embryo a normal hyperthelism, and this can only be explained by the
descent of man from lower primates (lemurs) with several pairs.

But the milk-gland of the mammal has a great morphological interest
from another point of view. This organ for feeding the young in man and the
higher mammals is, as is known, found in both sexes. However, it is usually
active only in the female sex, and yields the valuable “mother’s milk”; in
the male sex it is small and inactive, a real rudimentary organ of no
physiological interest. Nevertheless, in certain cases we find the breast as
fully developed in man as in woman, and it may give milk for feeding the
young.
We have a striking instance of this gynecomastism (large milk-giving
breasts in a male) in Fig. 104. I owe the photograph (taken from life) to the
kindness of Dr. Ornstein, of Athens, a German physician, who has rendered
service by a number of anthropological observations, (for instance, in
several cases of tailed men). The gynecomast in question is a Greek recruit
in his twentieth year, who has both normally developed male organs and
very pronounced female breasts. It is noteworthy that the other features of
his structure are in accord with the softer forms of the female sex. It
reminds us of the marble statues of hermaphrodites which the ancient Greek
and Roman sculptors often produced. But the man would only be a real
hermaphrodite if he had ovaries internally besides the (externally visible)
testicles.

Fig. 104—A Greek gynecomast.

I observed a very similar case during my stay in Ceylon (at Belligemma)
in 1881. A young Cinghalese in his twenty-fifth year was brought to me as a
curious hermaphrodite, half-man and half-woman. His large breasts gave

plenty of milk; he was employed as “male nurse” to suckle a new-born
infant whose mother had died at birth. The outline of his body was softer
and more feminine than in the Greek shown in Fig. 104. As the Cinghalese
are small of stature and of graceful build, and as the men often resemble the
women in clothing (upper part of the body naked, female dress on the lower
part) and the dressing of the hair (with a comb), I first took the beardless
youth to be a woman. The illusion was greater, as in this remarkable case
gynecomastism was associated with cryptorchism—that is to say, the
testicles had kept to their original place in the visceral cavity, and had not
travelled in the normal way down into the scrotum. (Cf. Chapter XXIX.)
Hence the latter was very small, soft, and empty. Moreover, one could feel
nothing of the testicles in the inguinal canal. On the other hand, the male
organ was very small, but normally developed. It was clear that this
apparent hermaphrodite also was a real male.
Another case of practical gynecomastism has been described by
Alexander von Humboldt. In a South American forest he found a solitary
settler whose wife had died in child-birth. The man had laid the new-born
child on his own breast in despair; and the continuous stimulus of the
child’s sucking movements had revived the activity of the mammary glands.
It is possible that nervous suggestion had some share in it. Similar cases
have been often observed in recent years, even among other male mammals
(such as sheep and goats).
The great scientific interest of these facts is in their bearing on the
question of heredity. The stem-history of the mammarium rests partly on its
embryology (Chapter XXIV.) and partly on the facts of comparative
anatomy and physiology. As in the lower and higher mammals (the
monotremes, and most of the marsupials) the whole lactiferous apparatus is
only found in the female; and as there are traces of it in the male only in a
few younger marsupials, there can be no doubt that these important organs
were originally found only in the female mammal, and that they were
acquired by these through a special adaptation to habits of life.
Later, these female organs were communicated to both sexes by heredity;
and they have been maintained in all persons of either sex, although they
are not physiologically active in the males. This normal permanence of the
female lactiferous organs in both sexes of the higher mammals and man is

independent of any selection, and is a fine instance of the much-disputed
“inheritance of acquired characters.”

Chapter XII.
EMBRYONIC SHIELD AND GERMINATIVE
AREA

The three higher classes of vertebrates which we call the amniotes—the
mammals, birds, and reptiles—are notably distinguished by a number of
peculiarities of their development from the five lower classes of the stem—
the animals without an amnion (the anamnia). All the amniotes have a
distinctive embryonic membrane known as the amnion (or “water-
membrane”), and a special embryonic appendage—the allantois. They have,
further, a large yelk-sac, which is filled with food-yelk in the reptiles and
birds, and with a corresponding clear fluid in the mammals. In consequence
of these later-acquired structures, the original features of the development
of the amniotes are so much altered that it is very difficult to reduce them to
the palingenetic embryonic processes of the lower amnion-less vertebrates.
The gastræa theory shows us how to do this, by representing the
embryology of the lowest vertebrate, the skull-less amphioxus, as the
original form, and deducing from it, through a series of gradual
modifications, the gastrulation and cœlomation of the craniota.
It was somewhat fatal to the true conception of the chief embryonic
processes of the vertebrate that all the older embryologists, from Malpighi
(1687) and Wolff (1750) to Baer (1828) and Remak (1850), always started
from the investigation of the hen’s egg, and transferred to man and the other
vertebrates the impressions they gathered from this. This classical object of
embryological research is, as we have seen, a source of dangerous errors.
The large round food-yelk of the bird’s egg causes, in the first place, a flat
discoid expansion of the small gastrula, and then so distinctive a
development of this thin round embryonic disk that the controversy as to its
significance occupies a large part of embryological literature.

One of the most unfortunate errors that this led to was the idea of an
original antithesis of germ and yelk. The latter was regarded as a foreign
body, extrinsic to the real germ, whereas it is properly a part of it, an
embryonic organ of nutrition. Many authors said there was no trace of the
embryo until a later stage, and outside the yelk; sometimes the two-layered
embryonic disk itself, at other times only the central portion of it (as
distinguished from the germinative area, which we will describe presently),
was taken to be the first outline of the embryo. In the light of the gastræa
theory it is hardly necessary to dwell on the defects of this earlier view and
the erroneous conclusions drawn from it. In reality, the first segmentation-
cell, and even the stem-cell itself and all that issues therefrom, belong to the
embryo. As the large original yelk-mass in the undivided egg of the bird
only represents an inclosure in the greatly enlarged ovum, so the later
contents of its embryonic yelk-sac (whether yet segmented or not) are only
a part of the entoderm which forms the primitive gut. This is clearly shown
by the ova of the amphibia and cyclostoma, which explain the transition
from the yelk-less ova of the amphioxus to the large yelk-filled ova of the
reptiles and birds.

Fig. 105—Severance of the discoid mammal embryo from the yelk-
sac, in transverse section (diagrammatic). A The germinal disk (h, hf) lies
flat on one side of the branchial-gut vesicle (kb). B In the middle of the
germinal disk we find the medullary groove (mr), and underneath it the
chorda (ch). C The gut-fibre-layer (df) has been enclosed by the gut-gland-
layer (dd). D The skin-fibre-layer (hf) and gut-fibre-layer (df) divide at the

periphery; the gut (d) begins to separate from the yelk-sac or umbilical
vesicle (nb). E The medullary tube (mr) is closed; the body-cavity (c)
begins to form. F The provertebræ (w) begin to grow round the medullary
tube (mr) and the chorda (ch): the gut (d) is cut off from the umbilical
vesicle (nb). H The vertebræ (w) have grown round the medullary tube (mr)
and chorda; the body-cavity is closed, and the umbilical vesicle has
disappeared. The amnion and serous membrane are omitted. The letters
have the same meaning throughout: h horn-plate, mr medullary tube, hf
skin-fibre-layer, w provertebræ, ch chorda, c body-cavity or cœloma, df gut-
fibre-layer, dd gut-gland-layer, d gut-cavity, nb umbilical vesicle.

It is precisely in the study of these difficult features that we see the
incalculable value of phylogenetic considerations in explaining complex
ontogenetic facts, and the need of separating cenogenetic phenomena from
palingenetic. This is particularly clear as regards the comparative
embryology of the vertebrates, because here the phylogenetic unity of the
stem has been already established by the well-known facts of paleontology
and comparative anatomy. If this unity of the stem, on the basis of the
amphioxus, were always borne in mind, we should not have these errors
constantly recurring.
In many cases the cenogenetic relation of the embryo to the food-yelk
has until now given rise to a quite wrong idea of the first and most
important embryonic processes in the higher vertebrates, and has
occasioned a number of false theories in connection with them. Until thirty
years ago the embryology of the higher vertebrates always started from the
position that the first structure of the embryo is a flat, leaf-shaped disk; it
was for this reason that the cell-layers that compose this germinal disk (also
called germinative area) are called “germinal layers.” This flat germinal
disk, which is round at first and then oval, and which is often described as
the tread or cicatricula in the laid hen’s egg, is found at a certain part of the
surface of the large globular food-yelk. I am convinced that it is nothing
else than the discoid, flattened gastrula of the birds. At the beginning of
germination the flat embryonic disk curves outwards, and separates on the
inner side from the underlying large yelk-ball. In this way the flat layers are
converted into tubes, their edges folding and joining together (Fig. 105). As
the embryo grows at the expense of the food-yelk, the latter becomes
smaller and smaller; it is completely surrounded by the germinal layers.

Later still, the remainder of the food-yelk only forms a small round sac, the
yelk-sac or umbilical vesicle (Fig. 105 nb). This is enclosed by the visceral
layer, is connected by a thin stalk, the yelk-duct, with the central part of the
gut-tube, and is finally, in most of the vertebrates, entirely absorbed by this
(H). The point at which this takes place, and where the gut finally closes, is
the visceral navel. In the mammals, in which the remainder of the yelk-sac
remains without and atrophies, the yelk-duct at length penetrates the outer
ventral wall. At birth the umbilical cord proceeds from here, and the point
of closure remains throughout life in the skin as the navel.
As the older embryology of the higher vertebrates was mainly based on
the chick, and regarded the antithesis of embryo (or formative-yelk) and
food-yelk (or yelk-sac) as original, it had also to look upon the flat leaf-
shaped structure of the germinal disk as the primitive embryonic form, and
emphasise the fact that hollow grooves were formed of these flat layers by
folding, and closed tubes by the joining together of their edges.
This idea, which dominated the whole treatment of the embryology of the
higher vertebrates until thirty years ago, was totally false. The gastræa
theory, which has its chief application here, teaches us that it is the very
reverse of the truth. The cup-shaped gastrula, in the body-wall of which the
two primary germinal layers appear from the first as closed tubes, is the
original embryonic form of all the vertebrates, and all the multicellular
invertebrates; and the flat germinal disk with its superficially expanded
germinal layers is a later, secondary form, due to the cenogenetic formation
of the large food-yelk and the gradual spread of the germ-layers over its
surface. Hence the actual folding of the germinal layers and their
conversion into tubes is not an original and primary, but a much later and
tertiary, evolutionary process. In the phylogeny of the vertebrate embryonic
process we may distinguish the following three stages:—
A. First stage: B. Second stage: C. Third stage:
Primary Secondary Tertiary
(palingenic) (cenogenetic) (cenogenetic)
embryonic process. embryonic process. embryonic process.
The germinal layers form The germinal layers The germinal layers
from the first closed spread out leaf-wise, form a flat germinal
tubes, the one-layered food-yelk gathering in disk, the borders of
blastula being converted the ventral entoderm, which join together and

into the two-layered and a large yelk-sac form closed tubes,
gastrula by invagination. being formed from the separating from the
No food-yelk. middle of the gut-tube. central yelk-sac.
(Amphioxus.) (Amphibia.) (Amniotes.)
As this theory, a logical conclusion from the gastræa theory, has been
fully substantiated by the comparative study of gastrulation in the last few
decades, we must exactly reverse the hitherto prevalent mode of treatment.
The yelk-sac is not to be treated, as was done formerly, as if it were
originally antithetic to the embryo, but as an essential part of it, a part of its
visceral tube. The primitive gut of the gastrula has, on this view, been
divided into two parts in the higher animals as a result of the cenogenetic
formation of the food-yelk—the permanent gut (metagaster), or permanent
alimentary canal, and the yelk-sac (lecithoma), or umbilical vesicle. This is
very clearly shown by the comparative ontogeny of the fishes and
amphibia. In these cases the whole yelk undergoes cleavage at first, and
forms a yelk-gland, composed of yelk-cells, in the ventral wall of the
primitive gut. But it afterwards becomes so large that a part of the yelk does
not divide, and is used up in the yelk-sac that is cut off outside.
When we make a comparative study of the embryology of the
amphioxus, the frog, the chick, and the rabbit, there cannot, in my opinion,
be any further doubt as to the truth of this position, which I have held for
thirty years. Hence in the light of the gastræa theory we must regard the
features of the amphioxus as the only and real primitive structure among all
the vertebrates, departing very little from the palingenetic embryonic form.
In the cyclostoma and the frog these features are, on the whole, not much
altered cenogenetically, but they are very much so in the chick, and most of
all in the rabbit. In the bell-gastrula of the amphioxus and in the hooded
gastrula of the lamprey and the frog the germinal layers are found to be
closed tubes or vesicles from the first. On the other hand, the chick-embryo
(in the new laid, but not yet hatched, egg) is a flat circular disk, and it was
not easy to recognise this as a real gastrula. Rauber and Goette have,
however, achieved this. As the discoid gastrula grows round the large
globular yelk, and the permanent gut then separates from the outlying yelk-
sac, we find all the processes which we have shown (diagrammatically) in
Figure 1.108—processes that were hitherto regarded as principal acts,
whereas they are merely secondary.

Fig. 106—The visceral embryonic vesicle (blastocystis or gastrocystis)
of a rabbit (the “blastula” or vesicula blastodermica of other writers), a
outer envelope (ovolemma), b skin-layer or ectoderm, forming the entire
wall of the yelk-vesicle, c groups of dark cells, representing the visceral
layer or entoderm.
Fig. 107—The same in section. Letters as above. d cavity of the vesicle.
(From Bischoff.)

The oldest, oviparous mammals, the monotremes, behave in the same
way as the reptiles and birds. But the corresponding embryonic processes in
the viviparous mammals, the marsupials and placentals, are very elaborate
and distinctive. They were formerly quite misinterpreted; it was not until
the publication of the studies of Edward van Beneden (1875) and the later
research of Selenka, Kuppfer, Rabl, and others, that light was thrown on
them, and we were in a position to bring them into line with the principles
of the gastræa theory and trace them to the embryonic forms of the lower
vertebrates. Although there is no independent food-yelk, apart from the
formative yelk, in the mammal ovum, and although its segmentation is total
on that account, nevertheless a large yelk-sac is formed in their embryos,
and the “embryo proper” spreads leaf-wise over its surface, as in the reptiles
and birds, which have a large food-yelk and partial segmentation. In the
mammals, as well as in the latter, the flat, leaf-shaped germinal disk
separates from the yelk-sac, and its edges join together and form tubes.
How can we explain this curious anomaly? Only as a result of very
characteristic and peculiar cenogenetic modifications of the embryonic
process, the real causes of which must be sought in the change in the
rearing of the young on the part of the viviparous mammals. These are
clearly connected with the fact that the ancestors of the viviparous
mammals were oviparous amniotes like the present monotremes, and only

gradually became viviparous. This can no longer be questioned now that it
has been shown (1884) that the monotremes, the lowest and oldest of the
mammals, still lay eggs, and that these develop like the ova of the reptiles
and birds. Their nearest descendants, the marsupials, formed the habit of
retaining the eggs, and developing them in the oviduct; the latter was thus
converted into a womb (uterus). A nutritive fluid that was secreted from its
wall, and passed through the wall of the blastula, now served to feed the
embryo, and took the place of the food-yelk. In this way the original food-
yelk of the monotremes gradually atrophied, and at last disappeared so
completely that the partial ovum-segmentation of their descendants, the rest
of the mammals, once more became total. From the discogastrula of the
former was evolved the distinctive epigastrula of the latter.
It is only by this phylogenetic explanation that we can understand the
formation and development of the peculiar, and hitherto totally
misunderstood, blastula of the mammal. The vesicular condition of the
mammal embryo was discovered 200 years ago (1677) by Regner de Graaf.
He found in the uterus of a rabbit four days after impregnation small, round,
loose, transparent vesicles, with a double envelope. However, Graaf’s
discovery passed without recognition. It was not until 1827 that these
vesicles were rediscovered by Baer, and then more closely studied in 1842
by Bischoff in the rabbit (Figs. 106, 107). They are found in the womb of
the rabbit, the dog, and other small mammals, a few days after copulation.
The mature ova of the mammal, when they have left the ovary, are fertilised
either here or in the oviduct immediately afterwards by the invading sperm-
cells.[25] (As to the womb and oviduct see Chapter XXIX) The cleavage and
formation of the gastrula take place in the oviduct. Either here in the
oviduct or after the mammal gastrula has passed into the uterus it is
converted into the globular vesicle which is shown externally in Fig. 106,
and in section in Fig. 107. The thick, outer, structureless envelope that
encloses it is the original ovolemma or zona pellucida, modified, and
clothed with a layer of albumin that has been deposited on the outside.
From this stage the envelope is called the external membrane, the primary
chorion or prochorion (a). The real wall of the vesicle enclosed by it
consists of a simple layer of ectodermic cells (b), which are flattened by
mutual pressure, and generally hexagonal; a light nucleus shines through
their fine-grained protoplasm (Fig. 108). At one part (c) inside this hollow

ball we find a circular disc, formed of darker, softer, and rounder cells, the
dark-grained entodermic cells (Fig. 109).
[25] In man and the other mammals the fertilisation of the ova probably takes
place, as a rule, in the oviduct; here the ova, which issue from the female ovary
in the shape of the Graafian follicle, and enter the inner aperture of the oviduct,
encounter the mobile sperm-cells of the male seed, which pass into the uterus at
copulation, and from this into the external aperture of the oviduct. Impregnation
rarely takes place in the ovary or in the womb.

Fig. 108—Four entodermic cells from the embryonic vesicle of the
rabbit.
Fig. 109—Two entodermic cells from the embryonic vesicle of the rabbit.

The characteristic embryonic form that the developing mammal now
exhibits has up to the present usually been called the “blastula” (Bischoff),
“sac-shaped embryo” (Baer), “vesicular embryo” (vesicula blastodermica,
or, briefly, blastosphæra). The wall of the hollow vesicle, which consists of
a single layer of cells, was called the “blastoderm,” and was supposed to be
equivalent to the cell-layer of the same name that forms the wall of the real
blastula of the amphioxus and many of the invertebrates (such as
Monoxenia, Fig. 29 F, G). Formerly this real blastula was generally
believed to be equivalent to the embryonic vesicle of the mammal.
However, this is by no means the case. What is called the “blastula” of the
mammal and the real blastula of the amphioxus and many of the
invertebrates are totally different embryonic structures. The latter (blastula)
is palingenetic, and precedes the formation of the gastrula. The former
(blastodermic vesicle) is cenogenetic, and follows gastrulation. The
globular wall of the blastula is a real blastoderm, and consists of
homogeneous (blastodermic) cells; it is not yet differentiated into the two
primary germinal layers. But the globular wall of the mammal vesicle is the
differentiated ectoderm, and at one point in it we find a circular disk of

quite different cells—the entoderm. The round cavity, filled with fluid,
inside the real blastula is the segmentation-cavity. But the similar cavity
within the mammal vesicle is the yelk-sac cavity, which is connected with
the incipient gut-cavity. This primitive gut-cavity passes directly into the
segmentation-cavity in the mammals, in consequence of the peculiar
cenogenetic changes in their gastrulation, which we have considered
previously (Chapter IX). For these reasons it is very necessary to recognise
the secondary embryonic vesicle in the mammal (gastrocystis or
blastocystis) as a characteristic structure peculiar to this class, and
distinguish it carefully from the primary blastula of the amphioxus and the
invertebrates.

Fig. 110—Ovum of a rabbit from the uterus, one sixth of an inch in
diameter. The embryonic vesicle (b) has withdrawn a little from the smooth
ovolemma (a). In the middle of the ovolemma we see the round germinal
disk (blastodiscus, c), at the edge of which (at d) the inner layer of the
embryonic vesicle is already beginning to expand. (Figs. 110–114 from
Bischoff.
Fig. 111—The same ovum, seen in profile. Letters as in Fig. 110.
Fig. 112—Ovum of a rabbit from the uterus, one-fourth of an inch in
diameter. The blastoderm is already for the most part two-layered (b). The
ovolemma, or outer envelope, is tufted (a).
Fig. 113—The same ovum, seen in profile. Letters as in Fig. 112.
Fig. 114—Ovum of a rabbit from the uterus, one-third of an inch in

diameter. The embryonic vesicle is now nearly everywhere two-layered (k)
only remaining one-layered below (at d).

The small, circular, whitish, and opaque spot which the gastric disk (Fig.
106) forms at a certain part of the surface of the clear and transparent
embryonic vesicle has long been known to science, and compared to the
germinal disk of the birds and reptiles. Sometimes it has been called the
germinal disk, sometimes the germinal spot, and usually the germinative
area. From the area the further development of the embryo proceeds.
However, the larger part of the embryonic vesicle of the mammal is not
directly used for building up the later body, but for the construction of the
temporary umbilical vesicle. The embryo separates from this in proportion
as it grows at its expense; the two are only connected by the yelk-duct (the
stalk of the yelk-sac), and this maintains the direct communication between
the cavity of the umbilical vesicle and the forming visceral cavity (Fig.
105).

Fig. 115—Round germinative area of the rabbit, divided into the
central light area (area pellucida) and the peripheral dark area (area opaca).
The light area seems darker on account of the dark ground appearing
through it.
Fig. 116—Oval area, with the opaque whitish border of the dark area
without.

The germinative area or gastric disk of the animal consists at first (like
the germinal disk of birds and reptiles) merely of the two primary germinal
layers, the ectoderm and entoderm. But soon there appears in the middle of

the circular disk between the two a third stratum of cells, the rudiment of
the middle layer or fibrous layer (mesoderm). This middle germinal layer
consists from the first, as we have seen in Chapter X, of two separate
epithelial plates, the two layers of the cœlom-pouches (parietal and
visceral). However, in all the amniotes (on account of the large formation of
yelk) these thin middle plates are so firmly pressed together that they seem
to represent a single layer. It is thus peculiar to the amniotes that the middle
of the germinative area is composed of four germinal layers, the two
limiting (or primary) layers and the middle layers between them (Figs. 96,
97). These four secondary germinal layers can be clearly distinguished as
soon as what is called the sickle-groove (or “embryonic sickle”) is seen at
the hind border of the germinative area. At the borders, however, the
germinative area of the mammal only consists of two layers. The rest of the
wall of the embryonic vesicle consists at first (but only for a short time in
most of the mammals) of a single layer, the outer germinal layer.
From this stage, however, the whole wall of the embryonic vesicle
becomes two-layered. The middle of the germinative area is much
thickened by the growth of the cells of the middle layers, and the inner layer
expands at the same time, and increases at the border of the disk all round.
Lying close on the outer layer throughout, it grows over its inner surface at
all points, covers first the upper and then the lower hemisphere, and at last
closes in the middle of the inner layer (Figs. 110–114). The wall of the
embryonic vesicle now consists throughout of two layers of cells, the
ectoderm without and the entoderm within. It is only in the centre of the
circular area, which becomes thicker and thicker through the growth of the
middle layers, that it is made up of all four layers. At the same time, small
structureless tufts or warts are deposited on the surface of the outer
ovolemma or prochorion, which has been raised above the embryonic
vesicle (Figs. 112–114 a).
We may now disregard both the outer ovolemma and the greater part of
the vesicle, and concentrate our attention on the germinative area and the
four-layered embryonic disk. It is here alone that we find the important
changes which lead to the differentiation of the first organs. It is immaterial
whether we examine the germinative area of the mammal (the rabbit, for
instance) or the germinal disk of a bird or a reptile (such as a lizard or
tortoise). The embryonic processes we are now going to consider are
essentially the same in all members of the three higher classes of

vertebrates which we call the amniotes. Man is found to agree in this
respect with the rabbit, dog, ox, etc.; and in all these animals the
germinative area undergoes essentially the same changes as in the birds and
reptiles. They are most frequently and accurately studied in the chick,
because we can have incubated hens’ eggs in any quantity at any stage of
development. Moreover, the round germinal disk of the chick passes
immediately after the beginning of incubation (within a few hours) from the
two-layered to the four-layered stage, the two-layered mesoderm
developing from the median primitive groove between the ectoderm and
entoderm (Figs. 82–95).

Fig. 117—Oval germinal disk of the rabbit, magnified. As the delicate,
half-transparent disk lies on a black ground, the pellucid area looks like a
dark ring, and the opaque area (lying outside it) like a white ring. The oval
shield in the centre also looks whitish, and in its axis we see the dark
medullary groove. (From Bischoff.)
Fig. 118—Pear-shaped germinal shield of the rabbit (eight days old),
magnified. rf medullary groove. pr primitive groove (primitive mouth).
(From Kölliker.

The first change in the round germinal disk of the chick is that the cells at
its edges multiply more briskly, and form darker nuclei in their protoplasm.
This gives rise to a dark ring, more or less sharply set off from the lighter
centre of the germinal disk (Fig. 115). From this point the latter takes the
name of the “light area” (area pellucida), and the darker ring is called the
“dark area” (area opaca). (In a strong light, as in Figs. 115–117, the light
area seems dark, because the dark ground is seen through it; and the dark
area seems whiter). The circular shape of the area now changes into elliptic,

and then immediately into oval (Figs. 116, 117). One end seems to be
broader and blunter, the other narrower and more pointed; the former
corresponds to the anterior and the latter to the posterior section of the
subsequent body. At the same time, we can already trace the characteristic
bilateral form of the body, the antithesis of right and left, before and behind.
This will be made clearer by the “primitive streak,” which appears at the
posterior end.
At an early stage an opaque spot is seen in the middle of the clear
germinative area, and this also passes from a circular to an oval shape. At
first this shield-shaped marking is very delicate and barely perceptible; but
it soon becomes clearer, and now stands out as an oval shield, surrounded
by two rings or areas (Fig. 117). The inner and brighter ring is the
remainder of the pellucid area, and the dark outer ring the remainder of the
opaque area; the opaque shield-like spot itself is the first rudiment of the
dorsal part of the embryo. We give it briefly the name of embryonic shield
or dorsal shield. In most works this embryonic shield is described as “the
first rudiment or trace of the embryo,” or “primitive embryo.” But this is
wrong, though it rests on the authority of Baer and Bischoff.

Fig. 119—Median longitudinal section of the gastrula of four
vertebrates. (From Rabl.) A discogastrula of a shark (Pristiurus). B
amphigastrula of a sturgeon (Accipenser). C amphigastrula of an

amphibium (Triton). D epigastrula of an amniote (diagram). a ventral, b
dorsal lip of the primitive mouth.

As a matter of fact, we already have the embryo in the stem-cell, the
gastrula, and all the subsequent stages. The embryonic shield is simply the
first rudiment of the dorsal part, which is the earliest to develop. As the
older names of “embryonic rudiment” and “germinative area” are used in
many different senses—and this has led to a fatal confusion in embryonic
literature—we must explain very clearly the real significance of these
important embryonic parts of the amniote. It will be useful to do so in a
series of formal principles:—
1. The so-called ”first trace of the embryo” in the amniotes, or the
embryonic shield, in the centre of the pellucid area, consists merely of an
early differentiation and formation of the middle dorsal parts.
2. Hence the best name for it is ”the dorsal shield,” as I proposed long
ago.
3. The germinative area, in which the first embryonic blood-vessels
appear at an early stage, is not opposed as an external area to the ”embryo
proper,” but is a part of it.
4. In the same way, the yelk-sac or the umbilical vesicle is not a foreign
external appendage of the embryo, but an outlying part of its primitive gut.
5. The dorsal shield gradually separates from the germinative area and
the yelk-sac, its edges growing downwards and folding together to form
ventral plates.
6. The yelk-sac and vessels of the germinative area, which soon spread
over its whole surface, are, therefore, real embryonic organs, or temporary
parts of the embryo, and have a transitory importance in connection with
the nutrition of the growing later body; the latter may be called the
”permanent body” in contrast to them.
The relation of these cenogenetic features of the amniotes to the
palingenetic structures of the older non-amniotic vertebrates may be
expressed in the following theses: The original gastrula, which completely
passes into the embryonic body in the acrania, cyclostoma, and amphibia, is
early divided into two parts in the amniotes—the embryonic shield, which
represents the dorsal outline of the permanent body; and the temporary
embryonic organs of the germinative area and its blood-vessels, which soon

grow over the whole of the yelk-sac. The differences which we find in the
various classes of the vertebrate stem in these important particulars can only
be fully understood when we bear in mind their phylogenetic relations on
the one hand, and, on the other, the cenogenetic modifications of structure
that have been brought about by changes in the rearing of the young and the
variation in the mass of the food-yelk.
We have already described in Chapter IX the changes which this increase
and decrease of the nutritive yelk causes in the form of the gastrula, and
especially in the situation and shape of the primitive mouth. The primitive
mouth or prostoma is originally a simple round aperture at the lower pole of
the long axis; its dorsal lip is above and ventral lip below. In the amphioxus
this primitive mouth is a little eccentric, or shifted to the dorsal side (Fig.
39). The aperture increases with the growth of the food-yelk in the
cyclostoma and ganoids; in the sturgeon it lies almost on the equator of the
round ovum, the ventral lip (a) in front and the dorsal lip (b) behind (Fig.
119 b). In the wide-mouthed, circular discoid gastrula of the selachii or
primitive fishes, which spreads quite flat on the large food-yelk, the anterior
semi-circle of the border of the disk is the ventral, and the posterior
semicircle the dorsal lip (Fig. 119 A). The amphiblastic amphibia are
directly connected with their earlier fish-ancestors, the dipneusts and
ganoids, and further the oldest selachii (Cestracion); they have retained
their total unequal segmentation, and their small primitive mouth (Fig. 119
C, ab), blocked up by the yelk-stopper, lies at the limit of the dorsal and
ventral surface of the embryo (at the lower pole of its equatorial axis), and
there again has an upper dorsal and a lower ventral lip (a, b). The formation
of a large food-yelk followed again in the stem-forms of the amniotes, the
protamniotes or proreptilia, descended from the amphibia (Fig. 119 D). But
here the accumulation of the food-yelk took place only in the ventral wall of
the primitive-gut, so that the narrow primitive mouth lying behind was
forced upwards, and came to lie on the back of the discoid ”epigastrula” in
the shape of the ”primitive groove”; thus (in contrast to the case of the
selachii, Fig. 119 A) the dorsal lip (b) had to be in front, and the ventral lip
(a) behind (Fig. 119 D). This feature was transmitted to all the amniotes,
whether they retained the large food-yelk (reptiles, birds, and monotremes),
or lost it by atrophy (the viviparous mammals).
This phylogenetic explanation of gastrulation and cœlomation, and the
comparative study of them in the various vertebrates, throw a clear and full

light on many ontogenetic phenomena, as to which the most obscure and
confused opinions were prevalent thirty years ago. In this we see especially
the high scientific value of the biogenetic law and the careful separation of
palingenetic from cenogenetic processes. To the opponents of this law the
real explanation of these remarkable phenomena is impossible. Here, and in
every other part of embryology, the true key to the solution lies in
phylogeny.

Chapter XIII.
DORSAL BODY AND VENTRAL BODY

The earliest stages of the human embryo are, for the reasons already
given, either quite unknown or only imperfectly known to us. But as the
subsequent embryonic forms in man behave and develop just as they do in
all the other mammals, there cannot be the slightest doubt that the preceding
stages also are similar. We have been able to see in the cœlomula of the
human embryo (Fig. 97), by transverse sections through its primitive
mouth, that its two cœlom-pouches are developed in just the same way as in
the rabbit (Fig. 96); moreover, the peculiar course of the gastrulation is just
the same.

Fig. 120—Embryonic vesicle of a seven-days-old rabbit with oval
embryonic shield (ag). A seen from above, B from the side. (From
Kölliker.) ag dorsal shield or embryonic spot. In B the upper half of the
vesicle is made up of the two primary germinal layers, the lower (up to ge)
only from the outer layer.

The germinative area forms in the human embryo in the same way as in
the other mammals, and in the middle part of this we have the embryonic
shield, the purport of which we considered in Chapter XII. The next

changes in the embryonic disk, or the “embryonic spot,” take place in
corresponding fashion. These are the changes we are now going to consider
more closely.
The chief part of the oval embryonic shield is at first the narrow hinder
end; it is in the middle line of this that the primitive streak appears (Fig. 121
ps).The narrow longitudinal groove in it—the so-called “primitive
groove”—is, as we have seen, the primitive mouth of the gastrula. In the
gastrula-embryos of the mammals, which are much modified
cenogenetically, this cleft-shaped prostoma is lengthened so much that it
soon traverses the whole of the hinder half of the dorsal shield; as we find
in a rabbit embryo of six to eight days (Fig. 122 pr). The two swollen
parallel borders that limit this median furrow are the side lips of the
primitive mouth, right and left. In this way the bilateral-symmetrical type of
the vertebrate becomes pronounced. The subsequent head of the amniote is
developed from the broader and rounder fore-half of the dorsal shield.
In this fore-half of the dorsal shield a median furrow quickly makes its
appearance (Fig. 123 rf). This is the broader dorsal furrow or medullary
groove, the first beginning of the central nervous system. The two parallel
dorsal or medullary swellings that enclose it grow together over it
afterwards, and form the medullary tube. As is seen in transverse sections, it
is formed only of the outer germinal layer (Figs. 95 and 136). The lips of
the primitive mouth, however, lie, as we know, at the important point where
the outer layer bends over the inner, and from which the two cœlom
pouches grow between the primary germinal layers.

Fig. 121—Oval embryonic shield of the rabbit (A of six days eighteen
hours, B of eight days). (From Kölliker.) ps primitive streak, pr primitive
groove, arg area germinalis, sw sickle-shaped germinal growth.