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diff --git a/8700-0.txt b/8700-0.txt new file mode 100644 index 0000000..2b369ba --- /dev/null +++ b/8700-0.txt @@ -0,0 +1,24436 @@ +The Project Gutenberg EBook of The Evolution of Man, by Ernst Haeckel + +This eBook is for the use of anyone anywhere in the United States and most +other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms of +the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you'll have +to check the laws of the country where you are located before using this ebook. + +Title: The Evolution of Man + +Author: Ernst Haeckel + +Release Date: August 1, 2003 [EBook #8700] +[Most recently updated: April 5, 2020] + +Language: English + +Character set encoding: UTF-8 + +*** START OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN *** + + + + +Produced by Produced by Sue Asscher and Derek Thompson + + + + +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 + +From the painting by Franz von Lenbach, 1899 + + +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) 1. Unnucleated Bacteria +Protamæbæ Monera 2. Nucleated _a._ Rhizopoda Amœbina +Radiolaria _b._ Infusoria Flagellata Ciliata 3. +Cell-colonies Catallacta Blastæada + + + + +Unicellular animals (Protozoa) I Cœlenterata, +or Zoophytes. +Animals without +body-cavity, +blood, or anus. _a._ Gastræads Gastremaria Cyemaria _b._ +Sponges Protospongiæ Metaspongiæ _c._ Cnidaria (stinging animals) +Hydrozoa Polyps Medusæ _d._ Platodes (flat-worms) Platodaria +Turbullaria Trematoda Cestoda +II +Cœlomaria or +Bilaterals. +Animals with +body-cavity and +anus, and generally blood. _a._ Vermalia (worm-like) Rotatoria +Strongylaria Prosopygia Frontonia _b._ Molluscs Cochlides Conchades +Teuthodes _c._ Articulates Annelida Crustacea Tracheata _d._ +Echinoderms Monorchonia Pentorchonia _e._ Tunicates Copelata +Ascidiæ Thalidiæ _f._ Vertebrates I. Acrania-Lancelet (without +skull) II. Craniota (with skull) _a._ Cyclostomes (“round-mouthed”) +_b._ Fishes Selachii Ganoids 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 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. 1_c_ and Fig. 2_k_). 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 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. +2_k_). 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. Fig. 4 Five spiny or grooved cells. Fig. +5 Ten liver-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. 2_k_). 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. 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. 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. 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 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 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. + + +Fig. 11 Indirect or mitotic cell-division. 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. 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. 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. 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 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. 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. 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. 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. +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. 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. 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. 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. 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. 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. 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. 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. 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. + + + +Gastrulation of a coral. 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 Gastrula of a very simple primitive-gut animal or 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. 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 (olynthus). 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. 30–35_e_); 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. 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. 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. 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 + (primitive) segmentation. 1. Equal segmentation + (bell-gastrula). A. Total segmentation (without independent + food-yelk). II. Cenogenetic segmentation + (modified by adaptation). 2. Unequal segmentation (hooded + gastrula). 3. Discoid segmentation (discoid gastrula). B. + Partial segmentation (with independent food-yelk). 4. Superficial + segmentation + (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. Total +Segmentation. Holoblastic ova. + + + + Gastrula without separate food-yelk. +Hologastrula. I. Primitive +Segmentation. Archiblastic ova. + Bell-gastrula +(archigastrula.) _a._ Many lower zoophyta (sponges, hydrapolyps, +medusæ, simpler corals). _b._ Many lower annelids (sagitta, phoronis, + many nematoda, etc., terebratula, argiope, + pisidium). _c._ Some lower molluscs. +_d._ Many echinoderms. _e._ A few lower articulata (some brachiopods, +copepods: Tardigrades, pteromalina). _f._ Many tunicata. _g._ The +acrania (amphioxus). II. Unequal Segmentation. +Amphiblastic ova. + Hooded-gastrula (amphigastrula). _a._ Many zoophyta (sponges, + medusæ, + corals, siphonophoræ, ctenophora). _b._ Most worms. _c._ Most + molluscs. _d._ Many echinoderms (viviparous species and some + others). +_e._ Some of the lower articulata (both crustacea + and tracheata). _f._ Many tunicata. +_g._ Cyclostoma, the oldest fishes, amphibia, + mammals (not including man). II. Partial Segmentation. Meroblastic + ova. + Gastrula with +separate food-yelk. Merogastrula. III. Discoid Segmentation. +Discoblastic ova. + Discoid gastrula. _c._ Cephalopods or cuttlefish. _e._ Many + articulata, wood-lice, scorpions, etc. _g._ Primitive fishes, bony + fishes, reptiles, birds, monotremes. IV. Superficial +Segmentation. Periblastic ova. Spherical-gastrula. _e._ The great +majority of the articulata + (crustaceans, myriapods, arachnids, insects). + + + + +Chapter IX. +THE GASTRULATION OF THE VERTEBRATE[20] + + + [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. 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. 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._). + + +Blastula of the water-salamander. 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. 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. 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. 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 in the lamprey. 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. 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. 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. 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. 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. 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. 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. 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. 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 bastula of a hen. Fig. 59. The germinal +disk of the hen’s ovum at the beginning of gastrulation. Fig. 60. +Longitudinal section of the germinal disk of a siskin. 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. 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. 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. 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). 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. Fig. 66. Oval gastrula of the opossum (Didelphys), about +eight hours old. 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. 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). Fig. 69. +Incipient cleavage of the mammal ovum (from the rabbit). Fig. 70. The +first four segmentation-cells of the mammal ovum (from the rabbit). +Fig. 71. Mammal ovum with eight segmentation-cells (from the rabbit). +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). 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). 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). +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. 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. 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 terminal layers. 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. Coelomula of sagitta. 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 (outer limiting layer). I. Neural layer + (_neuroblast_). The two secondary germinal layers of the + body-wall: +I. Epithelial. II. Fibrous. 2. Skin-fibre-layer (outer middle +layer). II. Parietal layer + (_myoblast_). 3. Gut-fibre-layer (inner middle layer). III. + Visceral layer + (_genoblast_). The two secondary germinal layers of the + gut-wall: III. Fibrous. IV. Epithelial. 4. Gut-gland-layer + (inner limiting layer). IV. Enteral layer + (_enteroblast_) + +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. Coelomula of sagitta, in section. 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. 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-larvae. 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. 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 (_mk_1 parietal, _mk_2 +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. 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). 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 coelomula-embryos of +vertebrates. 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 coelomula embryos of triton. +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. 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, _mk_1 and _mk_2 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). 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). 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. + + +Transverse section of the primitive streak (primitive mouth) of the +chick. 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. 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 coelomula stage). 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. +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). 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. 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). 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: Primary +(palingenic) embryonic process. B. Second stage: Secondary +(cenogenetic) embryonic process. C. Third stage: +Tertiary (cenogenetic) embryonic process. The germinal layers form from +the first closed tubes, the one-layered blastula being converted into +the two-layered gastrula by invagination. No food-yelk. + (_Amphioxus._) The germinal layers spread out leaf-wise, + food-yelk gathering in the ventral entoderm, and a large yelk-sac + being formed from the middle of the gut-tube. (_Amphibia._) The + germinal layers form a flat germinal disk, the borders of which + join together and form closed tubes, separating from the central + yelk-sac. (_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. + + +Figs. 106 and 107. The visceral embryonnic vesicle (blastocystis or +gastrocystis) of a rabbit. 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 vesicle of the rabbit. Fig. +109. Two entodermic cells from the embryonic vesicle of the rabbit. +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. Fig. 111. The same ovum, seen in profile. Fig. 112. Ovum of a +rabbit from the uterus, one-fourth of an inch in diameter. Fig. 113. +The same ovum, seen in profile. Fig. 114. Ovum of a rabbit from the +uterus, one-third of an inch in diameter. 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. Fig. 116. Oval area, +with the opaque whitish border of the dark area without. 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. Fig. 118. +Pear-shaped germinal shield of the rabbit (eight days old), magnified. +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. 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). 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. 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. + + +Fig.122. Dorsal shield (ag) and germinative area of a rabbit-embryo of +eight days. Fig. 123. Embryonic shield of a rabbit of eight days. Fig. +122—Dorsal shield (_ag_) and germinative area of a rabbit-embryo of +eight days. (From _Kölliker._) _pr_ primitive groove, _rf_ dorsal +furrow. Fig. 123.—Embryonic shield of a rabbit of eight days. (From +_Van Beneden._) _pr_ primitive groove, _cn_ canalis neurentericus, _nk_ +nodus neurentericus (or “Hensen’s ganglion”), _kf_ head-process +(chorda). + + +Thus the median primitive furrow (_pr_) +in the hind-half and the median medullary furrow (_Rf_) in the +fore-half of the oval shield are totally different structures, although +the latter seems to a superficial observer to be merely the forward +continuation of the former. Hence they were formerly always confused. +This error was the more pardonable as immediately afterwards the two +grooves do actually pass into each other in a very remarkable way. The +point of transition is the remarkable neurenteric canal (Fig. 124 +_cn_). But the direct connection which is thus established does not +last long; the two are soon definitely separated by a partition. + + +Fig.124. Longitudinal section of the coelomula of amphioxus. Fig. +124—Longitudinal section of the cœlomula of amphioxus (from the left). +_i_ entoderm, _d_ primitive gut, _cn_ medullary duct, _n_ nerve tube, +_m_ mesoderm, _s_ first primitive segment, _c_ cœlom-pouches. (From +_Hatschek._) + + +The enigmatic _neurenteric canal_ is a very old embryonic organ, and of +great phylogenetic interest, because it arises in the same way in all +the chordonia (both tunicates and vertebrates). In every case it +touches or embraces like an arch the posterior end of the chorda, which +has been developed here in front out of the middle line of the +primitive gut (between the two cœlom-folds of the sickle groove) +(“head-process,” Fig. 123 _kf_). These very ancient and strictly +hereditary structures, which have no physiological significance to-day, +deserve (as “rudimentary organs”) our closest attention. The tenacity +with which the useless neurenteric canal has been transmitted down to +man through the whole series of vertebrates is of equal interest for +the theory of descent in general, and the phylogeny of the chordonia in +particular. + +The connection which the neurenteric canal (Fig. 123 _cn_) establishes +between the dorsal nerve-tube (_n_) and the ventral gut-tube (_d_) is +seen very plainly in the amphioxus in a longitudinal section of the +cœlomula, as soon as the primitive mouth is completely closed at its +hinder end. The medullary tube has still at this stage an opening at +the forward end, the neuroporus Fig. 83 _np_). This opening also is +afterwards closed. There are then two completely closed canals over +each other—the medullary tube above and the gastric tube below, the two +being separated by the chorda. The same features as in the acrania are +exhibited by the related tunicates, the ascidiæ. + + +Fig.125. Longitudinal section of the chordula of a frog. Fig. +125—Longitudinal section of the chordula of a frog. (From _Balfour._) +_nc_ nerve-tube, _x_ canalis neurentericus, _al_ alimentary canal, _yk_ +yelk-cells, _m_ mesoderm. + + +Again, we find the neurenteric canal in just the same form and +situation in the amphibia. A longitudinal section of a young tadpole +(Fig. 125) shows how we may penetrate from the still open primitive +mouth (_x_) either into the wide primitive gut-cavity (_al_) or the +narrow overlying nerve-tube. A little later, when the primitive mouth +is closed, the narrow neurenteric canal (Fig. 126 _ne_) represents the +arched connection between the dorsal medullary canal (_mc_) and the +ventral gastric canal. + +In the amniotes this original curved form of the neurenteric canal +cannot be found at first, because here the primitive mouth travels +completely over to the dorsal surface of the gastrula, and is converted +into the longitudinal furrow we call the primitive groove. Hence the +primitive groove (Fig. 128 _pr_), examined from above, appears to be +the straight +continuation of the fore-lying and younger medullary furrow (_me_). The +divergent hind legs of the latter embrace the anterior end of the +former. Afterwards we have the complete closing of the primitive mouth, +the dorsal swellings joining to form the medullary tube and growing +over it. The neurenteric canal then leads directly, in the shape of a +narrow arch-shaped tube (Fig. 129 _ne_), from the medullary tube (_sp_) +to the gastric tube (_pag_). Directly in front of it is the latter end +of the chorda (_cli_). + + +Fig.126. Longitudinal section of a frog-embryo. Fig. 126—Longitudinal +section of a frog-embryo. (From _Goette._) _m_ mouth, _l_ liver, _an_ +anus, _ne_ canalis neurentericus, _mc_ medullary-tube, _pn_ pineal body +(epiphysis), _ch_ chorda. + + +While these important processes are taking place in the axial part of +the dorsal shield, its external form also is changing. The oval form +(Fig. 117) becomes like the sole of a shoe or sandal, lyre-shaped or +finger-biscuit shaped (Fig. 130). The middle third does not grow in +width as quickly as the posterior, and still less than the anterior +third; thus the shape of the permanent body becomes somewhat narrow at +the waist. At the same time, the oval form of the germinative area +returns to a circular shape, and the inner pellucid area separates more +clearly from the opaque outer area (Fig. 131 _a_). The completion of +the circle in the area marks the limit of the formation of +blood-vessels in the mesoderm. + + +Figs. 127 and 128. Dorsal shield of the chick. Figs. 127 and 128—Dorsal +shield of the chick. (From _Balfour._) The medullary furrow (_me_), +which is not yet visible in Fig. 130, encloses with its hinder end the +fore end of the primitive groove (_pr_) in Fig. 131.) + + +The characteristic sandal-shape of the dorsal shield, which is +determined by the narrowness of the middle part, and which is compared +to a violin, lyre, or shoe-sole, persists for a long time in all the +amniotes. All mammals, birds, and reptiles have substantially the same +construction at this stage, and even for a longer or shorter +period after the division of the primitive segments into the +cœlom-folds has begun (Fig. 132). The human embryonic shield assumes +the sandal-form in the second week of development; towards the end of +the week our sole-shaped embryo has a length of about one-twelfth of an +inch (Fig. 133). + + +Fig.129. Longitudinal section of the hinder end of a chick. Fig. +129—Longitudinal section of the hinder end of a chick. (From +_Balfour._) _sp_ medullary tube, connected with the terminal gut +(_pag_) by the neurenteric canal (_ne_), _ch_ chorda, _pr_ neurenteric +(or Hensen’s) ganglion, _al_ allantois, _ep_ ectoderm, _hy_ entoderm, +_so_ parietal layer, _sp_ visceral layer, _an_ anus-pit, _am_ amnion. + + +The complete bilateral symmetry of the vertebrate body is very early +indicated in the oval form of the embryonic shield (Fig. 117) by the +median primitive streak; in the sandal-form it is even more pronounced +(Figs. 131–135). In the lateral parts of the embryonic shield a darker +central and a lighter peripheral zone become more obvious; the former +is called the stem-zone (Fig. 134 _stz_), and the latter the parietal +zone (_pz_); from the first we get the dorsal and from the second the +ventral half of the body-wall. The stem-zone of the amniote embryo +would be called more appropriately the dorsal zone or dorsal shield; +from it develops the whole of the dorsal half of the later body (or +permanent body)—that is to say, the dorsal body (_episoma_). Again, it +would be better to call the “parietal zone” the ventral zone or ventral +shield; from it develop the ventral “lateral plates,” which afterwards +separate from the embryonic vesicle and form the ventral body +(_hyposoma_)—that is to say, the ventral half of the permanent body, +together with the body-cavity and the gastric canal that it encloses. + + +Fig.130. Germinal area or germinal disk of the rabbit, with sole-shaped +embryonic shield. Fig. 130—Germinal area or germinal disk of the +rabbit, with sole-shaped embryonic shield, magnified. The clear +circular field (_d_) is the opaque area. The pellucid area (_c_) is +lyre-shaped, like the embryonic shield itself (_b_). In its axis is +seen the dorsal furrow or medullary furrow (_a_). (From _Bischoff_. + + +The sole-shaped germinal shields of all the amniotes are still, at the +stage of construction which Fig. 134 illustrates in the rabbit and Fig. +135 in the opossum, so like each other that we can either not +distinguish them at all or only by means of quite subordinate +peculiarities in the size of the various parts. Moreover, the human +sandal-shaped embryo cannot at this stage be distinguished from those +of other mammals, and it particularly resembles that of the rabbit. On +the other hand, the outer form of these flat sandal-shaped embryos is +very different from the corresponding form of the lower animals, +especially the acrania (amphioxus). Nevertheless, the body is just the +same in the essential features of its structure as that we find in the +chordula of the latter (Figs. 83–86), and in the embryonic forms which +immediately develop from it. The striking external difference is here +again due to the fact that in the palingenetic embryos of the amphioxus +(Figs. 83, 84) and the amphibia (Figs. 85, 86) the gut-wall and +body-wall form closed tubes from the first, whereas in the cenogenetic +embryos of the amniotes they are forced to expand leaf-wise on the +surface owing to the great extension of the food-yelk. + +It is all the more notable that the early separation of dorsal and +ventral halves takes place in the same rigidly hereditary fashion in +all the vertebrates. In both the acrania and the craniota the dorsal +body is about this period separated from the ventral body. In the +middle part of the body this division has already taken place by the +construction of the chorda between the dorsal nerve-tube and the +ventral canal. But in the outer or lateral part of the body it is only +brought about by the division of the coelom-pouches into two sections—a +dorsal _episomite_ (dorsal segment or provertebra) and a ventral +_hyposomite_ (or ventral segment) by a frontal constriction. In the +amphioxus each of the former makes a muscular pouch, and each of the +latter a sex-pouch or gonad. + + +Fig.131. Embryo of the opossum, sixty hours old. Fig. 132. +Sandal-shaped embryonic shield of a rabbit of eight days. Fig. +131—Embryo of the opossum, sixty hours old, one-sixth of an inch in +diameter. (From _Selenka_) _b_ the globular embryonic vesicle, _a_ the +round germinative area, _b_ limit of the ventral plates, _r_ dorsal +shield, _v_ its fore part, _u_ the first primitive segment, _ch_ +chorda, _chr_ its fore-end, _pr_ primitive groove (or mouth). Fig. +132—Sandal-shaped embryonic shield of a rabbit of eight days, with the +fore part of the germinative area (_ao_ opaque, _ap_ pellucid area). +(From _Kölliker._) _rf_ dorsal furrow, in the middle of the medullary +plate, _h, pr_ primitive groove (mouth), _stz_ dorsal (stem) zone, _pz_ +ventral (parietal) zone. In the narrow middle part the first three +primitive segments may be seen. + + +These important processes of differentiation in the mesoderm, which we +will consider more closely in the next chapter, proceed step by step +with interesting changes in the ectoderm, while the entoderm changes +little at first. We can study these processes best in transverse +sections, made vertically to the surface through the sole-shaped +embryonic shield. Such a transverse section of a chick embryo, at the +end of the first day of incubation, shows the gut-gland layer as a very +simple epithelium, which is spread like a leaf over the outer surface +of the food-yelk (Fig. 92). The chorda (_ch_) has separated from the +dorsal middle line of the entoderm; to the right and left of it are the +two halves of the mesoderm, or the two cœlom-folds. A narrow cleft in +the latter indicates the body-cavity (_uwh_); this separates the two +plates of the cœlom-pouches, the lower (visceral) and upper (parietal). +The broad dorsal furrow (_rf_) formed by the medullary plate (_m_) is +still wide open, but is divided from the lateral horn-plate +(_h_) by the parallel medullary swellings, which eventually close. + + +Fig.133. Human embryo at the sandal-stage. Fig. 134. Sandal-shaped +embryonic shield of a rabbit of nine days. Fig. 133—Human embryo at the +sandal-stage, one-twelfth of an inch long, from the end of the second +week, magnified. (From _Count Spee._) Fig. 134—Sandal-shaped embryonic +shield of a rabbit of nine days. (From _Kölliker._) (Back view from +above.) _stz_ stem-zone or dorsal shield (with eight pairs of primitive +segments), _pz_ parietal or ventral zone, _ap_ pellucid area, _af_ +amnion-fold, _h_ heart, _ph_ pericardial cavity, _vo_ +omphalo-mesenteric vein, _ab_ eye-vesicles, _vh_ fore brain, _mh_ +middle brain, _hh_ hind brain, _uw_ primitive segments (or vertebræ). + + +During these processes important changes are taking place in the outer +germinal layer (the “skin-sense layer”). The continued rise and growth +of the dorsal swellings causes their higher parts to bend together at +their free borders, approach nearer and nearer (Fig. 136 _w_), and +finally unite. Thus in the end we get from the open dorsal furrow, the +upper cleft of which becomes narrower and narrower, a closed +cylindrical tube (Fig. 137 _mr_). This tube is of the utmost +importance; it is the beginning of the central nervous system, the +brain and spinal marrow, the _medullary tube._ This embryonic fact was +formerly looked upon as very mysterious. We shall see presently that in +the light of the theory of descent it is a thoroughly natural process. +The phylogenetic explanation of it is that the central nervous system +is the organ by means of which all intercourse with the outer world, +all psychic action and sense-perception, are accomplished; hence it was +bound to develop originally from the outer and upper surface of the +body, or from the outer skin. The medullary tube afterwards separates +completely from the outer germinal layer, and is surrounded by the +middle parts of the provertebræ and forced inwards (Fig. 146).The +remaining portion of the skin-sense layer (Fig. 93 _h_) is now called +the horn-plate or horn-layer, because from it is developed the whole of +the outer skin or epidermis, with all its horny appendages (nails, +hair, etc.). + +A totally different organ, the _prorenal_ +(primitive kidney) _duct_ (_ung_), is found to be developed at an early +stage from the ectoderm. This is originally a quite simple, +tube-shaped, lengthy duct, or straight canal, which runs from front to +rear at each side of the provertebræ (on the outer side, Fig. 93 +_ung_). It originates, it seems, out of the horn-plate at the side of +the medullary tube, in the gap that we find between the provertebral +and the lateral plates. The prorenal duct is visible in this gap even +at the time of the severance of the medullary tube from the horn-plate. +Other observers think that the first trace of it does not come from the +skin-sense layer, but the skin-fibre layer. + + +Fig.135. Sandal-shaped embryonic shield of an opossum. Fig. +135—Sandal-shaped embryonic shield of an opossum (_Didelphys_), three +days old. (From _Selenka._) (Back view from above.) _stz_ stem-zone or +dorsal shield (with eight pairs of primitive segments), _pz_ parietal +or ventral zone, _ap_ pellucid area, _ao_ opaque area, _hh_ halves of +the heart, _v_ fore-end, _h_ hind-end. In the median line we see the +chorda (_ch_) through the transparent medullary tube (_m_). _u_ +primitive segment, _pr_ primitive streak (or primitive mouth). + + +The inner germinal layer, or the gut-fibre layer (Fig. 93 _dd_), +remains unchanged during these processes. A little later, however, it +shows a quite flat, groove-like depression in the middle line of the +embryonic shield, directly under the chorda. This depression is called +the gastric groove or furrow. This at once indicates the future lot of +this germinal layer. As this ventral groove gradually deepens, and its +lower edges bend towards each other, it is formed into a closed tube, +the alimentary canal, in the same way as the medullary groove grows +into the medullary tube. The gut-fibre layer (Fig. 137 _f_), which lies +on the gut-gland layer (_d_), naturally follows it in its folding. +Moreover, the incipient gut-wall consists from the first of two layers, +internally the gut-gland layer and externally the gut-fibre layer. + +The formation of the alimentary canal resembles that of the medullary +tube to this extent—in both cases a straight groove or furrow arises +first of all in the middle line of a flat layer. The edges of this +furrow then bend towards each other, and join to form a tube (Fig. +137). But the two processes are really very different. The medullary +tube closes in its whole length, and forms a cylindrical tube, whereas +the alimentary canal remains open in the middle, and its cavity +continues for a long time in connection with the cavity of the +embryonic vesicle. The open connection between the two cavities is only +closed at a very late stage, by the construction of the navel. The +closing of the medullary tube is effected from both sides, the edges of +the groove joining together from right and left. But the closing of the +alimentary canal is not only effected from right and left, but also +from front and rear, the edges of the ventral groove growing together +from every side towards the navel. Throughout the three higher classes +of vertebrates the whole of this process of the construction of the gut +is closely connected with the formation of the navel, or with the +separation of the embryo from the yelk-sac or umbilical vesicle. + +In order to get a clear idea of this, we must understand carefully the +relation of the embryonic shield to the germinative area and the +embryonic vesicle. This is done best by a comparison of the five stages +which are shown in longitudinal section in Figs. 138–142. The embryonic +shield (_c_), which at first projects very slightly over the surface of +the germinative area, soon begins to rise higher above it, and to +separate from the embryonic vesicle. At this point the embryonic +shield, looked at from the dorsal surface, shows still the original +simple sandal-shape (Figs. 133–135). We do not yet see any trace of +articulation into head, neck, trunk, etc., or limbs. But the embryonic +shield has increased greatly in thickness, especially in the anterior +part. It now has the appearance of a thick, oval swelling, strongly +curved over the surface of the germinative area. It begins to sever +completely from the embryonic vesicle, with which it is connected at +the ventral surface. As this severance proceeds, the back bends more +and more; in proportion as the embryo grows the embryonic vesicle +decreases, and at last it merely hangs as a small vesicle from the +belly of the embryo (Fig. 142 _ds_). In consequence of the +growth-movements which cause this severance, a groove-shaped depression +is formed at the surface of the vesicle, the _limiting furrow,_ which +surrounds the vesicle in the shape of a pit, and a circular mound or +dam (Fig. 139 ks) is formed at the outside of this pit by the elevation +of the contiguous parts of the germinal vesicle. + + +Fig.136. Transverse section of the embryonic disk of a chick at the end +of the first day of incubation. Fig. 136—Transverse section of the +embryonic disk of a chick at the end of the first day of incubation, +magnified. The edges of the medullary plate (_m_), the medullary +swellings (_w_), which separate the medullary from the horn-plate +(_h_), are bending towards each other. At each side of the chorda +(_ch_) the primitive segment plates (_u_) have separated from the +lateral plates (_sp_). A gut-gland layer. (From _Remak._) + + +In order to understand clearly this important process, we may compare +the embryo to a fortress with its surrounding rampart and trench. The +ditch consists of the outer part of the germinative area, and comes to +an end at the point where the area passes into the vesicle. The +important fold of the middle germinal layer that brings about the +formation of the body-cavity spreads beyond the borders of the embryo +over the whole germinative area. At first this middle layer reaches as +far as the germinative area; the whole of the rest of the embryonic +vesicle consists in the beginning only of the two original limiting +layers, the outer and inner germinal layers. Hence, as far as the +germinative area extends the germinal layer splits into the two plates +we have already recognised in it, the outer skin-fibre layer and the +inner gut-fibre layer. These two plates diverge considerably, a clear +fluid gathering between them (Fig. 140 _am_). The inner plate, the +gut-fibre layer, remains on the inner layer of the embryonic vesicle +(on the gut-gland layer). The +outer plate, the skin-fibre layer, lies close on the outer layer of the +germinative area, or the skin-sense layer, and separates together with +this from the embryonic vesicle. From these two united outer plates is +formed a continuous membrane. This is the circular mound that rises +higher and higher round the whole embryo, and at last joins above it +(Figs. 139–142 _am_). To return to our illustration of the fortress, we +must imagine the circular rampart to be extraordinarily high and +towering far above the fortress. Its edges bend over like the combs of +an overhanging wall of rock that would enclose the fortress; they form +a deep hollow, and at last join together above. In the end the fortress +lies entirely within the hollow that has been formed by the growth of +the edges of this large rampart. + + +Fig.137. Three diagrammatic transverse sections of the embryonic disk +of the higher vertebrate, to show the origin of the tubular organs from +the bending germinal layers. Fig. 137—Three diagrammatic transverse +sections of the embryonic disk of the higher vertebrate, to show the +origin of the tubular organs from the bending germinal layers. In Fig. +_A_ the medullary tube (_n_) and the alimentary canal (_a_) are still +open grooves. In Fig. _B_ the medullary tube (_n_) and the dorsal wall +are closed, but the alimentary canal (_a_) and the ventral wall are +open; the prorenal ducts (_u_) are cut off from the horn-plate (_h_) +and internally connected with segmental prorenal canals. In Fig. _C_ +both the medullary tube and the dorsal wall above and the alimentary +canal and ventral wall below are closed. All the open grooves have +become closed tubes; the primitive kidneys are directed inwards. The +letters have the same meaning in all three figures: _h_ skin-sense +layer, _n_ medullary tube, _u_ prorenal ducts, _x_ axial rod, _s_ +primitive-vertebra, _r_ dorsal wall, _b_ ventral wall, _c_ body-cavity +or cœloma, _f_ gut-fibre layer, _t_ primitive artery (aorta), _v_ +primitive vein (subintestinal vein), _d_ gut-fibre layer, _a_ +alimentary canal. + + +As the two outer layers of the germinative area thus rise in a fold +about the embryo, and join above it, they come at last to form a +spacious sac-like membrane about it. This envelope takes the name of +the germinative membrane, or water-membrane, or _amnion_ (Fig. 142 +_am_). The embryo floats in a watery fluid, which fills the space +between the embryo and the amnion, and is called the amniotic fluid +(Figs. 141, 142 _ah_). We will deal with this remarkable formation and +with the allantois later on (Chapter XV). In front of the allantois the +yelk-sac or umbilical vesicle (_ds_), the remainder of the original +embryonic vesicle, starts from the open belly of the embryo (Fig. 138 +_kh_). In more advanced embryos, in which the gastric wall and the +ventral wall are nearly closed, it hangs out of the navel-opening in +the shape of a small vesicle with a stalk (Figs. 141, 142 _ds_). The +more the embryo grows, the smaller becomes the vitelline (yelk) sac. At +first the embryo looks like a small appendage of the large embryonic +vesicle. Afterwards it is the yelk-sac, or the remainder of the +embryonic vesicle, that seems a small pouch-like appendage of the +embryo (Fig. 142 _ds_). It ceases to have any significance in the end. +The very wide opening, through which the gastric cavity at first +communicates with the umbilical vesicle, becomes narrower and narrower, +and at last disappears altogether. The _navel,_ the small pit-like +depression that we find in the developed man in the middle of the +abdominal wall, is the spot at which the remainder of the embryonic +vesicle (the umbilical vesicle) originally entered into the ventral +cavity, and joined on to the growing gut. + +The origin of the navel coincides with the complete closing of the +external ventral wall. In the amniotes the ventral wall originates in +the same way as the dorsal wall. Both are formed substantially from the +skin-fibre layer, and externally covered with the horn-plate, the +border section of the skin-sense layer. Both come into + +existence by the conversion of the four flat germinal layers of the +embryonic shield into a double tube by folding from opposite +directions; above, at the back, we have the vertebral canal which +encloses the medullary tube, and below, at the belly, the wall of the +body-cavity which contains the alimentary canal (Fig. 137). + + +Figs. 138 to 142. Five diagrammatic longitudinal sections of the +maturing mammal embryo and its envelopes. Figs. 138–142—Five +diagrammatic longitudinal sections of the maturing mammal embryo and +its envelopes. In Figs. 138–141 the longitudinal section passes through +the sagittal or middle plane of the body, dividing the right and left +halves; in Fig. 142 the embryo is seen from the left side. In Fig. 138 +the tufted it prochorion (_dd′_) encloses the germinal vesicle, the +wall of which consists of the two primary layers. Between the outer +(_a_) and inner (_i_) layer the middle layer (_m_) has been developed +in the region of the germinative area. In Fig. 139 the embryo (_e_) +begins to separate from the embryonic vesicle (_ds_), while the wall of +the amnion-fold rises about it (in front as head-sheath, _ks,_ behind +as tail-sheath, _ss_). In Fig. 140 the edges of the amniotic fold +(_am_) rise together over the back of the embryo, and form the amniotic +cavity (_ah_); as the embryo separates more completely from the +embryonic vesicle (_ds_) the alimentary canal (_dd_) is formed, from +the hinder end of which the allantois grows (_al_). In Fig. 141 the +allantois is larger; the yelk-sac (_ds_) smaller. In Fig. 142 the +embryo shows the gill-clefts and the outline of the two legs; the +chorion has formed branching villi (tufts.) In all four figures +_e_=embryo, _a_ outer germinal layer, _m_ middle germinal layer, _i_ +inner germinal layer, _am_ amnion (_ks_ head-sheath, _ss_ tail-sheath), +_ah_ amniotic cavity, _as_ amniotic sheath of the umbilical cord, _kh_ +embryonic vesicle, _ds_ yelk-sac (umbilical vesicle), _dg_ vitelline +duct, _df_ gut-fibre layer, _dd_ gut-gland layer, _al_ allantois, +_vl=hh_ place of heart, _d_ vitelline membrane (ovolemma or +prochorion), _d′_ tufts or villi of same, _sh_ serous membrane +(serolemma), _sz_ tufts of same, _ch_ chorion, _chz_ tufts or villi, +_st_ terminal vein, _r_ pericœlom or serocœlom (the space, filled with +fluid, between the amnion and chorion). (From _Kölliker._) + + +Figs. 143 and 144. Transverse sections of embryos (of chicks). Figs. +143–144—Transverse sections of embryos (of chicks). Fig. 143 of the +second, Fig. 144 of the third, Fig. 145 of the fourth, and Fig. 146 of +the fifth day of incubation. Fig. 143–145 from _Kölliker,_ magnified; +Fig. 146 from _Remak,_ magnified. _h_ horn-plate, _mr_ medullary tube, +_ung_ prorenal duct, _un_ prorenal vesicles, _hp_ skin-fibre layer, +_m=mu=mp_ muscle-plate, _uw_ provertebral plate (_wh_ cutaneous +rudiment of the body of the vertebra, _wb_ of the arch of the vertebra, +_wq_ the rib or transverse continuation), _uwh_ provertebral cavity, +_ch_ axial rod or chorda, _sh_ chorda-sheath, _bh_ ventral wall, _g_ +hind and _v_ fore root of the spinal nerves, _a=af=am_ amniotic fold, +_p_ body-cavity or cœloma, _df_ gut-fibre layer, _ao_ primitive aortas, +_sa_ secondary aorta, _vc_ cardinal veins, _d=dd_ gut-gland layer, _dr_ +gastric groove. In Fig. 143 the larger part of the right half, in Fig. +144 the larger part of the left half, of the section is omitted. Of the +yelk-sac or remainder of the embryonic vesicle only a small piece of +the wall is indicated below. + + +We will consider the formation of the dorsal wall first, and that of +the ventral wall afterwards (Figs. 143–147). In the middle of the +dorsal surface of the embryo there is originally, as we already know, +the medullary (_mr_) tube directly underneath the horn-plate (_h_), +from the middle part of which it has been developed. Later, however, +the provertebral plates (_uw_) grow over from the right and left +between these originally connected parts (Figs. 145, 146). The upper +and inner edges of the two provertebral plates push between the +horn-plate and medullary tube, force them away from each other, and +finally join between them in a seam that corresponds to the middle line +of the back. The coalescence of these two dorsal plates and the closing +in the middle of the dorsal wall take place in the same way as the +medullary tube, which is henceforth enclosed by the vertebral tube. +Thus is formed the dorsal wall, and the medullary tube takes up a +position inside the body. In the same way the provertebral mass grows +afterwards round the chorda, and forms the vertebral column. Below this +the inner and outer edge of the provertebral plate splits on each side +into two horizontal plates, of which the upper pushes between the +chorda and medullary tube, and the lower between the chorda and gastric +tube. As the plates meet from both sides above and below the chorda, +they completely enclose it, and so form the tubular, outer +chord-sheath, the sheath from which the vertebral column is formed +(_perichorda,_ Fig. 137 _C, s_; Figs. 145 _uwh,_ 146). + +We find in the construction of the ventral wall precisely the same +processes +as in the formation of the dorsal wall (Fig. 137 _B,_ Fig. 144 _hp,_ +Fig. 146 _bh_). It is formed on the flat embryonic shield of the +amniotes from the upper plates of the parietal zone. The right and left +parietal plates bend downwards towards each other, and grow round the +gut in the same way as the gut itself closes. The outer part of the +lateral plates forms the ventral wall or the lower wall of the body, +the two lateral plates bending considerably on the inner side of the +amniotic fold, and growing towards each other from right and left. +While the alimentary canal is closing, the body-wall also closes on all +sides. Hence the ventral wall, which encloses the whole ventral cavity +below, consists of two parts, two lateral plates that bend towards each +other. These approach each other all along, and at last meet at the +navel. We ought, therefore, really to distinguish two navels, an inner +and an outer one. The internal or intestinal navel is the definitive +point of the closing of the gut wall, which puts an end to the open +communication between the ventral cavity and the cavity of the yelk-sac +(Fig. 105). The external navel in the skin is the definitive point of +the closing of the ventral wall; this is visible in the developed body +as a small depression. + + +Figs. 145 and 146. Transverse sections of embryos (of chicks). Figs. +145–146—Transverse sections of embryos (of chicks). Fig. 143 of the +second, Fig. 144 of the third, Fig. 145 of the fourth, and Fig. 146 of +the fifth day of incubation. Fig. 143–145 from _Kölliker,_ magnified; +Fig. 146 from _Remak,_ magnified. _h_ horn-plate, _mr_ medullary tube, +_ung_ prorenal duct, _un_ prorenal vesicles, _hp_ skin-fibre layer, +_m=mu=mp_ muscle-plate, _uw_ provertebral plate (_wh_ cutaneous +rudiment of the body of the vertebra, _wb_ of the arch of the vertebra, +_wq_ the rib or transverse continuation), _uwh_ provertebral cavity, +_ch_ axial rod or chorda, _sh_ chorda-sheath, _bh_ ventral wall, _g_ +hind and _v_ fore root of the spinal nerves, _a=af=am_ amniotic fold, +_p_ body-cavity or cœloma, _df_ gut-fibre layer, _ao_ primitive aortas, +_sa_ secondary aorta, _vc_ cardinal veins, _d=dd_ gut-gland layer, _dr_ +gastric groove. In Fig. 143 the larger part of the right half, in Fig. +144 the larger part of the left half, of the section is omitted. Of the +yelk-sac or remainder of the embryonic vesicle only a small piece of +the wall is indicated below. + + +With the formation of the internal navel and the closing of the +alimentary canal is connected the formation of two cavities, which we +call the capital and the pelvic sections of the visceral cavity. As the +embryonic shield lies flat on the wall of the embryonic vesicle at +first, and only gradually separates from it, its fore and hind ends are +independent in the beginning; on the other hand, the middle part of the +ventral surface is connected with the yelk-sac by means of the +vitelline or umbilical duct (Fig. 147 _m_). This leads to a notable +curving of the dorsal surface; the head-end bends downwards towards the +breast and the tail-end towards the belly. We see this very clearly in +the excellent old diagrammatic illustration given by Baer (Fig. 147), a +median longitudinal section of the embryo of the chick, in which the +dorsal body or episoma is deeply shaded. The embryo seems to be trying +to roll up, like a hedgehog protecting itself from its pursuers. This +pronounced curve of the back is due to the more rapid growth of the +convex dorsal surface, and is directly connected with the severance of +the embryo from the yelk-sac. The further bending of the embryo leads +to the formation of the “head-cavity” of the gut (Fig. 148 above _D_) +and a similar one at the tail, known as its “pelvic cavity.” + + +Fig.147. Median longitudinal section of the embryo of a chick (fifth +day of incubation), seen from the right side (head to the right, tail +to the left). Fig. 147—Median longitudinal section of the embryo of a +chick (fifth day of incubation), seen from the right side (head to the +right, tail to the left). Dorsal body dark, with convex outline. _d_ +gut, _o_ mouth, _a_ anus, _l_ lungs, _h_ liver, _g_ mesentery, _v_ +auricle of the heart, _k_ ventricle of the heart, _b_ arch of the +arteries, _t_ aorta, _c_ yelk-sac, _m_ vitelline (yelk) duct, _u_ +allantois, _r_ pedicle (stalk) of the allantois, _n_ amnion, _w_ +amniotic cavity, _s_ serous membrane. (From _Baer._) + + +As a result of these processes the embryo attains a shape that may be +compared to a wooden shoe, or, better still, to an overturned canoe. +Imagine a canoe or boat with both ends rounded and a small covering +before and behind; if this canoe is turned upside down, so that the +curved keel is uppermost, we have a fair picture of the canoe-shaped +embryo (Fig. 147). The upturned convex keel corresponds to the middle +line of the back; the small chamber underneath the fore-deck represents +the capital cavity, and the small chamber under the rear-deck the +pelvic chamber of the gut (cf. Fig. 140). + +The embryo now, as it were, presses into the outer surface of the +embryonic vesicle with its free ends, while it moves away from it with +its middle part. As a result of this change the yelk-sac becomes +henceforth only a pouch-like outer appendage at the middle of the +ventral wall. The ventral appendage, growing smaller and smaller, is +afterwards called the umbilical (navel) vesicle. The cavity of the +yelk-sac or umbilical vesicle communicates with the corresponding +visceral cavity by a wide opening, which gradually contracts into a +narrow and long canal, the vitelline (yelk) duct (_ductus vitellinus,_ +Fig. 147 _m_). Hence, if we were to imagine ourselves in +the cavity of the yelk-sac, we could get from it through the yelk-duct +into the middle and still wide open part of the alimentary canal. If we +were to go forward from there into the head-part of the embryo, we +should reach the capital cavity of the gut, the fore-end of which is +closed up. + +The reader will ask: “Where are the mouth and the anus?” These are not +at first present in the embryo. The whole of the primitive gut-cavity +is completely closed, and is merely connected in the middle by the +vitelline duct with the equally closed cavity of the embryonic vesicle +(Fig. 140). The two later apertures of the alimentary canal—the anus +and the mouth—are secondary constructions, formed from the outer skin. +In the horn-plate, at the spot where the mouth is found subsequently, a +pit-like depression is formed, and this grows deeper and deeper, +pushing towards the blind fore-end of the capital cavity; this is the +mouth-pit. In the same way, at the spot in the outer skin where the +anus is afterwards situated a pit-shaped depression appears, grows +deeper and deeper, and approaches the blind hind-end of the pelvic +cavity; this is the anus-pit. In the end these pits touch with their +deepest and innermost points the two blind ends of the primitive +alimentary canal, so that they are now only separated from them by thin +membranous partitions. This membrane finally disappears, and henceforth +the alimentary canal opens in front at the mouth and in the rear by the +anus (Figs. 141, 147). Hence at first, if we penetrate into these pits +from without, we find a partition cutting them off from the cavity of +the alimentary canal, which gradually disappears. The formation of +mouth and anus is secondary in all the vertebrates. + + +Fig.148. Longitudinal section of the fore half of a chick-embryo at the +end of the first day of incubation (seen from the left side). Fig. +148—Longitudinal section of the fore half of a chick-embryo at the end +of the first day of incubation (seen from the left side). _k_ +head-plates, _ch_ chorda. Above it is the blind fore-end of the ventral +tube (_m_); below it the capital cavity of the gut. _d_ gut-gland +layer, _df_ gut-fibre layer, _h_ horn plate, _hh_ cavity of the heart, +_hk_ heart-capsule, _ks_ head-sheath, _kk_ head-capsule. (From +_Remak._) + + +During the important processes which lead to the formation of the +navel, and of the intestinal wall and ventral wall, we find a number of +other interesting changes taking place in the embryonic shield of the +amniotes. These relate chiefly to the prorenal ducts and the first +blood-vessels. The prorenal (primitive kidney) ducts, which at first +lie quite flat under the horn-plate or epiderm (Fig. 93 _ung_), soon +back towards each other in consequence of special growth movements +(Figs. 143–145 _ung_). They depart more and more from their point of +origin, and approach the gut-gland layer. In the end they lie deep in +the interior, on either side of the mesentery, underneath the chorda, +(Fig. 145 _ung_). At the same time, the two primitive aortas change +their position (cf. Figs. 138–145 _ao_); they travel inwards underneath +the chorda, and there coalesce at last to form a single secondary +aorta, which is found under the rudimentary vertebral column (Fig. 145 +_ao_). The cardinal veins, the first venous blood-vessels, also back +towards each other, and eventually unite immediately above the +rudimentary kidneys (Figs. 145 _vc,_ 152 _cav_). In the same spot, at +the inner side of the fore-kidneys, we soon see the first trace of the +sexual organs. The most important part of this apparatus (apart from +all its appendages) is the ovary in the female and the testicle + +in the male. Both develop from a small part of the cell-lining of the +body-cavity, at the spot where the skin-fibre layer and gut-fibre layer +touch. The connection of this embryonic gland with the prorenal ducts, +which lie close to it and assume most important relations to it, is +only secondary. + + +Fig.149. Longitudinal section of a human embryo of the fourth week, +one-fifth of an inch long. Fig. 149—Longitudinal section of a human +embryo of the fourth week, one-fifth of an inch long, magnified. (From +_Kollmann._) + + + +Fig.150. Transverse section of a human embryo of fourteen days. Fig. +151. Transverse section of a shark-embryo (or young selachius). Fig. +150—Transverse section of a human embryo of fourteen days. _mr_ +medullary tube, _ch_ chorda. _vu_ umbilical vein, _mt_ myotome, _mp_ +middle plate, _ug_ prorenal duct, _lh_ body-cavity, _e_ ectoderm, _bh_ +ventral skin, _hf_ skin-fibre layer, _df_ gut-fibre layer. (From +_Kollmann._) +Fig. 151—Transverse section of a shark-embryo (or young selachius). +_mr_ medullary tube, _ch_ chorda, _a_ aorta, _d_ gut, _vp_ principal +(or subintestinal) vein, _mt_ myotome, _mm_ muscular mass of the +provertebra, _mp_ middle plate, _ug_ prorenal duct, _lh_ body-cavity, +_e_ ectoderm of the rudimentary extremities, _mz_ mesenchymic cells, +_z_ point where the myotome and nephrotome separate. (From _H. E. +Ziegler._) + + +Fig.152. Transverse section of a duck-embryo with twenty-four primitive +segments. Fig. 152—Transverse section of a duck-embryo with twenty-four +primitive segments. (From _Balfour._) From a dorsal lateral joint of +the medullary tube (_spc_) the spinal ganglia (_spg_) grow out between +it and the horn-plate. _ch_ chorda, _ao_ double aorta, _hy_ gut-gland +layer, _sp_ gut-fibre layer, with blood-vessels in section, _ms_ muscle +plate, in the dorsal wall of the myocœl (episomite). Below the cardinal +vein (_cav_) is the prorenal duct (_wd_) and a segmental prorenal canal +(_st_). The skin-fibre layer of the body-wall (_so_) is continued in +the amniotic fold (_am_). Between the four secondary germinal layers +and the structures formed from them there is formed embryonic +connective matter with stellate cells and vascular structures +(Hertwig’s “mesenchym”). + + + + +Chapter XIV. +THE ARTICULATION OF THE BODY[26] + + + [26] The term articulation is used in this chapter to denote both + “segmentation” and “articulation” in the ordinary sense.—Translator. + + +The vertebrate stem, to which our race belongs as one of the latest and +most advanced outcomes of the natural development of life, is rightly +placed at the head of the animal kingdom. This privilege must be +accorded to it, not only because man does in point of fact soar far +above all other animals, and has been lifted to +the position of “lord of creation”; but also because the vertebrate +organism far surpasses all the other animal-stems in size, in +complexity of structure, and in the advanced character of its +functions. From the point of view of both anatomy and physiology, the +vertebrate stem outstrips all the other, or invertebrate, animals. + +There is only one among the twelve stems of the animal kingdom that can +in many respects be compared with the vertebrates, and reaches an +equal, if not a greater, importance in many points. This is the stem of +the articulates, composed of three classes: 1, the annelids +(earth-worms, leeches, and cognate forms); 2, the crustacea (crabs, +etc.); 3, the tracheata (spiders, insects, etc.). The stem of the +articulates is superior not only to the vertebrates, but to all other +animal-stems, in variety of forms, number of species, elaborateness of +individuals, and general importance in the economy of nature. + +When we have thus declared the vertebrates and the articulates to be +the most important and most advanced of the twelve stems of the animal +kingdom, the question arises whether this special position is accorded +to them on the ground of a peculiarity of organisation that is common +to the two. The answer is that this is really the case; it is their +segmental or transverse articulation, which we may briefly call +metamerism. In all the vertebrates and articulates the developed +individual consists of a series of successive members (segments or +metamera = “parts”); in the embryo these are called primitive segments +or somites. In each of these segments we have a certain group of organs +reproduced in the same arrangement, so that we may regard each segment +as an individual unity, or a special “individual” subordinated to the +entire personality. + +The similarity of their segmentation, and the consequent physiological +advance in the two stems of the vertebrates and articulates, has led to +the assumption of a direct affinity between them, and an attempt to +derive the former directly from the latter. The annelids were supposed +to be the direct ancestors, not only of the crustacea and tracheata, +but also of the vertebrates. We shall see later (Chapter XX) that this +annelid theory of the vertebrates is entirely wrong, and ignores the +most important differences in the organisation of the two stems. The +internal articulation of the vertebrates is just as profoundly +different from the external metamerism of the articulates as are their +skeletal structure, nervous system, vascular system, and so on. The +articulation has been developed in a totally different way in the two +stems. The unarticulated chordula (Figs. 83–86), which we have +recognised as one of the chief palingenetic embryonic forms of the +vertebrate group, and from which we have inferred the existence of a +corresponding ancestral form for all the vertebrates and tunicates, is +quite unthinkable as the stem-form of the articulates. + +All articulated animals came originally from unarticulated ones. This +phylogenetic principle is as firmly established as the ontogenetic fact +that every articulated animal-form develops from an unarticulated +embryo. But the organisation of the embryo is totally different in the +two stems. The chordula-embryo of all the vertebrates is characterised +by the dorsal medullary tube, the neurenteric canal, which passes at +the primitive mouth into the alimentary canal, and the axial chorda +between the two. None of the articulates, either annelids or arthropods +(crustacea and tracheata), show any trace of this type of organisation. +Moreover, the development of the chief systems of organs proceeds in +the opposite way in the two stems. Hence the segmentation must have +arisen independently in each. This is not at all surprising; we find +analogous cases in the stalk-articulation of the higher plants and in +several groups of other animal stems. + +The characteristic internal articulation of the vertebrates and its +importance in the organisation of the stem are best seen in the study +of the skeleton. Its chief and central part, the cartilaginous or bony +vertebral column, affords an obvious instance of vertebrate metamerism; +it consists of a series of cartilaginous or bony pieces, which have +long been known as _vertebræ_ (or _spondyli_). Each vertebra is +directly connected with a special section of the muscular system, the +nervous system, the vascular system, etc. Thus most of the “animal +organs” take part in this vertebration. But we saw, when we were +considering our own vertebrate character (in Chapter XI), that the same +internal articulation is also found in the lowest primitive +vertebrates, the acrania, although here the whole skeleton consists +merely of the simple chorda, and is not at all articulated. +Hence the articulation does not proceed primarily from the skeleton, +but from the muscular system, and is clearly determined by the more +advanced swimming-movements of the primitive chordonia-ancestors. + + +Figs. 153-155. Sole-shaped embryonic disk of the chick, in three +successive stages of development, looked at from the dorsal surface, +magnified, somewhat diagrammatic. Figs. 153–155—Sole-shaped embryonic +disk of the chick, in three successive stages of development, looked at +from the dorsal surface, magnified, somewhat diagrammatic. Fig. 153 +with six pairs of somites. Brain a simple vesicle (_hb_). Medullary +furrow still wide open from _x_; greatly widened at _z. mp_ medullary +plates, _sp_ lateral plates, _y_ limit of gullet-cavity (_sh_) and +fore-gut (_vd_). Fig. 154 with ten pairs of somites. Brain divided into +three vesicles: _v_ fore-brain, _m_ middle-brain, _h_ hind-brain, _c_ +heart, _dv_ vitelline-veins. Medullary furrow still wide open behind +(_z_). _mp_ medullary plates. Fig. 155 with sixteen pairs of somites. +Brain divided into five vesicles: _v_ fore-brain, _z_ +intermediate-brain, _m_ middle-brain, _h_ hind-brain, _n_ after-brain, +_a_ optic vesicles, _g_ auditory vesicles, _c_ heart, _ dv_ vitelline +veins, _mp_ medullary plate, _uw_ primitive vertebra. + + +It is, therefore, wrong to describe the first rudimentary segments in +the vertebrate embryo as primitive vertebræ or provertebræ; the fact +that they have been so called for some time has led to much error and +misunderstanding. Hence we shall give the name of “somites” or +primitive segments to these so-called “primitive vertebræ.” If the +latter name is retained at all, it should only be used of the +sclerotom—i.e., the small part of the somites from which the later +vertebra does actually develop. + +Articulation begins in all vertebrates at a very early embryonic stage, +and this indicates the considerable phylogenetic age of the process. +When the chordula (Figs. 83–86) has completed its characteristic +composition, often even a little earlier, we find in the amniotes, in +the +middle of the sole-shaped embryonic shield, several pairs of dark +square spots, symmetrically distributed on both sides of the chorda +(Figs. 131–135).Transverse sections (Fig. 93 _uw_) show that they +belong to the stem-zone (episoma) of the mesoderm, and are separated +from the parietal zone (hyposoma) by the lateral folds; in section they +are still quadrangular, almost square, so that they look something like +dice. These pairs of “cubes” of the mesoderm are the first traces of +the primitive segments or somites, the so-called “protovertebræ.” +(Figs. 153–155 _uw_). + + +Fig.156. Embryo of the amphioxus, sixteen hours old, seen from the +back. Fig. 156—Embryo of the amphioxus, sixteen hours old, seen from +the back. (From _Hatschek._) _d_ primitive gut, _u_ primitive mouth, +_p_ polar cells of the mesoderm, _ c_ cœlom-pouches, _m_ their first +segment, _n_ medullary tube, _i_ entoderm, _e_ ectoderm, _s_ first +segment-fold. + + +Among the mammals the embryos of the marsupials have three pairs of +somites (Fig. 131) after sixty hours, and eight pairs after seventy-two +hours (Fig. 135). They develop more slowly in the embryo of the rabbit; +this has three somites on the eighth day (Fig. 132), and eight somites +a day later (Fig. 134). In the incubated hen’s egg the first somites +make their appearance thirty hours after incubation begins (Fig. 153). +At the end of the second day the number has risen to sixteen or +eighteen (Fig. 155). The articulation of the stem-zone, to which the +somites owe their origin, thus proceeds briskly from front to rear, new +transverse constrictions of the “protovertebral plates” forming +continuously and successively. The first segment, which is almost +half-way down in the embryonic shield of the amniote, is the foremost +of all; from this first somite is formed the first cervical vertebra +with its muscles and skeletal parts. It follows from this, firstly, +that the multiplication of the primitive segments proceeds backwards +from the front, with a constant lengthening of the hinder end of the +body; and, secondly, that at the beginning of segmentation nearly the +whole of the anterior half of the sole-shaped embryonic shield of the +amniote belongs to the later head, while the whole of the rest of the +body is formed from its hinder half. We are reminded that in the +amphioxus (and in our hypothetic primitive vertebrate, Figs. 98–102) +nearly the whole of the fore half corresponds to the head, and the hind +half to the trunk. + + +Fig.157. Embryo of the amphioxus, twenty hours old, with five somites. +Fig. 157—Embryo of the amphioxus, twenty hours old, with five somites. +(Right view; for left view see Fig. 124.) (From _Hatschek._) _ V_ fore +end, _H_ hind end. _ak, mk, ik_ outer, middle, and inner germinal +layers; _dh_ alimentary canal, _n_ neural tube, _cn_ canalis +neurentericus, _ush_ cœlom-pouches (or primitive-segment cavities), +_us1_ first (and foremost) primitive segment. + + +The number of the metamera, and of the embryonic somites or primitive +segments from which they develop, varies considerably in the +vertebrates, according as the hind part of the body is short or is +lengthened by a tail. In the developed man the trunk (including the +rudimentary tail) consists of thirty-three metamera, the solid centre +of which is formed by that number of vertebræ in the vertebral column +(seven cervical, twelve dorsal, five lumbar, five sacral, and four +caudal). To these we must add at least nine head-vertebræ, which +originally (in all the craniota) constitute the skull. Thus the total +number of the primitive segments of the human +body is raised to at least forty-two; it would reach forty-five to +forty-eight if (according to recent investigations) the number of the +original segments of the skull is put at twelve to fifteen. In the +tailless or anthropoid apes the number of metamera is much the same as +in man, only differing by one or two; but it is much larger in the +long-tailed apes and most of the other mammals. In long serpents and +fishes it reaches several hundred (sometimes 400). + + +Figs. 158-160. Embryo of the amphioxus, twenty four hours old, with +eight somites. Figs. 158–160—Embryo of the amphioxus, twenty four hours +old, with eight somites. (From _Hatschek._) Figs. 158 and 159 lateral +view (from left). Fig. 160 seen from back. In Fig. 158 only the +outlines of the eight primitive segments are indicated, in Fig. 159 +their cavities and muscular walls. _V_ fore end, _H_ hind end, _d_ gut, +_du_ under and _ dd_ upper wall of the gut, _ne_ canalis neurentericus, +_ nv_ ventral, _nd_ dorsal wall of the neural tube, _np_ neuroporus, +_dv_ fore pouch of the gut, _ch_ chorda, _ mf_ mesodermic fold, _pm_ +polar cells of the mesoderm (_ms_), _e_ ectoderm. + + +In order to understand properly the real nature and origin of +articulation in the human body and that of the higher vertebrates, it +is necessary to compare it with that of the lower vertebrates, and bear +in mind always the genetic connection of all the members of the stem. +In this the simple development of the invaluable amphioxus once more +furnishes the key to the complex and cenogenetically modified embryonic +processes of the craniota. The articulation of the amphioxus begins at +an early stage—earlier than in the craniotes. The two cœlom-pouches +have hardly grown out of the primitive gut (Fig. 156 _c_) when the +blind fore part of it (farthest away from the primitive mouth, _u_) +begins to separate by a transverse fold (_s_): this is the first +primitive segment. Immediately afterwards the hind part of the +cœlom-pouches begins to divide into a series of pieces by new +transverse folds (Fig. 157). The foremost of these primitive segments +(_us_1) is the first and oldest; in Figs. 124 and 157 there are already +five formed. They separate so rapidly, one behind the other, that eight +pairs are formed within twenty-four hours of the beginning of +development, and seventeen pairs twenty-four hours later. The number +increases as the embryo grows and extends +backwards, and new cells are formed constantly (at the primitive mouth) +from the two primitive mesodermic cells (Figs. 159–160). + + +Figs. 161 and 162. Transverse section of shark-embryos (through the +region of the kidneys). Figs. 161 and 162—Transverse section of +shark-embryos (through the region of the kidneys). (From _Wijhe_ and +_Hertwig._) In Fig. 162 the dorsal segment-cavities (_h_) are already +separated from the body-cavity (_lh_), but they are connected a little +earlier (Fig. 161), _nr_ neural tube, _ch_ chorda, _sch_ subchordal +string, _ao_ aorta, _sk_ skeletal-plate, _mp_ muscle-plate, _cp_ +cutis-plate, _ w_ connection of latter (growth-zone), _vn_ primitive +kidneys, _ug_ prorenal duct, _uk_ prorenal canals, _ us_ point where +they are cut off, _tr_ prorenal funnel, _ mk_ middle germ-layer (_mk_1 +parietal, _ mk_2 visceral), _ik_ inner germ-layer (gut-gland layer). + + +This typical articulation of the two cœlom-sacs begins very early in +the lancelet, before they are yet severed from the primitive gut, so +that at first each segment-cavity (_us_) still communicates by a narrow +opening with the gut, like an intestinal gland. But this opening soon +closes by complete severance, proceeding regularly backwards. The +closed segments then extend more, so that their upper half grows +upwards like a fold between the ectoderm (_ak_) and neural tube (_n_), +and the lower half between the ectoderm and alimentary canal (_ch_; +Fig. 82 _d,_ left half of the figure). Afterwards the two halves +completely separate, a lateral longitudinal fold cutting between them +(_mk,_ right half of Fig. 82). The dorsal segments (_sd_) provide the +muscles of the trunk the whole length of the body (159): this cavity +afterwards disappears. On the other hand, the ventral parts give rise, +from their uppermost section, to the pronephridia or primitive-kidney +canals, and from the lower to the segmental rudiments of the sexual +glands or gonads. The partitions of the muscular dorsal pieces +(_myotomes_) remain, and determine the permanent articulation of the +vertebrate organism. But the partitions of the large ventral pieces +(_gonotomes_) become thinner, and afterwards disappear in part, so that +their cavities run together to form the metacœl, or the simple +permanent body-cavity. + +The articulation proceeds in substantially the same way in the other +vertebrates, the craniota, starting from the cœlom-pouches. But whereas +in the former case there is first a transverse division of the +cœlom-sacs (by vertical folds) and then the dorso-ventral division, the +procedure is reversed in the craniota; in their case each of the long +cœlom-pouches first divides into a dorsal (primitive segment plates) +and a ventral (lateral plates) section by a lateral longitudinal fold. +Only the former are then broken up into primitive segments by the +subsequent vertical folds; while the latter (segmented +for a time in the amphioxus) remain undivided, and, by the divergence +of their parietal and visceral plates, form a body-cavity that is +unified from the first. In this case, again, it is clear that we must +regard the features of the younger craniota as cenogenetically modified +processes that can be traced palingenetically to the older acrania. + +We have an interesting intermediate stage between the acrania and the +fishes in these and many other respects in the cyclostoma (the hag and +the lamprey, cf. Chapter XXI). + + +Fig.163. Frontal (or horizontal-longitudinal) section of a +triton-embryo with three pairs of primitive segments. Fig. 163—Frontal +(or horizontal-longitudinal) section of a triton-embryo with three +pairs of primitive segments. _ch_ chorda, _us_ primitive segments, +_ush_ their cavity, _ ak_ horn plate. + + +Among the fishes the selachii, or primitive fishes, yield the most +important information on these and many other phylogenetic questions +(Figs. 161 and 162). The careful studies of Rückert, Van Wijhe, H. E. +Ziegler, and others, have given us most valuable results. The products +of the middle germinal layer are partly clear in these cases at the +period when the dorsal primitive segment cavities (or myocœls, _h_) are +still connected with the ventral body-cavity (_lh_; Fig. 161). In Fig. +162, a somewhat older embryo, these cavities are separated. The outer +or lateral wall of the dorsal segment yields the cutis-plate (_cp_), +the foundation of the connective corium. From its inner or median wall +are developed the muscle-plate (_mp,_ the rudiment of the +trunk-muscles) and the skeletal plate, the formative matter of the +vertebral column (_sk_). + +In the amphibia, also, especially the water-salamander (_Triton_), we +can observe very clearly the articulation of the cœlom-pouches and the +rise of the primitive segments from their dorsal half (cf. Fig. 91, _A, +B, C_). A horizontal longitudinal section of the salamander-embryo +(Fig. 163) shows very clearly the series of pairs of these vesicular +dorsal segments, which have been cut off on each side from the ventral +side-plates, and lie to the right and left of the chorda. + + +Fig.164. The third cervical vertebra (human)> Fig. 165. The sixth +dorsal vertebra (human). Fig. 166. The second lumbar vertebra (human). +Fig. 164—The third cervical vertebra (human). Fig. 165—The sixth dorsal +vertebra (human). +Fig. 166—The second lumbar vertebra (human). + + +The metamerism of the amniotes agrees in all essential points with that +of the three lower classes of vertebrates we have considered; but it +varies considerably in detail, in consequence of cenogenetic +disturbances that are due in the first place (like the degeneration of +the cœlom-pouches) to the large development of the food-yelk. As the +pressure of this seems to force the two middle layers together from the +start, and as the solid structure of the mesoderm apparently belies the +original hollow character of the sacs, the two sections of the +mesoderm, which are at that time divided by the lateral fold—the dorsal +segment-plates and ventral side-plates—have the appearance at first of +solid layers of cells (Figs. 94–97). And when the articulation of the +somites begins in the sole-shaped embryonic shield, and a couple of +protovertebræ are developed in succession, constantly increasing in +number towards the rear, these cube-shaped somites (formerly called +protovertebræ, or primitive vertebræ) have the appearance of solid +dice, made up of mesodermic cells (Fig. 93). Nevertheless, there is for +a time a ventral cavity, or provertebral cavity, even in these solid +“protovertebræ” (Fig. 143 _uwh_). This vesicular condition of the +provertebra is of the greatest phylogenetic interest; we must, +according to the cœlom theory, regard it as an hereditary reproduction +of the hollow dorsal somites of the amphioxus (Figs. 156–160) and the +lower vertebrates (Fig. 161–163). This rudimentary “provertebral +cavity” has no physiological significance whatever in the +amniote-embryo; it soon disappears, being filled up with cells of the +muscular plate. + + +Fig.167. Head of a shark embryo. Fig. 167—Head of a shark embryo +(_Pristiurus_), one-third of an inch long, magnified. (From _Parker._) +Seen from the ventral side. + + +The innermost median part of the primitive segment plates, which lies +immediately on the chorda (Fig. 145 _ch_) and the medullary tube (_m_), +forms the vertebral column in all the higher vertebrates (it is wanting +in the lowest); hence it may be called the _skeleton_ plate. In each of +the provertebræ it is called the “sclerotome” (in opposition to the +outlying muscular plate, the “myotome”). From the phylogenetic point of +view the myotomes are much older than the sclerotomes. The lower or +ventral part of each sclerotome (the inner and lower edge of the +cube-shaped provertebra) divides into two plates, which grow round the +chorda, and thus form the foundation of the body of the vertebra +(_wh_). The upper plate presses between the chorda and the medullary +tube, the lower between the chorda and the alimentary canal (Fig. 137 +_C_). As the plates of two opposite provertebral pieces unite from the +right and left, a circular sheath is formed round this part of the +chorda. From this develops the _body_ of a vertebra—that is to say, the +massive lower or ventral half of the bony ring, which is called the +“vertebra” proper and surrounds the medullary tube (Figs. 164–166). The +upper or dorsal half of this bony ring, the vertebral arch (Fig. 145 +_wb_), arises in just the same way from the upper part of the skeletal +plate, and therefore from the inner and upper edge of the cube-shaped +primitive vertebra. As the upper edges of two opposing somites grow +together over the medullary tube from right and left, the vertebra-arch +becomes closed. + +The whole of the secondary vertebra, which is thus formed from the +union of the skeletal plates of two provertebral pieces +and encloses a part of the chorda in its body, consists at first of a +rather soft mass of cells; this afterwards passes into a firmer, +cartilaginous stage, and finally into a third, permanent, bony stage. +These three stages can generally be distinguished in the greater part +of the skeleton of the higher vertebrates; at first most parts of the +skeleton are soft, tender, and membranous; they then become +cartilaginous in the course of their development, and finally bony. + + +Figs. 168 and 169. Head of a chick embryo, of the third day. Fig. 168 +and 169—Head of a chick embryo, of the third day. Fig. 168 from the +front, Fig. 169 from the right. _n_ rudimentary nose (olfactory pit), +_l_ rudimentary eye (optic pit, lens-cavity), _g_ rudimentary ear +(auditory pit), _v_ fore-brain, _gl_ eye-cleft. Of the three pairs of +gill-arches the first has passed into a process of the upper jaw (_o_) +and of the lower jaw (_u_). (From _Kölliker._) + + +At the head part of the embryo in the amniotes there is not generally a +cleavage of the middle germinal layer into provertebral and lateral +plates, but the dorsal and ventral somites are blended from the first, +and form what are called the “head-plates” (Fig. 148 _k_). From these +are formed the skull, the bony case of the brain, and the muscles and +corium of the body. The skull develops in the same way as the +membranous vertebral column. The right and left halves of the head +curve over the cerebral vesicle, enclose the foremost part of the +chorda below, and thus finally form a simple, soft, membranous capsule +about the brain. This is afterwards converted into a cartilaginous +primitive skull, such as we find permanently in many of the fishes. +Much later this cartilaginous skull becomes the permanent bony skull +with its various parts. The bony skull in man and all the other +amniotes is more highly differentiated and modified than that of the +lower vertebrates, the amphibia and fishes. But as the one has arisen +phylogenetically from the other, we must assume that in the former no +less than the latter the skull was originally formed from the +sclerotomes of a number of (at least nine) head-somites. + + +Fig.170. Head of a dog embryo, seen from the front. Fig. 170—Head of a +dog embryo, seen from the front. _ a_ the two lateral halves of the +foremost cerebral vesicle, _ b_ rudimentary eye, _c_ middle cerebral +vesicle, _de_ first pair of gill-arches (_e_ upper-jaw process, _d_ +lower-jaw process), _f, f′, f″,_ second, third, and fourth pairs of +gill-arches, _g h i k_ heart (_g_ right, _ h_ left auricle; _i_ left, +_k_ right ventricle), _ l_ origin of the aorta with three pairs of +arches, which go to the gill-arches. (From _Bischoff._) + + +While the articulation of the vertebrate body is always obvious in the +_episoma_ or dorsal body, and is clearly expressed in the segmentation +of the muscular plates and vertebræ, it is more latent in the +_hyposoma_ or ventral body. Nevertheless, the hyposomites of the +vegetal half of the body are not less important than the episomites of +the animal half. The segmentation in the ventral cavity affects the +following principal systems of organs: 1, the gonads or sex-glands +(gonotomes); 2, the nephridia or kidneys (nephrotomes); and 3, the +head-gut with its gill-clefts (branchiotomes). + +The metamerism of the hyposoma is less conspicuous because in all the +craniotes the cavities of the ventral segments, in the walls of which +the sexual products are developed, have long since coalesced, and +formed a single large body-cavity, owing to the disappearance of the +partition. This cenogenetic process is so old that the cavity seems to +be unsegmented from the first in all the craniotes, and the rudiment of +the gonads also is almost always unsegmented. It is the more +interesting to learn that, according to the important discovery of +Rückert, this sexual structure is at first segmental even in the actual +selachii, and the several +gonotomes only blend into a simple sexual gland on either side +secondarily. + +Amphioxus, the sole surviving representative of the acrania, once more +yields us most interesting information; in this case the sexual glands +remain segmented throughout life. The sexually mature lancelet has, on +the right and left of the gut, a series of metamerous sacs, which are +filled with ova in the female and sperm in the male. These segmental +gonads are originally nothing else than the real gonotomes, separate +body-cavities, formed from the hyposomites of the trunk. + + +Fig.171. Human embryo of the fourth week (twenty-six days old). Fig. +171—Human embryo of the fourth week (twenty-six days old), one-fourth +of an inch in length, magnified. (From _Moll._) The rudiments of the +cerebral nerves and the roots of the spinal nerves are especially +marked. Underneath the four gill-arches (left side) is the heart (with +auricle, _V,_ and ventricle, _K_), under this again the liver (_L_). + + +The gonads are the most important segmental organs of the hyposoma, in +the sense that they are phylogenetically the oldest. We find sexual +glands (as pouch-like appendages of the gastro-canal system) in most of +the lower animals, even in the medusæ, etc., which have no kidneys. The +latter appear first (as a pair of excretory tubes) in the platodes +(turbellaria), and have probably been inherited from these by the +articulates +(annelids) on the one hand and the unarticulated prochordonia on the +other, and from these passed to the articulated vertebrates. The oldest +form of the kidney system in this stem are the segmental pronephridia +or prorenal canals, in the same arrangement as Boveri found them in the +amphioxus. They are small canals that lie in the frontal plane, on each +side of the chorda, between the episoma and hyposoma (Fig. 102 _n_); +their internal funnel-shaped opening leads into the various +body-cavities, their outer opening is the lateral furrow of the +epidermis. Originally they must have had a double function, the +carrying away of the urine from the episomites and the release of the +sexual cells from the hyposomites. + +The recent investigations of Ruckert and Van Wijhe on the mesodermic +segments of the trunk and the excretory system of the selachii show +that these “primitive fishes” are closely related to the amphioxus in +this further respect. The transverse section of the shark-embryo in +Fig. 161 shows this very clearly. + +In other higher vertebrates, also, the kidneys develop (though very +differently formed later on) from similar structures, which have been +secondarily derived from the segmental pronephridia of the acrania. The +parts of the mesoderm at which the first traces of them are found are +usually called the middle or mesenteric plates. As the first traces of +the gonads make their appearance in the lining of these middle plates +nearer inward (or the middle) from the inner funnels of the +nephro-canals, it is better to count this part of the mesoderm with the +hyposoma. + +The chief and oldest organ of the vertebrate hyposoma, the alimentary +canal, is generally described as an unsegmented organ. But we could +just as well say that it is the oldest of all the segmented organs of +the vertebrate; the double row of the cœlom-pouches grows out of the +dorsal wall of the gut, on either side of the chorda. In the brief +period during which these segmental cœlom-pouches are still openly +connected with the gut, they look just like a double chain of segmented +visceral glands. But apart from this, we have originally in all +vertebrates an important articulation of the fore-gut, that is wanting +in the lower gut, the segmentation of the branchial (gill) gut. + + +Fig.172. Transverse section of the shoulder and fore-limb (wing) of a +chick-embryo of the fourth day. Fig. 172—Transverse section of the +shoulder and fore-limb (wing) of a chick-embryo of the fourth day, +magnified about twenty times. Beside the medullary tube we can see on +each side three clear streaks in the dark dorsal wall, which advance +into the rudimentary fore-limb or wing (_e_). The uppermost of them is +the muscular plate; the middle is the hind and the lowest the fore root +of a spinal nerve. Under the chorda in the middle is the single aorta, +at each side of it a cardinal vein, and below these the primitive +kidneys. The gut is almost closed. The ventral wall advances into the +amnion, which encloses the embryo. (From _ Remak._) + + +The gill-clefts, which originally in the older acrania pierced the wall +of the fore-gut, and the gill-arches that separated them, were +presumably also segmental, and distributed among the various metamera +of the chain, like the gonads in the after-gut and the nephridia. In +the amphioxus, too, they are still segmentally formed. Probably there +was a division of labour of the hyposomites in the older (and long +extinct) acrania, in such wise that those of the fore-gut took over the +function of breathing and those of the after-gut that of reproduction. +The former developed into gill-pouches, the latter into sex-pouches. +There may have been primitive kidneys in both. Though the gills have +lost their function in the higher animals, certain parts of them have +been generally maintained in the embryo by a tenacious heredity. At a +very early stage we notice in the embryo of man and the other amniotes, +at each side of the head, the remarkable and important structures which +we call the gill-arches and gill-clefts (Figs. 167–170 _f_). They +belong to the characteristic and inalienable organs of the +amniote-embryo, and are found always in the same +spot and with the same arrangement and structure. There are formed to +the right and left in the lateral wall of the fore-gut cavity, in its +foremost part, first a pair and then several pairs of sac-shaped +inlets, that pierce the whole thickness of the lateral wall of the +head. They are thus converted into clefts, through which one can +penetrate freely from without into the gullet. The wall thickens +between these branchial folds, and changes into an arch-like or +sickle-shaped piece—the gill, or gullet-arch. In this the muscles and +skeletal parts of the branchial gut separate; a blood-vessel arch rises +afterwards on their inner side (Fig. 98 _ka_). The number of the +branchial arches and the clefts that alternate with them is four or +five on each side in the higher vertebrates (Fig. 170 _d, f, f′, f″_). +In some of the fishes (selachii) and in the cyclostoma we find six or +seven of them permanently. + + +Fig.173. Transverse section of the pelvic region and hind legs of a +chick-embryo of the fourth day. Fig. 173—Transverse section of the +pelvic region and hind legs of a chick-embryo of the fourth day, +magnified. _h_ horn-plate, _w_ medullary tube, _n_ canal of the tube, +_u_ primitive kidneys, _x_ chorda, _e_ hind legs, _b_ allantoic canal +in the ventral wall, _t_ aorta, _ v_ cardinal veins, _a_ gut, _d_ +gut-gland layer, _ f_ gut-fibre layer, _g_ embryonic epithelium, _r_ +dorsal muscles, _c_ body-cavity or cœloma. (From _ Waldeyer._) + + +These remarkable structures had originally the function of respiratory +organs—gills. In the fishes the water that serves for breathing, and is +taken in at the mouth, still always passes out by the branchial clefts +at the sides of the gullet. In the higher vertebrates they afterwards +disappear. The branchial arches are converted partly into the jaws, +partly into the bones of the tongue and the ear. From the first +gill-cleft is formed the tympanic cavity of the ear. + +There are few parts of the vertebrate organism that, like the outer +covering or integument of the body, are not subject to metamerism. The +outer skin (_epidermis_) is unsegmented from the first, and proceeds +from the continuous horny plate. Moreover, the underlying _cutis_ is +also not metamerous, although it develops from the segmental structure +of the cutis-plates (Figs. 161, 162 _cp_). The vertebrates are +strikingly and profoundly different from the articulates in these +respects also. + +Further, most of the vertebrates still have a number of unarticulated +organs, which have arisen locally, by adaptation of particular parts of +the body to certain special functions. Of this character are the +sense-organs in the episoma, and the limbs, the heart, the spleen, and +the large visceral glands—lungs, liver, pancreas, etc.—in the hyposoma. +The heart is originally only a local spindle-shaped enlargement of the +large ventral blood-vessel or principal vein, at the point where the +subintestinal passes into the branchial artery, at the limit of the +head and trunk (Figs. 170, 171). The three higher sense-organs—nose, +eye, and ear—were originally developed in the same form in all the +craniotes, as three pairs of small depressions in the skin at the side +of the head. + +The organ of smell, the nose, has the appearance of a pair of small +pits above the mouth-aperture, in front of the head (Fig. 169 _n_). The +organ of sight, the eye, is found at the side of the head, also in the +shape of a depression (Figs. 169 _l_, 170 _b_), to which corresponds a +large outgrowth of the foremost cerebral vesicle on each side. Farther +behind, at each side of the head, there is a third depression, the +first trace of the organ of hearing (Fig. 169 _g_). As yet we can see +nothing of the later elaborate structure of these organs, nor of the +characteristic build of the face. + +When the human embryo has reached +When the human embryo has reached this stage of development, it can +still scarcely be distinguished from that of any other higher +vertebrate. All the chief parts of the body are now laid down: the head +with the primitive skull, the rudiments of the three higher +sense-organs and the five cerebral vesicles, and the gill-arches and +clefts; the trunk with the spinal cord, the rudiment of the vertebral +column, the chain of metamera, the heart and chief blood-vessels, and +the kidneys. At this stage man is a higher vertebrate, but shows no +essential morphological difference from the embryos of the mammals, the +birds, the reptiles, etc. This is an ontogenetic fact of the utmost +significance. From it we can gather the most important phylogenetic +conclusions. + + +Fig.174. Development of the lizard’s legs. Fig. 174—Development of the +lizard’s legs (_Lacerta agilis_), with special relation to their +blood-vessels. _1, 3, 5, 7, 9, 11_ right fore-leg; _13, 15_ left +fore-leg; _2, 4, 6, 8, 10, 12_ right hind-leg; _ 14, 16_ left hind-leg; +_SRV_ lateral veins of the trunk, _VU_ umbilical vein. (From _F. +Hochstetter._) + + +There is still no trace of the limbs. Although head and trunk are +separated and all the principal internal organs are laid down, there is +no indication whatever of the “extremities” at this stage; they are +formed later on. Here again we have a fact of the utmost interest. It +proves that the older vertebrates had no feet, as we find to be the +case in the lowest living vertebrates (amphioxus and the cyclostoma). +The descendants of these ancient footless vertebrates only acquired +extremities—two fore-legs and two hind-legs—at a much later stage of +development. +These were at first all alike, though they afterwards vary considerably +in structure—becoming fins (of breast and belly) in the fishes, wings +and legs in the birds, fore and hind legs in the creeping animals, arms +and legs in the apes and man. All these parts develop from the same +simple original structure, which forms secondarily from the trunk-wall +(Figs. 172, 173). They have always the appearance of two pairs of small +buds, which represent at first simple roundish knobs or plates. +Gradually each of these plates becomes a large projection, in which we +can distinguish a small inner part and a broader outer part. The latter +is the rudiment of the foot or hand, the former that of the leg or arm. +The similarity of the original rudiment of the limbs in different +groups of vertebrates is very striking. + + +Fig.175. Human embryo, five weeks old, half an inch long, seen from the +right. Fig. 175—Human embryo, five weeks old, half an inch long, seen +from the right, magnified. (From _Russel Bardeen_ and _Harmon Lewis._) +In the undissected head we see the eye, mouth, and ear. In the trunk +the skin and part of the muscles have been removed, so that the +cartilaginous vertebral column is free; the dorsal root of a spinal +nerve goes out from each vertebra (towards the skin of the back). In +the middle of the lower half of the figure part of the ribs and +intercostal muscles are visible. The skin and muscles have also been +removed from the right limbs; the internal rudiments of the five +fingers of the hand, and five toes of the foot, are clearly seen within +the fin-shaped plate, and also the strong network of nerves that goes +from the spinal cord to the extremities. The tail projects under the +foot, and to the right of it is the first part of the umbilical cord. + + +How the five fingers or toes with their +blood-vessels gradually differentiate within the simple fin-like +structure of the limbs can be seen in the instance of the lizard in +Fig. 174. They are formed in just the same way in man: in the human +embryo of five weeks the five fingers can clearly be distinguished +within the fin-plate (Fig. 175). + +The careful study and comparison of human embryos with those of other +vertebrates at this stage of development is very instructive, and +reveals more mysteries to the impartial student than all the religions +in the world put together. For instance, if we compare attentively the +three successive stages of development that are represented, in twenty +different amniotes we find a remarkable likeness. When we see that as a +fact twenty different amniotes of such divergent characters develop +from the same embryonic form, we can easily understand that they may +all descend from a common ancestor. + + +Figs. 176-178. Embryos of the bat (Vespertilio murinus) at three +different stages. Figs. 176–178—Embryos of the bat (_Vespertilio +murinus_) at three different stages. (From _Oscar Schultze._) Fig. 176: +Rudimentary limbs (_v_ fore-leg, _ h_ hind-leg). _l_ lenticular +depression, _r_ olfactory pit, _ok_ upper jaw, _uk_ lower jaw, _ k_2, +_k_3, _k_4 first, second and third gill-arches, _a_ amnion, _n_ +umbilical vessel, _d_ yelk-sac. Fig. 177: Rudiment of flying membrane, +membranous fold between fore and hind leg. _n_ umbilical vessel, _o_ +ear-opening, _f_ flying membrane. Fig. 178: The flying membrane +developed and stretched across the fingers of the hands, which cover +the face. + + +In the first stage of development, in which the head with the five +cerebral vesicles is already clearly indicated, but there are no limbs, +the embryos of all the vertebrates, from the fish to man, are only +incidentally or not at all different from each other. In the second +stage, which shows the limbs, we begin to see differences between the +embryos of the lower and higher vertebrates; but the human embryo is +still hardly distinguishable from that of the higher mammals. In the +third stage, in which the gill-arches have disappeared and the face is +formed, the differences become more pronounced. These are facts of a +significance that cannot be exaggerated.[27] + + [27] Because they show how the most diverse structures may be + developed from a common form. As we actually see this in the case of + the embryos, we have a right to assume it in that of the stem-forms. + Nevertheless, this resemblance, however great, is never a real + identity. Even the embryos of the different individuals of one species + are usually not really identical. If the reader can consult the + complete edition of this work at a library, he will find six plates + illustrating these twenty embryos. + + +If there is an intimate causal connection between the processes of +embryology and stem-history, as we must assume in virtue of the laws of +heredity, several important phylogenetic conclusions follow at once +from these ontogenetic facts. The profound and remarkable similarity in +the embryonic development of man and the other vertebrates can only be +explained when we admit their descent from a common ancestor. As a +fact, this common descent is now accepted by all competent scientists; +they have substituted the natural evolution for the supernatural +creation of organisms. + + + + +Chapter XV. +FŒTAL MEMBRANES AND CIRCULATION + + +Among the many interesting phenomena that we have encountered in the +course of human embryology, there is an especial importance in the fact +that the development of the human body follows from the beginning just +the same lines as that of the other viviparous mammals. As a fact, all +the embryonic peculiarities that distinguish the mammals from other +animals are found also in man; even the ovum with its distinctive +membrane (_zona pellucida,_ Fig. 14) shows the same typical +structure in all mammals (apart from the older oviparous monotremes). +It has long since been deduced from the structure of the developed man +that his natural place in the animal kingdom is among the mammals. +Linné (1735) placed him in this class with the apes, in one and the +same order (_primates_), in his _Systema Naturæ._ This position is +fully confirmed by comparative embryology. We see that man entirely +resembles the higher mammals, and most of all the apes, in embryonic +development as well as in anatomic structure. And if we seek to +understand this ontogenetic agreement in the light of the biogenetic +law, we find that it proves clearly and necessarily the descent of man +from a series of other mammals, and proximately from the primates. The +common origin of man and the other mammals from a single ancient +stem-form can no longer be questioned; nor can the immediate +blood-relationship of man and the ape. + + +Fig.179. Human embryos from the second to the fifteenth week, seen from +the left. Fig. 179—Human embryos from the second to the fifteenth week, +seen from the left, the curved back turned towards the right. (Mostly +from _ Ecker._) II of fourteen days. III of three weeks. IV of four +weeks. V of five weeks. VI of six weeks. VII of seven weeks. VIII of +eight weeks. XII of twelve weeks. XV of fifteen weeks. + + +The essential agreement in the whole bodily form and inner structure is +still visible in the embryo of man and the other mammals at the late +stage of development at which the mammal-body can be recognised as +such. But at a somewhat earlier stage, in which the limbs, gill-arches, +sense-organs, etc., are already outlined, we cannot yet recognise the +mammal embryos as such, or distinguish them from those of birds and +reptiles. When we consider still earlier stages of development, we are +unable to discover any essential difference in bodily structure between +the embryos of these higher vertebrates and those of the lower, the +amphibia and fishes. If, in fine, we go back to the construction of the +body out of the four germinal layers, we are astonished to perceive +that these four layers are the same in all vertebrates, and everywhere +take a similar part in the building-up of the fundamental organs of the +body. If we inquire as to the origin of these four secondary layers, we +learn that they always arise in the same way from the two primary +layers; and the latter have the same significance in all the metazoa +(_i.e.,_ all animals except the unicellulars). Finally, we see that the +cells which make up the primary germinal layers owe their origin in +every case to the repeated cleavage of a single simple cell, the +stem-cell or fertilised ovum. + + +Fig.180. Very young human embryo of the fourth week, one-fourth of an +inch long. Fig. 180—Very young human embryo of the fourth week, +one-fourth of an inch long (taken from the womb of a suicide eight +hours after death). (From _Rabl._) _n_ nasal pits, _ a_ eye, _u_ lower +jaw, _z_ arch of hyoid bone, _ k3_ and _k4_ third and fourth gill-arch, +_h_ heart; _s_ primitive segments, _vg_ fore-limb (arm), _hg_ hind-limb +(leg), between the two the ventral pedicle. + + +It is impossible to lay too much stress on this remarkable agreement in +the chief embryonic features in man and the other animals. We shall +make use of it later on for our monophyletic theory of descent—the +hypothesis of a common descent of man and all the metazoa from the +gastræa. The first rudiments of the principal parts of the body, +especially the oldest organ, the alimentary canal, are the same +everywhere; they have always the same extremely simple form. All the +peculiarities that distinguish the various groups of animals from each +other only appear gradually in the course of embryonic development; and +the closer the relation of the various groups, the later they are +found. We may formulate this phenomenon in a definite law, which may in +a sense be regarded as an appendix to our biogenetic law. This is the +law of the ontogenetic connection of related animal forms. It runs: The +closer the +relation of two fully-developed animals in respect of their whole +bodily structure, and the nearer they are connected in the +classification of the animal kingdom, the longer do their embryonic +forms retain their identity, and the longer is it impossible (or only +possible on the ground of subordinate features) to distinguish between +their embryos. This law applies to all animals whose embryonic +development is, in the main, an hereditary summary of their ancestral +history, or in which the original form of development has been +faithfully preserved by heredity. When, on the other hand, it has been +altered by cenogenesis, or disturbance of development, we find a +limitation of the law, which increases in proportion to the +introduction of new features by adaptation (cf. Chapter I, pp. 4–6). +Thus the apparent exceptions to the law can always be traced to +cenogenesis. + + +Fig.181. Human embryo of the middle of the fifth week, one-third of an +inch long. Fig. 181—Human embryo of the middle of the fifth week, +one-third of an inch long. (From _Rabl._) Letters as in Fig. 180, +except _sk_ curve of skull, _ok_ upper jaw, _ hb_ neck-indentation. + + +When we apply to man this law of the ontogenetic connection of related +forms, and run rapidly over the earliest stages of human development +with an eye to it, we notice first of all the structural identity of +the ovum in man and the other mammals at the very beginning (Figs. 1, +14). The human ovum possesses all the distinctive features of the ovum +of the viviparous mammals, especially the characteristic formation of +its membrane (_zona pellucida_), which clearly distinguishes it from +the ovum of all other animals. When the human fœtus has attained the +age of fourteen days, it forms a round vesicle (or “embryonic vesicle”) +about a quarter of an inch in diameter. A thicker part of its border +forms a simple sole-shaped embryonic shield one-twelfth of an inch long +(Fig. 133). On its dorsal side we find in the middle line the straight +medullary furrow, bordered by the two parallel dorsal or medullary +swellings. Behind, it passes by the neurenteric canal into the +primitive gut or primitive groove. From this the folding of the two +cœlom-pouches proceeds in the same way as in the other mammals (cf. +Fig. 96, 97). In the middle of the sole-shaped embryonic shield the +first primitive segments immediately begin to make their appearance. At +this age the human embryo cannot be distinguished from that of other +mammals, such as the hare or dog. + +A week later (or after the twenty-first day) the human embryo has +doubled its length; it is now about one-fifth of an inch long, and, +when seen from the side, shows the characteristic bend of the back, the +swelling of the head-end, the first outline of the three higher +sense-organs, and the rudiments of the gill-clefts, which pierce the +sides of the neck (Fig. 179, III). The allantois has grown out of the +gut behind. The embryo is already entirely enclosed in the amnion, and +is only connected in the middle of the belly by the vitelline duct with +the embryonic vesicle, which changes into the yelk-sac. There are no +extremities or limbs at this stage, no trace of arms or legs. The +head-end has been strongly differentiated from the tail-end; and the +first outlines of the cerebral vesicles in front, and the heart below, +under the fore-arm, are already more or less clearly seen. There is as +yet no real face. Moreover, we seek in vain at this stage a special +character that may distinguish the human embryo from that of other +mammals. + +A week later (after the fourth week, on the twenty-eighth to thirtieth +day of development) the human embryo has +reached a length of about one-third of an inch (Fig 179 IV). We can now +clearly distinguish the head with its various parts; inside it the five +primitive cerebral vesicles (fore-brain, middle-brain, +intermediate-brain, hind-brain, and after-brain); under the head the +gill-arches, which divide the gill-clefts; at the sides of the head the +rudiments of the eyes, a couple of pits in the outer skin, with a pair +of corresponding simple vesicles growing out of the lateral wall of the +fore-brain (Figs. 180, 181 _a_). Far behind the eyes, over the last +gill-arches, we see a vesicular rudiment of the auscultory organ. The +rudimentary limbs are now clearly outlined—four simple buds of the +shape of round plates, a pair of fore (_vg_) and a pair of hind legs +(_hg_), the former a little larger than the latter. The large head +bends over the trunk, almost at a right angle. The latter is still +connected in the middle of its ventral side with the embryonic vesicle; +but the embryo has still further severed itself from it, so that it +already hangs out as the yelk-sac. The hind part of the body is also +very much curved, so that the pointed tail-end is directed towards the +head. The head and face-part are sunk entirely on the still open +breast. The bend soon increases so much that the tail almost touches +the forehead (Fig. 179 V.; Fig. 181). We may then distinguish three or +four special curves on the round dorsal surface—namely, a skull-curve +in the region of the second cerebral vesicle, a neck-curve at the +beginning of the spinal cord, and a tail-curve at the fore-end. This +pronounced curve is only shared by man and +the higher classes of vertebrates (the amniotes); it is much slighter, +or not found at all, in the lower vertebrates. At this age (four weeks) +man has a considerable tail, twice as long as his legs. A vertical +longitudinal section through the middle plane of this tail (Fig. 182) +shows that the hinder end of the spinal marrow extends to the point of +the tail, as also does the underlying chorda (_ch_), the terminal +continuation of the vertebral column. Of the latter, the rudiments of +the seven coccygeal (or lowest) vertebræ are visible—thirty-two +indicates the third and thirty-six the seventh of these. Under the +vertebral column we see the hindmost ends of the two large +blood-vessels of the tail, the principal artery (_aorta caudalis_ or +_arteria sacralis media, Ao_), and the principal vein (_vena caudalis_ +or _sacralis media_). Underneath is the opening of the anus (_an_) and +the urogenital sinus (_S.ug_). From this anatomic structure of the +human tail it is perfectly clear that it is the rudiment of an +ape-tail, the last hereditary relic of a long hairy tail, which has +been handed down from our tertiary primate ancestors to the present +day. + + +Fig.182. Median longitudinal section of the tail of a human embryo, +two-thirds of an inch long. Fig. 182—Median longitudinal section of the +tail of a human embryo, two-thirds of an inch long. (From _Ross +Granville Harrison._) _Med_ medullary tube, _Ca.fil_ caudal filament, +_ch_ chorda, _ao_ caudal artery, _V.c.i_ caudal vein, _an_ anus, _S.ug_ +sinus urogenitalis. + + +Fig.183. Human embryo, four weeks old, opened on the ventral side. Fig. +183—Human embryo, four weeks old, opened on the ventral side. Ventral +and dorsal walls are cut away, so as to show the contents of the +pectoral and abdominal cavities. All the appendages are also removed +(amnion, allantois, yelk-sac), and the middle part of the gut. _n_ eye, +_3_ nose, _4_ upper jaw, _5_ lower jaw, _6_ second, _6″_ third +gill-arch, _ov_ heart (_o_ right, _o′_ left auricle; _v_ right, _v′_ +left ventricle), _b_ origin of the aorta, _f_ liver (_u_ umbilical +vein), _e_ gut (with vitelline artery, cut off at _a′_), _j′_ vitelline +vein, _m_ primitive kidneys, _t_ rudimentary sexual glands, _ r_ +terminal gut (cut off at the mesentery _z_), _n_ umbilical artery, _u_ +umbilical vein, _9_ fore-leg, _ 9′_ hind-leg. (From _Coste._) + + +Human embryo, five weeks old, opened from the ventral side. Fig. +184—Human embryo, five weeks old, opened from the ventral side (as in +Fig. 183). Breast and belly-wall and liver are removed. _3_ outer nasal +process, _4_ upper jaw, _5_ lower jaw, _z_ tongue, _v_ right, _v′_ left +ventricle of heart, _o′_ left auricle, _b_ origin of aorta, _b′, b″, +b‴_ first, second, and third aorta-arches, _c, c′, c″_ vena cava, _ae_ +lungs (_y_ pulmonary artery), _e_ stomach, _m_ primitive kidneys (_j_ +left vitelline vein, _s_ cystic vein, _a_ right vitelline artery, _n_ +umbilical artery, _u_ umbilical vein), _ x_ vitelline duct, _i_ rectum, +_8_ tail, _9_ fore-leg, _9′_ hind-leg. (From _Coste._) + + +It sometimes happens that we find even external relics of this tail +growing. According to the illustrated works of +Surgeon-General Bernhard Ornstein, of Greece, these tailed men are not +uncommon; it is not impossible that they gave rise to the ancient +fables of the satyrs. A great number of such cases are given by Max +Bartels in his essay on “Tailed Men” (1884, in the _Archiv für +Anthropologie,_ Band XV), and critically examined. These atavistic +human tails are often mobile; sometimes they contain only muscles and +fat, sometimes also rudiments of caudal vertebræ. They have a length of +eight to ten inches and more. Granville Harrison has very carefully +studied one of these cases of “pigtail,” which he removed by operation +from a six months old child in 1901. The tail moved briskly when the +child cried or was excited, and was drawn up when at rest. + + +Fig.185. The head of Miss Julia Pastrana. Fig. 185—The head of Miss +Julia Pastrana. (From a photograph by _Hintze._) + + +Human ovum of twelve to thirteen days. Fig. 186—Human ovum of twelve to +thirteen days (?). (From _Allen Thomson._) 1. Not opened. 2. Opened and +magnified. Within the outer chorion the tiny curved fœtus lies on the +large embryonic vesicle, to the left above. + + +Fig.187. Human ovum of ten days. Fig. 188. Human foetus of ten days, +taken from the preceding ovum, magnified. Fig. 187—Human ovum of ten +days. (From _Allen Thomson._) Opened; the small fœtus in the right +half, above. +Fig. 188—Human fœtus of ten days, taken from the preceding ovum, +magnified, _a_ yelk-sac, _b_ neck (the medullary groove already +closed), _ c_ head (with open medullary groove), _d_ hind part (with +open medullary groove), _e_ a shred of the amnion. + + +Fig.189. Human ovum of twenty to twenty-two days. Fig. 190. Human +foetus of twenty to twenty-two days, taken from the preceding ovum, +magnified. Fig. 189—Human ovum of twenty to twenty-two days. (From +_Allen Thomson._) Opened. The chorion forms a spacious vesicle, to the +inner wall of which the small fœtus (to the right above) is attached by +a short umbilical cord. +Fig. 190—Human fœtus of twenty to twenty-two days, taken from the +preceding ovum, magnified. _a_ amnion, _b_ yelk-sac, _c_ lower-jaw +process of the first gill-arch, _d_ upper-jaw process of same, _e_ +second gill-arch (two smaller ones behind). Three gill-clefts are +clearly seen. _f_ rudimentary fore-leg, _ g_ auditory vesicle, _h_ eye, +_i_ heart. + + +In the opinion of some travellers and anthropologists, the atavistic +tail-formation is hereditary in certain isolated tribes (especially in +south-eastern Asia and the archipelago), so that we might speak of a +special race or “species” of tailed men +(_Homo caudatus_). Bartels has “no doubt that these tailed men will be +discovered in the advance of our geographical and ethnographical +knowledge of the lands in question” (_Archiv für Anthropologie,_ Band +XV, p. 129). + + +Fig.191. Human embryo of sixteen to eighteen days. Fig. 191—Human +embryo of sixteen to eighteen days. (From _Coste._) Magnified. The +embryo is surrounded by the amnion, (_a_), and lies free with this in +the opened embryonic vesicle. The belly is drawn up by the large +yelk-sac (_d_), and fastened to the inner wall of the embryonic +membrane by the short and thick pedicle (_b_). Hence the normal convex +curve of the back (Fig. 190) is here changed into an abnormal concave +surface. _h_ heart, _ m_ parietal mesoderm. The spots on the outer wall +of the serolemma are the roots of the branching chorion-villi, which +are free at the border. + + +When we open a human embryo of one month (Fig. 183), we find the +alimentary canal formed in the body-cavity, and for the most part cut +off from the embryonic vesicle. There are both mouth and anus +apertures. But the mouth-cavity is not yet separated from the nasal +cavity, and the face not yet shaped. The heart shows all its four +sections; it is very large, and almost fills the whole of the pectoral +cavity (Fig. 183 _ov_). Behind it are the very small rudimentary lungs. +The primitive kidneys (_m_) are very large; they fill the greater part +of the abdominal cavity, and extend from the liver (_f_) to the pelvic +gut. Thus at the end of the first month all the chief organs are +already outlined. But there are at this stage no features by which the +human embryo materially differs from that of the dog, the hare, the ox, +or the horse—in a word, of any other higher mammal. All these embryos +have the same, or at least a very similar, form; they can at the most +be +distinguished from the human embryo by the total size of the body or +some other insignificant difference in size. Thus, for instance, in man +the head is larger in proportion to the trunk than in the ox. The tail +is rather longer in the dog than in man. These are all negligible +differences. On the other hand, the whole internal organisation and the +form and arrangement of the various organs are essentially the same in +the human embryo of four weeks as in the embryos of the other mammals +at corresponding stages. + + +Fig.192. Human embryo of the fourth week, one-third of an inch long, +lying in the dissected chorion. Fig. 192—Human embryo of the fourth +week, one-third of an inch long, lying in the dissected chorion. + + +Fig.193. Human embryo of the fourth week, with its membranes, like Fig. +192, but a little older. Fig. 193—Human embryo of the fourth week, with +its membranes, like Fig. 192, but a little older. The yelk-sac is +rather smaller, the amnion and chorion larger. + + +It is otherwise in the second month of human development. Fig. 179 +represents a human embryo of six weeks (VI), one of seven weeks (VII), +and one of eight weeks (VIII), at natural size. The differences which +mark off the human embryo from that of the dog and the lower mammals +now begin to be more pronounced. We can see important differences at +the sixth, and still more at the eighth week, especially in the +formation of the head. The size of the various sections of the brain is +greater in man, and the tail is shorter. + +Other differences between man and the lower mammals are found in the +relative size of the internal organs. But even at this stage the human +embryo differs very little from that of the nearest related mammals—the +apes, especially the anthropomorphic apes. + +The features by means of which we distinguish between them are not +clear until later on. Even at a much more advanced stage of +development, when we can distinguish the human fœtus from that of the +ungulates at a glance, it still closely resembles that of the higher +apes. At last we get the distinctive features, and +we can distinguish the human embryo confidently at the first glance +from that of all other mammals during the last four months of fœtal +life—from the sixth to the ninth month of pregnancy. Then we begin to +find also the differences between the various races of men, especially +in regard to the formation of the skull and the face. (Cf. Chapter +XXIII.) + + +Fig.194. Human embryo with its membranes, six weeks old. Fig. 194—Human +embryo with its membranes, six weeks old. The outer envelope of the +whole ovum is the chorion, thickly covered with its branching villi, a +product of the serous membrane. The embryo is enclosed in the delicate +amnion-sac. The yelk-sac is reduced to a small pear-shaped umbilical +vesicle; its thin pedicle, the long vitelline duct, is enclosed in the +umbilical cord. In the latter, behind the vitelline duct, is the much +shorter pedicle of the allantois, the inner lamina of which (the +gut-gland layer) forms a large vesicle in most of the mammals, while +the outer lamina is attached to the inner wall of the outer embryonic +coat, and forms the placenta there. (Half diagrammatic.) + + +The striking resemblance that persists so long between the embryo of +man and of the higher apes disappears much earlier in the lower apes. +It naturally remains longest in the large anthropomorphic apes +(gorilla, chimpanzee, orang, and gibbon). The physiognomic similarity +of these animals, which we find so great in their earlier years, +lessens with the increase of age. On the other hand, it remains +throughout life in the remarkable long-nosed ape of Borneo (_Nasalis +larvatus_). Its finely-shaped nose would be regarded with envy by many +a man who has too little of that organ. If we compare the face of the +long-nosed ape with that of abnormally ape-like human beings (such as +the famous Miss Julia Pastrana, Fig. 185), it will be admitted to +represent a higher stage of development. There are still people among +us who look especially to the face for the “image of God in man.” The +long-nosed ape would have more claim to this than some of the +stumpy-nosed human individuals one meets. + +This progressive divergence of the human from the animal form, which is +based on the law of the ontogenetic connection between related forms, +is found in the structure of the internal organs as well as in external +form. It is also expressed in the construction of the envelopes and +appendages that we find surrounding the fœtus externally, and that we +will now consider more closely. Two of these appendages—the amnion and +the allantois—are only found in the three higher classes of +vertebrates, while the third, the yelk-sac, is found in most of the +vertebrates. This is a circumstance of great importance, and it gives +us valuable data for constructing man’s genealogical tree. + +As regards the external membrane that encloses the ovum in the mammal +womb, +we find it just the same in man as in the higher mammals. The ovum is, +the reader will remember, first surrounded by the transparent +structureless _ovolemma_ or _zona pellucida_ (Figs. 1, 14). But very +soon, even in the first week of development, this is replaced by the +permanent chorion. This is formed from the external layer of the +amnion, the _serolemma,_ or “serous membrane,” the formation of which +we shall consider presently; it surrounds the fœtus and its appendages +as a broad, completely closed sac; the space between the two, filled +with clear watery fluid, is the _serocœlom,_ or interamniotic cavity +(“extra-embryonic body-cavity”). But the smooth surface of the sac is +quickly covered with numbers of tiny tufts, which are really hollow +outgrowths like the fingers of a glove (Figs. 186, 191, 198 _chz_). +They ramify and push into the corresponding depressions that are formed +by the tubular glands of the mucous membrane of the maternal womb. +Thus, the ovum secures its permanent seat (Fig. 186–194). + + +Fig.195. Diagram of the embryonic organs of the mammal (foetal +membranes and appendages). Fig. 195—Diagram of the embryonic organs of +the mammal (fœtal membranes and appendages). (From _Turner._) _E, M, H_ +outer, middle, and inner germ layer of the embryonic shield, which is +figured in median longitudinal section, seen from the left. _am_ +amnion. _AC_ amniotic cavity, _UV_ yelk-sac or umbilical vesicle, _ALC_ +allantois, _al_ pericœlom or serocœlom (inter-amniotic cavity), _ sz_ +serolemma (or serous membrane), _pc_ prochorion (with villi).) + + +In human ova of eight to twelve days this external membrane, the +chorion, is already covered with small tufts or villi, and forms a ball +or spheroid of one-fourth to one-third of an inch in diameter (Figs. +186–188). As a large quantity of fluid gathers inside it, the chorion +expands more and more, so that the embryo only occupies a small part of +the space within the vesicle. The villi of the chorion grow larger and +more numerous. They branch out more and more. At first the villi cover +the whole surface, but they afterwards disappear from the greater part +of it; they then develop with proportionately greater vigour at a spot +where the placenta is formed from the allantois. + +When we open the chorion of a human embryo of three weeks, we find on +the ventral side of the fœtus a large round sac, filled with fluid. +This is the yelk-sac, or “umbilical vesicle,” the origin of which we +have considered previously. The larger the embryo becomes the smaller +we find the yelk-sac. In the end we find the remainder of it in the +shape of a small pear-shaped vesicle, fastened to a long thin stalk (or +pedicle), and hanging from the open belly of the fœtus (Fig. 194). This +pedicle is the vitelline duct, and is separated from the body at the +closing of the navel. + +Behind the yelk-sac a second appendage, +of much greater importance, is formed at an early stage at the belly of +the mammal embryo. This is the allantois or “primitive urinary sac,” an +important embryonic organ, only found in the three higher classes of +vertebrates. In all the amniotes the allantois quickly appears at the +hinder end of the alimentary canal, growing out of the cavity of the +pelvic gut (Fig. 147 _r, u,_ Fig. 195 _ ALC_). + +The further development of the allantois varies considerably in the +three sub-classes of the mammals. The two lower sub-classes, monotremes +and marsupials, retain the simpler structure of their ancestors, the +reptiles. The wall of the allantois and the enveloping serolemma +remains smooth and without villi, as in the birds. But in the third +sub-class of the mammals the serolemma forms, by invagination at its +outer surface, a number of hollow tufts or villi, from which it takes +the name of the _chorion_ or _mallochorion._ The gut-fibre layer of the +allantois, richly supplied with branches of the umbilical vessel, +presses into these tufts of the primary chorion, and forms the +“secondary chorion.” Its embryonic blood-vessels are closely correlated +to the contiguous maternal blood-vessels of the environing womb, and +thus is formed the important nutritive apparatus of the embryo which we +call the placenta. + + +Fig.196. Diagrammatic frontal section of the pregnant human womb. Fig. +196—Diagrammatic frontal section of the pregnant human womb. (From +_Longet._) The embryo hangs by the umbilical cord, which encloses the +pedicle of the allantois (_al_). _ nb_ umbilical vessel, _am_ amnion, +_ch_ chorion, _ ds_ decidua serotina, _dv_ decidua vera, _dr_ decidua +reflexa, _z_ villi of the placenta, _c_ cervix uteri, _ u_ uterus.) + + +The pedicle of the allantois, which connects the embryo with the +placenta and conducts the strong umbilical vessels from the former to +the latter, is covered by the amnion, and, with this amniotic sheath +and the pedicle of the yelk-sac, forms what is called the _umbilical +cord_ (Fig. 196 _al_). As the large and blood-filled vascular network +of the fœtal allantois attaches itself closely to the mucous lining of +the maternal womb, and the partition between the blood-vessels of +mother and child becomes much thinner, we get that remarkable nutritive +apparatus of the fœtal body which is characteristic of the placentalia +(or choriata). We shall return afterwards to the closer consideration +of this (cf. Chapter XXIII). + +In the various orders of mammals the placenta undergoes many +modifications, and these are in part of great evolutionary importance +and useful in classification. There is only one of these that need be +specially mentioned—the important fact, established by Selenka in 1890, +that the distinctive human placentation is confined to the anthropoids. +In this most advanced group of the mammals the allantois is very small, +soon loses its cavity, and then, in common with the amnion, undergoes +certain peculiar changes. The umbilical cord develops in this case from +what is called the “ventral pedicle.” Until very recently this was +regarded as a structure peculiar to man. We now know from Selenka that +the much-discussed ventral pedicle is merely the pedicle of the +allantois, combined with the pedicle of the amnion and the rudimentary +pedicle of the yelk-sac. It has just the same structure in the orang +and gibbon (Fig. 197) and very probably in the chimpanzee and gorilla, +as in man; it is, therefore, not a _disproof,_ but a striking fresh +proof, of the blood-relationship of man and the anthropoid apes. + +We find only in the anthropoid apes—the gibbon and orang of Asia and +the chimpanzee and gorilla of Africa—the peculiar and elaborate +formation of the placenta that characterises man (Fig. 198). +In this case there is at an early stage an intimate blending of the +chorion of the embryo and the part of the mucous lining of the womb to +which it attaches. The villi of the chorion with the blood-vessels they +contain grow so completely into the tissue of the uterus, which is rich +in blood, that it becomes impossible to separate them, and they form +together a sort of cake. This comes away as the “afterbirth” at +parturition; at the same time, the part of the mucous lining of the +womb that has united inseparably with the chorion is torn away; hence +it is called the _decidua_ (“falling-away membrane”), and also the +“sieve-membrane,” because it is perforated like a sieve. We find a +decidua of this kind in most of the higher placentals; but it is only +in man and the anthropoid apes that it divides into three parts—the +outer, inner, and placental decidua. The external or true decidua (Fig. +196 _du,_ Fig. 199 _g_) is the part of the mucous lining of the womb +that clothes the inner surface of the uterine cavity wherever it is not +connected with the placenta. The placental or spongy decidua +(_placentalis_ or _serotina,_ Fig. 196 _ds,_ Fig. 199 _d_) is really +the placenta itself, or the maternal part of it (_placenta +uterina_)—namely, that part of the mucous lining of the womb which +unites intimately with the chorion-villi of the fœtal placenta. The +internal or false decidua (_interna_ or _reflexa,_ Fig. 196 _dr,_ Fig. +199 _f_) is that part of the mucous lining of the womb which encloses +the remaining surface of the ovum, the smooth chorion (_chorion læve_), +in the shape of a special thin membrane. The origin of these three +different deciduous membranes, in regard to which quite erroneous views +(still retained in their names) formerly prevailed, is now quite clear, +The external _ decidua vera_ is the specially modified and subsequently +detachable superficial stratum of the original mucous lining of the +womb. The placental _decidua serotina_ is that part of the preceding +which is completely transformed by the ingrowth of the chorion-villi, +and is used for constructing the placenta. The inner _decidua reflexa_ +is formed by the rise of a circular fold of the mucous lining (at the +border of the _decidua vera_ and _ serotina_), which grows over the +fœtus (like the anmnion) to the end. + + +Fig.197. Male embryo of the Siamang-gibbon (Hylobates siamanga) of +Sumatra. Fig. 197—Male embryo of the Siamang-gibbon (_Hylobates +siamanga_) of Sumatra; to the left the dissected uterus, of which only +the dorsal half is given. The embryo has been taken out, and the limbs +folded together; it is still connected by the umbilical cord with the +centre of the circular placenta which is attached to the inside of the +womb. This embryo takes the head-position in the womb, and this is +normal in man also. + + +The peculiar anatomic features that characterise the human fœtal +membranes are found in just the same way in the higher +apes. Until recently it was thought that the human embryo was +distinguished by its peculiar construction of a solid allantois and a +special ventral pedicle, and that the umbilical cord developed from +this in a different way than in the other mammals. The opponents of the +unwelcome “ape-theory” laid great stress on this, and thought they had +at last discovered an important indication that separated man from all +the other placentals. But the remarkable discoveries published by the +distinguished zoologist Selenka in 1890 proved that man shares these +peculiarities of placentation with the anthropoid apes, though they are +not found in the other apes. Thus the very feature which was advanced +by our critics as a disproof became a most important piece of evidence +in favour of our pithecoid origin.) + + +Fig.198. Frontal section of the pregnant human womb. Fig. 198—Frontal +section of the pregnant human womb. (From _Turner._) The embryo (a +month old) hangs in the middle of the amniotic cavity by the ventral +pedicle or umbilical cord, which connects it with the placenta (above). + + +Of the three vesicular appendages of the amniote embryo which we have +now described the amnion has no blood-vessels at any moment of its +existence. But the other two vesicles, the yelk-sac and the allantois, +are equipped with large blood-vessels, and these effect the nourishment +of the embryonic body. We may take the opportunity to make a few +general observations on the first circulation in the embryo and its +central organ, the heart. The first blood-vessels, the heart, and the +first blood itself, are formed from the gut-fibre layer. Hence it was +called by earlier embryologists the “vascular layer.” In a sense the +term is quite correct. But it must not be understood as if all the +blood-vessels in the body came from this layer, or as if the whole of +this layer were taken up only with the formation of blood-vessels. +Neither of these suppositions is true. Blood-vessels may be formed +independently in other parts, especially in the various products of the +skin-fibre layer. + + +Fig.199. Human foetus, twelve weeks old, with its membranes. Fig. +199—Human fœtus, twelve weeks old, with its membranes. The umbilical +cord goes from its navel to the placenta. _b_ amnion, _c_ chorion, _d_ +placenta, _d_ apostrophe, relics of villi on smooth chorion, _f_ +internal or reflex decidua, _g_ external or true decidua. (From _B. +Schultze._) + + +Fig.200. Mature human foetus (at the end of the pregnancy, in its +natural position, taken out of the uterine cavity). Fig. 200—Mature +human fœtus (at the end of pregnancy, in its natural position, taken +out of the uterine cavity). On the inner surface of the latter (to the +left) is the placenta, which is connected by the umbilical cord with +the child’s navel. (From _Bernhard Schultze._) + + +The first blood-vessels of the mammal embryo have been considered by us +previously, and we shall study the development of the heart in the +second volume. + +In every vertebrate it lies at first in the ventral wall of the +fore-gut, or in the ventral (or cardiac) mesentery, by which it is +connected for a time with the wall of the body. But it soon severs +itself from the place of its origin, and lies freely in a cavity—the +cardiac cavity. For a short time it is still connected with the former +by the thin plate of the mesocardium. Afterwards it lies quite free in +the cardiac cavity, and is only directly connected with the gut-wall by +the vessels which issue from it. + + +Fig.201. Vitelline vessels in the germinative area of a chick-embryo, +at the close of the third day of incubation. Fig. 201—Vitelline vessels +in the germinative area of a chick-embryo, at the close of the third +day of incubation. (From _Balfour._) The detached germinative area is +seen from the ventral side: the arteries are dark, the veins light. _H_ +heart, _AA_ aorta-arches, _Ao_ aorta, _R.of.A_ right omphalo-mesenteric +artery, _S.T._ sinus terminalis, _ L.Of_ and _R.Of_ right and left +omphalo-mesenteric veins, _S.V._ sinus venosus, _D.C._ ductus Cuvieri, +_ S.Ca.V._ and _V.Ca._ fore and hind cardinal veins. + + +The fore-end of the spindle-shaped tube, which soon bends into an +S-shape (Figure 1.202), divides into a right and left branch. These +tubes are bent upwards arch-wise, and represent the first arches of the +aorta. They rise in the wall of the fore-gut, which they enclose in a +sense, and then unite above, in the upper wall of the fore gut-cavity, +to form a large single artery, that runs backward immediately under the +chorda, and is called the aorta (Fig. 201 _Ao_). The first pair of +aorta-arches rise on the inner wall of the first pair of gill-arches, +and so lie between the first gill-arch (_k_) and the fore-gut (_d_), +just as we find them throughout life in the fishes. The single aorta, +which results from the conjunction of these two first vascular arches, +divides again immediately into two parallel branches, which run +backwards on either side of the chorda. These are the primitive aortas +which we have already mentioned; they are also called the posterior +vertebral arteries. These two arteries now give off at each side, +behind, at right angles, four or five branches, and these pass from the +embryonic body to the germinative area, they +are called omphalo-mesenteric or vitelline arteries. They represent the +first beginning of a fœtal circulation. Thus, the first blood-vessels +pass over the embryonic body and reach as far as the edge of the +germinative area. At first they are confined to the dark or “vascular” +area. But they afterwards extend over the whole surface of the +embryonic vesicle. In the end, the whole of the yelk-sac is covered +with a vascular net-work. These vessels have to gather food from the +contents of the yelk-sac and convey it to the embryonic body. This is +done by the veins, which pass first from the germinative area, and +afterwards from the yelk-sac, to the farther end of the heart. They are +called vitelline, or, frequently, omphalo-mesenteric, veins. + +These vessels naturally atrophy with the degeneration of the umbilical +vesicle, and the vitelline circulation is replaced by a second, that of +the allantois. Large blood-vessels are developed in the wall of the +urinary sac or the allantois, as before, from the gut-fibre layer. +These vessels grow larger and larger, and are very closely connected +with the vessels that develop in the body of the embryo itself. Thus, +the secondary, allantoic circulation gradually takes the place of the +original vitelline circulation. When the allantois has attached itself +to the inner wall of the chorion and been converted into the placenta, +its blood-vessels alone effect the nourishment of the embryo. They are +called umbilical vessels, and are originally double—a pair of umbilical +arteries and a pair of umbilical veins. The two umbilical veins (Fig. +183 _u_), which convey blood from the placenta to the heart, open it +first into the united vitelline veins. The latter then disappear, and +the right umbilical vein goes with them, so that henceforth a single +large vein, the left umbilical vein, conducts all the blood from the +placenta to the heart of the embryo. The two arteries of the allantois, +or the umbilical arteries (Figs. 183 _n_, 184 _n_), are merely the +ultimate terminations of the primitive aortas, which are strongly +developed afterwards. This umbilical circulation is retained until the +nine months of embryonic life are over, and the human embryo enters +into the world as the independent individual. The umbilical cord (Fig. +196 _al_), in which these large blood-vessels pass from the embryo to +the placenta, comes away, together with the latter, in the after-birth, +and with the use of the lungs begins an entirely new form of +circulation, which is confined to the body of the infant. + + +Fig.202. Boat-shaped embryo of the dog, from the ventral side, +magnified. Fig. 202—Boat-shaped embryo of the dog, from the ventral +side, magnified. In front under the forehead we can see the first pair +of gill-arches; underneath is the S-shaped heart, at the sides of which +are the auditory vesicles. The heart divides behind into the two +vitelline veins, which expand in the germinative area (which is torn +off all round). On the floor of the open belly lie, between the +protovertebræ, the primitive aortas, from which five pairs of vitelline +arteries are given off. (From _ Bischoff._) + + +There is a great phylogenetic significance in the perfect agreement +which we find between man and the anthropoid apes in these important +features of embryonic circulation, and the special construction of the +placenta and the umbilical cord. We must infer from it a close +blood-relationship of man and the anthropomorphic apes—a common descent +of them from one and the same extinct group of lower apes. Huxley’s +“pithecometra-principle” applies to these ontogenetic features as much +as to any other morphological relations: “The differences in +construction of any part of the body are less between man and the +anthropoid apes than between the latter and the lower apes.” + +This important Huxleian law, the chief consequence of which is “the +descent of man from the ape,” has lately been confirmed in an +interesting and unexpected way from the side of the experimental +physiology of the blood. The experiments of Hans Friedenthal at Berlin +have shown that human blood, mixed with the blood of lower apes, has a +poisonous effect on the latter; the serum of the one destroys the +blood-cells of the other. But this does not happen when human blood is +mixed with that of the anthropoid ape. As we know from many other +experiments that the mixture of two different kinds of blood is only +possible without injury in the case of two closely related animals of +the same family, we have another proof of the close blood-relationship, +in the literal sense of the word, of man and the anthropoid ape. + + +Fig.203. Lar or white-handed gibbon (Hylobates lar or albimanus), from +the Indian mainland. Fig. 203—Lar or white-handed gibbon (_Hylobates +lar_ or _albimanus_), from the Indian mainland (From _Brehm._) + + +Fig.204. Young orang (Satyrus orang), asleep. Fig. 204—Young orang +(_Satyrus orang_), asleep. + + +The existing anthropoid apes are only a small remnant of a large family +of eastern apes (or _Catarrhinæ_), from which man was evolved about the +end of the Tertiary period. They fall into two geographical groups—the +Asiatic and the African anthropoids. In each group we can distinguish +two genera. The oldest of these four genera is the gibbon _Hylobates,_ +Fig. 203); there are from eight to twelve species of it in the East +Indies. I made observations of four of them during my voyage in the +East Indies (1901), and had a specimen of the ash-grey gibbon +(_Hylobates leuciscus_) living for several months in the garden of my +house in Java. I have described the interesting habits of this ape +(regarded by the Malays as the wild descendant of men who had lost +their way) in my _Malayischen_ +_Reisebriefen_ (chap. xi). Psychologically, he showed a good deal of +resemblance to the children of my Malay hosts, with whom he played and +formed a very close friendship. + + +Fig.205. Wild orang (Dyssatyrus auritus). Fig. 205—Wild orang +(_Dyssatyrus auritius_). (From _R. Fick_ and _Leutemann._). + + +The second, larger and stronger, genus of Asiatic anthropoid ape is the +orang (_Satyrus_); he is now found only in the islands of Borneo and +Sumatra. Selenka, who has published a very thorough _Study of the +Development and Cranial Structure of the Anthropoid Apes_ (1899), +distinguishes ten races of the orang, which may, however, also be +regarded as “local varieties or species.” They fall into two sub-genera +or genera: one group, _Dyssatyrus_ (orang-bentang, Fig. 205), is +distinguished for the strength of its limbs, and the formation of very +peculiar and salient cheek-pads in the elderly male; these are wanting +in the other group, the ordinary orang-outang (_Eusatyrus_). + +Several species have lately been distinguished in the two genera of the +black African anthropoid apes (chimpanzee and gorilla). In the genus +_Anthropithecus_ (or _Anthropopithecus,_ formerly _Troglodytes_), the +bald-headed chimpanzee, _A. calvus_ (Fig. 206), and the gorilla-like +_A. mafuca_ differ very strikingly from the ordinary _Anthropithecus +niger_ (Fig. 207), not only in the size and proportion of many parts of +the body, but also in the peculiar shape of the head, especially the +ears and lips, and in the hair and colour. The controversy that still +continues as to whether these different forms of + +chimpanzee and orang are “merely local varieties” or “true species” is +an idle one; as in all such disputes of classifiers there is an utter +absence of clear ideas as to what a species really is. + + +Fig.206. The bald-headed chimpanzee (Anthropithecus calvus). Female. +Fig. 206—The bald-headed chimpanzee (_Anthropithecus calvus_). Female. +This fresh species, described by Frank Beddard in 1897 as Troglodytes +calvus, differs considerably from the ordinary _A. niger_ Fig. 207) in +the structure of the head, the colouring, and the absence of hair in +parts. + + +Of the largest and most famous of all the anthropoid apes, the gorilla, +Paschen has lately discovered a giant-form in the interior of the +Cameroons, which seems to differ from the ordinary species (_Gorilla +gina_ Fig. 208), not only by its unusual size and strength, but also by +a special formation of the skull. This giant gorilla (_Gorilla gigas,_ +Fig. 209) is six feet eight inches long; the span of its great arms is +about nine feet; its powerful chest is twice as broad as that of a +strong man. + + +Fig.207. Female chimpanzee (Anthropithecus niger). Fig. 207—Female +chimpanzee (_Anthropithecus niger_). (From _ Brehm._) + + +The whole structure of this huge anthropoid ape is not merely very +similar to that of man, but it is substantially the same. “The same 200 +bones, arranged in the same way, form our internal skeleton; the same +300 muscles effect our movements; the same hair covers our skin; the +same groups of ganglionic cells compose the ingenious mechanism of our +brain; the same four-chambered heart is the central pump of our +circulation.” The really existing differences in the shape and size of +the various parts are explained by differences in their growth, due to +adaptation to different habits of life and unequal use of the various +organs. This of itself proves morphologically the descent of man from +the ape. We will return to the point in Chapter XXIII. But I wanted to +point already to this important solution of “the question of +questions,” because that agreement +in the formation of the embryonic membranes and in fœtal circulation +which I have described affords a particularly weighty proof of it. It +is the more instructive as even cenogenetic structures may in certain +circumstances have a high phylogenetic value. In conjunction with the +other facts, it affords a striking confirmation of our biogenetic law. + + +Fig.208. Female gorilla. Fig. 208—Female gorilla. (From _Brehm_). + + +Fig.209. Male giant-gorilla (Gorilla gigas), from Yaunde, in the +interior of the Cameroons. Killed by H. Paschen, stuffed by Umlauff. +Fig. 209—Male giant-gorilla (_Gorilla gigas_), from Yaunde, in the +interior of the Cameroons. Killed by H. Paschen, stuffed by Umlauff. + + + + +Chapter XVI. +STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT + + +In turning from the embryology to the phylogeny of man—from the +development of the individual to that of the species—we must bear in +mind the direct causal connection that exists between these two main +branches of the science of human evolution. This important causal nexus +finds its simplest expression in “the fundamental law of organic +development,” the content and purport of which we have fully considered +in the first chapter. According to this biogenetic law, ontogeny is a +brief and condensed recapitulation of phylogeny. If this compendious +reproduction were complete in all cases, it would be very easy to +construct the whole story of evolution on an embryonic basis. When we +wanted to know the ancestors of any higher organism, and, therefore, of +man—to know from what forms the race as a whole has been evolved we +should merely have to follow the series of forms in the development of +the individual from the ovum; we could then regard each of the +successive forms as the representative of an extinct ancestral form. +However, this direct application of ontogenetic facts to phylogenetic +ideas is possible, without limitations, only in a very small section of +the animal kingdom. There are, it is true, still a number of lower +invertebrates (for instance, some of the Zoophyta and Vermalia) in +which we are justified in recognising at once each embryonic form as +the historical reproduction, or silhouette, as it were, of an extinct +ancestor. But in the great majority of the animals, and in the case of +man, this is impossible, because the embryonic forms themselves have +been modified through the change of the conditions of existence, and +have lost their original character to some extent. During the +immeasurable course of organic history, the many millions of years +during which life was developing on our planet, secondary changes of +the embryonic forms have taken place in most animals. The young of +animals (not only detached larvæ, but also the embryos enclosed in the +womb) may be modified by the influence of the environment, just as well +as the mature organisms are by adaptation to the conditions of life; +even species are altered during the embryonic development. Moreover, it +is an advantage for all higher organisms (and the advantage is greater +the more advanced they are) to curtail and simplify the original course +of development, and thus to obliterate the traces of their ancestors. +The higher the individual organism is in the animal kingdom, the less +completely does it reproduce in its embryonic development the series of +its ancestors, for reasons that are as yet only partly known to us. The +fact is easily proved by comparing the different developments of higher +and lower animals in any single stem. + +In order to appreciate this important feature, we have distributed the +embryological phenomena in two groups, _ palingenetic_ and +_cenogenetic._ Under palingenesis we count those facts of embryology +that we can directly regard as a faithful synopsis of the corresponding +stem-history. By cenogenesis we understand those embryonic processes +which we cannot directly correlate with corresponding evolutionary +processes, but must regard as modifications or falsifications of them. +With this careful discrimination between palingenetic and cenogenetic +phenomena, our biogenetic law assumes the following more precise +shape:—The rapid and brief development of the individual (ontogeny) is +a condensed synopsis of the long and slow history of the stem +(phylogeny): this synopsis is the more faithful and complete in +proportion as the original features have been preserved by heredity, +and modifications have not been introduced by adaptation. + + +In order to distinguish correctly between palingenetic and cenogenetic +phenomena in embryology, and deduce sound conclusions in connection +with stem-history, we must especially make a comparative study of the +former. In doing this it is best to employ the methods that have long +been used by geologists for the purpose of establishing the succession +of the sedimentary rocks in the crust of the earth. This solid crust, +which encloses the glowing central mass like a thin shell, is composed +of different kinds of rocks: there are, firstly, the volcanic rocks +which were formed directly by the cooling at the surface of the molten +mass of the earth; secondly, there are the sedimentary rocks, that have +been made out of the former by the action of water, and have been laid +in successive strata at the bottom of the sea. Each of these +sedimentary strata was at first a soft layer of mud; but in the course +of thousands of years it condensed into a solid, hard mass of stone +(sandstone, limestone, marl, etc.), and at the same time permanently +preserved the solid and imperishable bodies that had chanced to fall +into the soft mud. Among these bodies, which were either fossilised or +left characteristic impressions of their forms in the soft slime, we +have especially the more solid parts of the animals and plants that +lived and died during the deposit of the slimy strata. + +Hence each of the sedimentary strata has its characteristic fossils, +the remains of the animals and plants that lived during that particular +period of the earth’s history. When we make a comparative study of +these strata, we can survey the whole series of such periods. All +geologists are now agreed that we can demonstrate a definite historical +succession in the strata, and that the lowest of them were deposited in +very remote, and the uppermost in comparatively recent, times. However, +there is no part of the earth where we find the series of strata in its +entirety, or even approximately complete. The succession of strata and +of corresponding historical periods generally given in geology is an +ideal construction, formed by piecing together the various partial +discoveries of the succession of strata that have been made at +different points of the earth’s surface (cf. Chapter XVIII). + +We must act in this way in constructing the phylogeny of man. We must +try to piece together a fairly complete picture of the series of our +ancestors from the various phylogenetic fragments that we find in the +different groups of the animal kingdom. We shall see that we are really +in a position to form an approximate picture of the evolution of man +and the mammals by a proper comparison of the embryology of very +different animals—a picture that we could never have framed from the +ontogeny of the mammals alone. As a result of the above-mentioned +cenogenetic processes—those of disturbed and curtailed heredity—whole +series of lower stages have dropped out in the embryonic development of +man and the other mammals especially from the earliest periods, or been +falsified by modification. But we find these lower stages in their +original purity in the lower vertebrates and their invertebrate +ancestors. Especially in the lowest of all the vertebrates, the +lancelet or Amphioxus, we have the oldest stem-forms completely +preserved in the embryonic development. We also find important evidence +in the fishes, which stand between the lower and higher vertebrates, +and throw further light on the course of evolution in certain periods. +Next to the fishes come the amphibia, from the embryology of which we +can also draw instructive conclusions. They represent the transition to +the higher vertebrates, in which the middle and older stages of +ancestral development have been either distorted or curtailed, but in +which we find the more recent stages of the phylogenetic process well +preserved in ontogeny. We are thus in a position to form a fairly +complete idea of the past development of man’s ancestors within the +vertebrate stem by putting together and comparing the embryological +developments of the various groups of vertebrates. And when we go below +the lowest vertebrates and compare their embryology with that of their +invertebrate relatives, we can follow the genealogical tree of our +animal ancestors much farther, down to the very lowest groups of +animals. + +In entering the obscure paths of this phylogenetic labyrinth, clinging +to the Ariadne-thread of the biogenetic law and guided by the light of +comparative anatomy, we will first, in accordance with the methods we +have adopted, discover and arrange those fragments from the manifold +embryonic developments of very different animals from which the +stem-history of man can be composed. I would call attention +particularly to the fact that +we can employ this method with the same confidence and right as the +geologist. No geologist has ever had ocular proof that the vast rocks +that compose our Carboniferous or Jurassic or Cretaceous strata were +really deposited in water. Yet no one doubts the fact. Further, no +geologist has ever learned by direct observation that these various +sedimentary formations were deposited in a certain order; yet all are +agreed as to this order. This is because the nature and origin of these +rocks cannot be rationally understood unless we assume that they were +so deposited. These hypotheses are universally received as safe and +indispensable “geological theories,” because they alone give a rational +explanation of the strata. + +Our evolutionary hypotheses can claim the same value, for the same +reasons. In formulating them we are acting on the same inductive and +deductive methods, and with almost equal confidence, as the geologist. +We hold them to be correct, and claim the status of “biological +theories” for them, because we cannot understand the nature and origin +of man and the other organisms without them, and because they alone +satisfy our demand for a knowledge of causes. And just as the +geological hypotheses that were ridiculed as dreams at the beginning of +the nineteenth century are now universally admitted, so our +phylogenetic hypotheses, which are still regarded as fantastic in +certain quarters, will sooner or later be generally received. It is +true that, as will soon appear, our task is not so simple as that of +the geologist. It is just as much more difficult and complex as man’s +organisation is more elaborate than the structure of the rocks. + +When we approach this task, we find an auxiliary of the utmost +importance in the comparative anatomy and embryology of two lower +animal-forms. One of these animals is the lancelet (_Amphioxus_), the +other the sea-squirt (_Ascidia_). Both of these animals are very +instructive. Both are at the border between the two chief divisions of +the animal kingdom—the vertebrates and invertebrates. The vertebrates +comprise the already mentioned classes, from the Amphioxus to man +(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and +mammals). Following the example of Lamarck, it is usual to put all the +other animals together under the head of invertebrates. But, as I have +often mentioned already, the group is composed of a number of very +different stems. Of these we have no interest just now in the +echinoderms, molluscs, and articulates, as they are independent +branches of the animal-tree, and have nothing to do with the +vertebrates. On the other hand, we are greatly concerned with a very +interesting group that has only recently been carefully studied, and +that has a most important relation to the ancestral tree of the +vertebrates. This is the stem of the Tunicates. One member of this +group, the sea-squirt, very closely approaches the lowest vertebrate, +the Amphioxus, in its essential internal structure and embryonic +development. Until 1866 no one had any idea of the close connection of +these apparently very different animals; it was a very fortunate +accident that the embryology of these related forms was discovered just +at the time when the question of the descent of the vertebrates from +the invertebrates came to the front. In order to understand it +properly, we must first consider these remarkable animals in their +fully-developed forms and compare their anatomy. + +We begin with the lancelet—after man the most important and interesting +of all animals. Man is at the highest summit, the lancelet at the +lowest root, of the vertebrate stem. + +It lives on the flat, sandy parts of the Mediterranean coast, partly +buried in the sand, and is apparently found in a number of seas.[28] It +has been found in the North Sea (on the British and Scandinavian coasts +and in Heligoland), and at various places on the Mediterranean (for +instance, at Nice, Naples, and Messina). It is also found on the coast +of Brazil and in the most distant parts of the Pacific Ocean (the coast +of Peru, Borneo, China, Australia, etc.). Recently eight to ten species +of the amphioxus have been determined, distributed in two or three +genera. + + [28] See the ample monograph by Arthur Willey, _ Amphioxus and the + Ancestry of the Vertebrates_; Boston, 1894. + + +Johannes Müller classed the lancelet with the fishes, although he +pointed out that the differences between this simple vertebrate and the +lowest fishes are much greater than between the fishes and the +amphibia. But this was far from expressing the real significance of the +animal. We may confidently lay down the following principle: The +Amphioxus differs more from the fishes than the fishes do from +man and the other vertebrates. As a matter of fact, it is so different +from all the other vertebrates in its whole organisation that the laws +of logical classification compel us to distinguish two divisions of +this stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, +the Craniota (man and the other vertebrates). The first and lower +division comprises the vertebrates that have no vertebræ or skull +(_cranium_). Of these the only living representatives are the Amphioxus +and Paramphioxus, though there must have been a number of different +species at an early period of the earth’s history. + + +Fig.210. The lancelet (Amphioxus lanceolatus), left view. Fig. 211. +Transverse section of the head of the Amphioxus. Fig. 210—The lancelet +(_Amphioxus lanceolatus_), left view. The long axis is vertical; the +mouth-end is above, the tail-end below; _a_ mouth, surrounded by +threads of beard; _b_ anus, _c_ gill-opening (_porus branchialis_), _d_ +gill-crate, _ e_ stomach, _f_ liver, _g_ small intestine, _h_ branchial +cavity, _i_ chorda (axial rod), underneath it the aorta; _k_ aortic +arches, _l_ trunk of the branchial artery, _m_ swellings on its +branches, _n_ vena cava, _ o_ visceral vein. +Fig. 211—Transverse section of the head of the Amphioxus. (From +_Boveri._) Above the branchial gut (_kd_) is the chorda, above this the +neural tube (in which we can distinguish the inner grey and the outer +white matter); above again is the dorsal fin (_fh_). To the right and +left above (in the episoma) are the thick muscular plates (_m_); below +(in the hyposoma) the gonads (_g_). _ ao_ aorta (here double), _c_ +corium, _ec_ endostyl, _f_ fascie, _gl_ glomerulus of the kidneys, _k_ +branchial vessel, _ld_ partition between the cœloma (_sc_) and atrium +(_p_), _mt_ transverse ventral muscle, _n_ renal canals, of upper and +_uf_ lower canals in the mantle-folds, _p_ peribranchial cavity, +(atrium), _ sc_ cœloma (subchordal body-cavity), _si_ principal (or +subintestinal) vein, _sk_ perichorda (skeletal layer). + + +Opposed to the Acrania is the second division of the vertebrates, which +comprises all the other members of the stem, from the fishes up to man. +All these vertebrates have a head quite distinct from the trunk, with a +skull (_cranium_) and brain; all have a centralised heart, fully-formed +kidneys, etc. Hence they are called the _Craniota._ These Craniotes +are, however, without a skull in their earlier period. As we already +know from embryology, even man, like every other mammal, passes in the +earlier course of his development through the important stage which we +call the chordula; at this lower stage the animal has neither vertebræ +nor skull nor limbs (Figs. 83–86). And even after the formation of the +primitive vertebræ has begun, the segmented fœtus of the amniotes still +has for a long time the simple form of a lyre-shaped disk or a sandal, +without limbs or extremities. When we compare this embryonic condition, +the sandal-shaped fœtus, with the developed lancelet, we may say that +the amphioxus is, in a certain sense, a permanent sandal-embryo, or a +permanent embryonic form of the Acrania; it never rises above a low +grade of development which we have long since passed. + +The fully-developed lancelet (Fig. 210) is about two inches long, is +colourless or of a light red tint, and has the shape of a narrow +lancet-formed leaf. The body is pointed at both ends, but much +compressed at the sides. There is no trace of limbs. The outer skin is +very thin and delicate, naked, transparent, and composed of two +different layers, a simple external stratum of cells, the epidermis, +and a thin underlying cutis-layer. Along the middle line of the back +runs a narrow fin-fringe which expands behind into an oval tail-fin, +and is continued below in a short anus-fin. The fin-fringe is supported +by a number of square elastic fin-plates. + +In the middle of the body we find a thin string of cartilage, which +goes the whole length of the body from front to back, and is pointed at +both ends (Fig. 210 _i_). This straight, cylindrical rod (somewhat +compressed for a time) is the axial rod or the _chorda dorsalis_; in +the lancelet this is the only trace of a vertebral column. The chorda +develops no further, but retains its original simplicity throughout +life. It is enclosed by a firm membrane, the chorda-sheath or +_perichorda._ The real features of this and of its dependent formations +are best seen in the transverse section of the Amphioxus (Fig. 211). +The perichorda forms a cylindrical tube immediately over the chorda, +and the central nervous system, the medullary tube, is enclosed in it. +This important psychic organ also remains in its simplest shape +throughout life, as a cylindrical tube, terminating with almost equal +plainness at either end, and enclosing a narrow canal in its thick +wall. However, the fore end is a little rounder, and contains a small, +almost imperceptible bulbous swelling of the canal. This must be +regarded as the beginning of a rudimentary brain. At the foremost end +of it there is a small black pigment-spot, a rudimentary eye; and a +narrow canal leads to a superficial sense-organ. In the vicinity of +this optic spot we find at the left side a small ciliated depression, +the single olfactory organ. There is no organ of hearing. This +defective development of the higher sense-organs is probably, in the +main, not an original feature, but a result of degeneration. + +Underneath the axial rod or chorda runs a very simple alimentary canal, +a tube that opens on the ventral side of the animal by a mouth in front +and anus behind. The oval mouth is surrounded by a ring of cartilage, +on which there are twenty to thirty cartilaginous threads (organs of +touch, Fig. 210 _a_). The alimentary canal divides into sections of +about equal length by a constriction in the middle. The fore section, +or head-gut, serves for respiration; the hind section, or trunk-gut, +for digestion. The limit of the two alimentary regions is also the +limit of the two parts of the body, the head and the trunk. The +head-gut or branchial gut forms a broad gill-crate, the grilled wall of +which is pierced by numbers of gill-clefts (Fig. 210 _d_). The fine +bars of the gill-crate between the clefts are strengthened with firm +parallel rods, and these are connected in pairs by cross-rods. The +water that enters the mouth of the Amphioxus passes through these +clefts into the large surrounding branchial cavity or _ atrium,_ and +then pours out behind through a hole in it, the respiratory pore +(_porus branchialis,_ Fig. 210 _c_). Below, on the ventral side of the +gill-crate, there is in the middle +line a ciliated groove with a glandular wall (the hypobranchial +groove), which is also found in the Ascidia and the larvæ of the +Cyclostoma. It is interesting because the thyroid gland in the larynx +of the higher vertebrates (underneath the “Adam’s apple”) has been +developed from it. + +Behind the respiratory part of the gut we have the digestive section, +the trunk or liver (hepatic) gut. The small particles that the +Amphioxus takes in with the water—infusoria, diatoms, particles of +decomposed plants and animals, etc.—pass from the gill-crate into the +digestive part of the canal, and are used up as food. From a somewhat +enlarged portion, that corresponds to the stomach (Fig. 210 _e_), a +long, pouch-like blind sac proceeds straight forward (_f_); it lies +underneath on the left side of the gill-crate, and ends blindly about +the middle of it. This is the liver of the Amphioxus, the simplest kind +of liver that we meet in any vertebrate. In man also the liver +develops, as we shall see, in the shape of a pouch-like blind sac, that +forms out of the alimentary canal behind the stomach. + + +Fig.212. Transverse section of an Amphioxus-larva, with five +gill-clefts, through the middle of the body. Fig. 213. Diagram of the +preceding. Fig. 212—Transverse section of an Amphioxus-larva, with five +gill-clefts, through the middle of the body. Fig. 213—Diagram of the +preceding. (From _ Hatschek._) _A_ epidermis, _B_ medullary tube, _ C_ +chorda, _C_1 inner chorda-sheath, _D_ visceral epithelium, _E_ +sub-intestinal vein. _1_ cutis, _2_ muscle-plate (myotome), _3_ +skeletal plate (sclerotome), _4_ cœloseptum (partition between dorsal +and ventral cœloma), _5_ skin-fibre layer, _6_ gut-fibre layer, _I_ +myocœl (dorsal body-cavity), _ II_ splanchnocœl (ventral body-cavity).) + + +The formation of the circulatory system in this animal is not less +interesting. All the other vertebrates have a compressed, thick, +pouch-shaped heart, which develops from the wall of the gut at the +throat, and from which the blood-vessels proceed; in the Amphioxus +there is no special centralised heart, driving the blood by its +pulsations. This movement is effected, as in the annelids, by the thin +blood-vessels themselves, which discharge the function of the heart, +contracting and pulsating in their whole length, and thus driving the +colourless blood through the entire body. On the under-side of the +gill-crate, in the middle line, there is the trunk of a large vessel +that corresponds to the heart of the other vertebrates and the trunk of +the branchial artery that proceeds from it; this drives the blood into +the gills (Fig. 210 _l_). A number of small vascular arches arise on +each side from this branchial artery, and form little heart-shaped +swellings or _ bulbilla_ (_m_) at their points of departure; they +advance along the branchial arches, between the gill-clefts and the +fore-gut, and unite, as branchial veins, above the gill-crate in a +large trunk blood-vessel that runs under the chorda dorsalis. This is +the principal artery or primitive aorta (Fig. 214 _D_). The branches +which it gives off to all parts of the body unite again in a larger +venous vessel at the underside of the gut, called the subintestinal +vein (Figs. 210 _o,_ 212 _E_). This single main vessel of the Amphioxus +goes like a closed circular water-conduit along the alimentary canal +through the whole body, and pulsates in its whole length above and +below. When the upper tube contracts the lower one is filled with +blood, and _vice versa._ In the upper tube the blood flows from front +to rear, then back from rear to front in the lower vessel. The whole of +the long tube that runs along the ventral side of the alimentary canal +and contains venous blood may be called the “principal vein,” and may +be compared to the ventral vessel in the worms. On the other hand, the +long +straight vessel that runs along the dorsal line of the gut above, +between it and the chorda, and contains arterial blood, is clearly +identical with the aorta or principal artery of the other vertebrates; +and on the other side it may be compared to the dorsal vessel in the +worms. + +The cœloma or body-cavity has some very important and distinctive +features in the Amphioxus. The embryology of it is most instructive in +connection with the stem-history of the body-cavity in man and the +other vertebrates. As we have already seen (Chapter X), in these the +two cœlom-pouches are divided at an early stage by transverse +constrictions into a double row of primitive segments (Fig. 124), and +each of these subdivides, by a frontal or lateral constriction, into an +upper (dorsal) and lower (ventral) pouch. + + +Fig.214. Transverse section of a young Amphioxus, immediately after +metamorphosis. Fig. 215. Diagram of preceding. Fig. 214—Transverse +section of a young Amphioxus, immediately after metamorphosis, through +the hindermost third (between the atrium-cavity and the anus). Fig. +215—Diagram of preceding. (From _Hatschek._) _A_ epidermis, _B_ +medullary tube, _C_ chorda, _ D_ aorta, _E_ visceral epithelium, _F_ +subintestinal vein. _1_ corium-plate, _2_ muscle-plate, _3_ +fascie-plate, _4_ outer chorda-sheath, _5_ myoseptum, _ 6_ skin-fibre +plate, _7_ gut-fibre plate, _I_ myocœl, _II_ splanchnocœl, _I_1 dorsal +fin, _I_2 anus-fin.) + + +These important structures are seen very clearly in the trunk of the +amphioxus (the latter third, Figs. 212–215), but it is otherwise in the +head, the foremost third (Fig. 216). Here we find a number of +complicated structures that cannot be understood until we have studied +them on the embryological side in the next chapter (cf. Fig. 81). The +branchial gut lies free in a spacious cavity filled with water, which +was wrongly thought formerly to be the body-cavity (Fig. 216 _A_). As a +matter of fact, this atrium (commonly called the peribranchial cavity) +is a secondary structure formed by the development of a couple of +lateral mantle-folds or gill-covers (_M_1, _U_). The real body-cavity +(_Lh_) is very narrow and entirely closed, lined with epithelium. The +peribranchial cavity (_A_) is full of water, and its walls are lined +with the skin-sense layer; it opens outwards in the rear through the +respiratory pore (Fig. 210 _ c_). + +On the inner surface of these mantle-folds (_M_1), in the ventral half +of the wide mantle cavity (atrium), we find the sex-organs of the +Amphioxus. At each side of the branchial gut there are between twenty +and thirty roundish four-cornered sacs, which can clearly be seen from +without with the naked eye, as they shine through the thin transparent +body-wall. These sacs are the sexual glands they are the same size and +shape in both sexes, only differing in contents. In the female they +contain a quantity of simple ova (Fig. 219 _g_); in the male a number +of much smaller cells that change into mobile ciliated cells +(sperm-cells). Both sacs lie on the inner wall of the atrium, and have +no special outlets. When the ova of the female and the sperm of the +male are ripe, they fall into the atrium, pass through the gill-clefts +into the +fore-gut, and are ejected through the mouth. + + +Fig.216. Transverse section of the lancelet, in the fore half. Fig. +216—Transverse section of the lancelet, in the fore half. (From +_Ralph._) The outer covering is the simple cell-layer of the epidermis +(_E_). Under this is the thin corium, the subcutaneous tissue of which +is thickened; it sends connective-tissue partitions between the muscles +(_M_1) and to the chorda-sheath. (_N_ medullary tube, _Ch_ chorda, _Lh_ +body-cavity, _A_ atrium, _L_ upper wall of same, _E_1 inner wall, _E_2 +outer wall, _Lh_1 ventral remnant of same, _Kst_ gill-reds, _M_ ventral +muscles, _R_ seam of the joining of the ventral folds (gill-covers), +_G_ sexual glands. + + +Above the sexual glands, at the dorsal angle of the atrium, we find the +kidneys. These important excretory organs could not be found in the +Amphioxus for a long time, on account of their remote position and +their smallness; they were discovered in 1890 by Theodor Boveri (Fig. +217 _x_). They are short segmented canals; corresponding to the +primitive kidneys of the other vertebrates (Fig. 218 _B_). Their +internal aperture (Fig. 217 _B_) opens into the body-cavity; their +outer aperture into the atrium (_C_). The prorenal canals lie in the +middle of the line of the head, outwards from the uppermost section of +the gill-arches, and have important relations to the branchial vessels +(_H_). For this reason, and in their whole arrangement, the primitive +kidneys of the Amphioxus +show clearly that they are equivalent to the prorenal canals of the +Craniotes (Fig. 218 _B_). The prorenal duct of the latter (Fig. 218 +_C_) corresponds to the branchial cavity or atrium of the former (Fig. +217 _C_). + + +Fig.217. Transverse section through the middle of the Amphioxus. Fig. +218. Transverse section of a primitive fish embryo. Fig. 217—Transverse +section through the middle of the Amphioxus. (From _Boveri._) On the +left a gill-rod has been struck, and on the right a gill-cleft; +consequently on the left we see the whole of a prorenal canal (_x_), on +the right only the section of its fore-leg. _A_ genital chamber +(ventral section of the gonocœl), _x_ pronephridium, _B_ its +cœlom-aperture, _C_ atrium, _D_ body-cavity, _E_ visceral cavity, _F_ +subintestinal vein, _G_ aorta (the left branch connected by a branchial +vessel with the subintestinal vein), _H_ renal vessel. Fig. +218—Transverse section of a primitive fish embryo (Selachii-embryo, +from _Boveri._). To the left pronephridia (_B_), the right primitive +kidneys (_A_). The dotted lines on the right indicate the later opening +of the primitive kidney canals (_A_) into the prorenal duct (_C_). _ D_ +body-cavity, _E_ visceral cavity, _F_ subintestinal vein, _G_ aorta, +_H_ renal vessel. + + +If we sum up the results of our anatomic study of the Amphioxus, and +compare them with the familiar organisation of man, we shall find an +immense distance between the two. As a fact, the highest summit of the +vertebrate organisation which man represents is in every respect so far +above the lowest stage, at which the lancelet remains, that one would +at first scarcely believe it possible to class both animals in the same +division of the animal kingdom. Nevertheless, this classification is +indisputably just. Man is only a more advanced stage of the vertebral +type that we find unmistakably in the Amphioxus in its characteristic +features. We need only recall the picture of the ideal Primitive +Vertebrate given in a former chapter, and compare it with the lower +stages of human embryonic development, to convince ourselves of our +close relationship to the lancelet. (Cf. Chapter XI) + +It is true that the Amphioxus is far below all other living +vertebrates. It is true that it has no separate head, no developed +brain or skull, the characteristic feature of the other vertebrates. +It is (probably as a result of degeneration) without the auscultory +organ and the centralised heart that all the others have; and it has no +fully-formed kidneys. Every single organ in it is simpler and less +advanced than in any of the others. Yet the characteristic connection +and arrangement of all the organs is just the same as in the other +vertebrates. All these, moreover, pass, during their embryonic +development, through a stage in which their whole organisation is no +higher than that of the Amphioxus, but is substantially identical with +it. + + +Fig.219. Transverse section of the head of the Amphioxus. Fig. +219—Transverse section of the head of the Amphioxus (at the limit of +the first and second third of the body). (From _Boveri_) _a_ aorta +(here double), _b_ atrium, _c_ chorda, _co_ umlaut cœloma +(body-cavity), _e_ endostyl (hypobranchial groove), _g_ gonads +(ovaries), _kb_ gill-arches, _kd_ branchial gut, _l_ liver-tube (on the +right, one-sided), _m_ muscles, _n_ renal canals, _r_ spinal cord, _sn_ +spinal nerves, _sp_ gill-clefts. + + +In order to see this quite clearly, it is particularly useful to +compare the Amphioxus with the youthful forms of those vertebrates that +are classified next to it. This is the class of the Cyclostoma. There +are to-day only a few species of this once extensive class, and these +may be distributed in two groups. One group comprises the hag-fishes or +Myxinoides. The other group are the Petromyzontes, or lampreys, which +are a familiar delicacy in their marine form. These Cyclostoma are +usually classified with the fishes. But they are far below the true +fishes, and form a very interesting connecting-group between them and +the lancelet. One can see how closely they approach the latter by +comparing a young lamprey with the Amphioxus. The chorda is of the same +simple character in both; also the medullary tube, that lies above the +chorda, and the alimentary canal below it. However, in the lamprey the +spinal cord swells in front into a simple pear-shaped cerebral vesicle, +and at each side of it there are a very simple eye and a rudimentary +auditory vesicle. The nose is a single pit, as in the Amphioxus. The +two sections of the gut are also just the same and very rudimentary in +the lamprey. On the other hand, we see a great advance in the structure +of the heart, which is found underneath the gills in the shape of a +centralised muscular tube, and is divided into an auricle and a +ventricle. Later on the lamprey advances still further, and gets a +skull, five cerebral vesicles, a series of independent gill-pouches, +etc. This makes all the more interesting the striking resemblance of +its immature larva to the developed and sexually mature Amphioxus. + +While the Amphioxus is thus connected through the Cyclostoma with the +fishes, and so with the series of the higher vertebrates, it is, on the +other hand, very closely related to a lowly invertebrate marine animal, +from which it seems to be entirely remote at first glance. This +remarkable animal is the sea-squirt or Ascidia, which was formerly +thought to be closely related to the mussel, and so classed in the +molluscs. But since the remarkable embryology of these animals was +discovered in 1866, there can be no question that they have nothing to +do with the molluscs. To the great astonishment of zoologists, they +were found, in their whole individual development, to be closely +related to the vertebrates. When fully developed the Ascidiæ are +shapeless lumps that would not, at first sight, be taken for animals at +all. The oval body, frequently studded with knobs or uneven and lumpy, +in which we can discover no special external organs, is attached at one +end to marine plants, rocks, or the floor of the sea. Many species look +like potatoes, others like melon-cacti, others like prunes. Many of the +Ascidiæ form transparent crusts or +deposits on stones and marine plants. Some of the larger species are +eaten like oysters. Fishermen, who know them very well, think they are +not animals, but plants. They are sold in the fish markets of many of +the Italian coast-towns with other lower marine animals under the name +of “sea-fruit” (_frutti di mare_). There is nothing about them to show +that they are animals. When they are taken out of the water with the +net the most one can perceive is a slight contraction of the body that +causes water to spout out in two places. The bulk of the Ascidiæ are +very small, at the most a few inches long. A few species are a foot or +more in length. There are many species of them, and they are found in +every sea. As in the case of the Acrania, we have no fossilised remains +of the class, because they have no hard and fossilisable parts. +However, they must be of great antiquity, and must go back to the +primordial epoch. + +The name of “Tunicates” is given to the whole class to which the +Ascidiæ belong, because the body is enclosed in a thick and stiff +covering like a mantle (_tunica_). This mantle—sometimes soft like +jelly, sometimes as tough as leather, and sometimes as stiff as +cartilage—has a number of peculiarities. The most remarkable of them is +that it consists of a woody matter, cellulose—the same vegetal +substance that forms the stiff envelopes of the plant-cells, the +substance of the wood. The tunicates are the only class of animals that +have a real cellulose or woody coat. Sometimes the cellulose mantle is +brightly coloured, at other times colourless. Not infrequently it is +set with needles or hairs, like a cactus. Often we find a mass of +foreign bodies—stone, sand, fragments of mussel-shells, etc.—worked +into the mantle. This has earned for the Ascidia the name of “the +microcosm.” + + +Fig.220. Organisation of an Ascidia (left view). Fig. 220—Organisation +of an Ascidia (left view); the dorsal side is turned to the right and +the ventral side to the left, the mouth (_o_) above; the ascidia is +attached at the tail end. The branchial gut (_br_), which is pierced by +a number of clefts, continues below in the visceral gut. The rectum +opens through the anus (_a_) into the atrium (_cl_), from which the +excrements are ejected with the respiratory water through the +mantle-hole or cloaca (_a_); _m_ mantle. (From _ Gegenbaur._ + + +The hind end, which corresponds to the tail of the Amphioxus, is +usually attached, often by means of regular roots. The dorsal and +ventral sides differ a good deal internally, but frequently cannot be +distinguished externally. If we open the thick tunic or mantle in order +to examine the internal organisation, we first find a spacious cavity +filled with water—the mantle-cavity or respiratory cavity (Fig. 220 +_cl_). It is also called the branchial cavity and the cloaca, because +it receives the excrements and sexual products as well as the +respiratory water. The greater part of the respiratory cavity is +occupied by the large grated branchial sac (_br_). This is so like the +gill-crate of the Amphioxus in its whole arrangement that the +resemblance was pointed out by the English naturalist Goodsir, years +ago, before anything was known of the relationship of the two animals. +As a fact, even in the Ascidia the mouth (_o_) opens first into this +wide branchial sac. The respiratory water passes through the +lattice-work of the branchial sac into the branchial cavity, and is +ejected from this by the respiratory pore (_a_′). Along the ventral +side of the branchial sac runs a ciliated groove—the hypobranchial +groove which we have previously found at the same spot in the +Amphioxus. The food of the Ascidia also +consists of tiny organisms, infusoria, diatoms, parts of decomposed +marine plants and animals; etc. These pass with the water into the +gill-crate and the digestive part of the gut at the end of it, at first +into an enlargement of it that represents the stomach. The adjoining +small intestine usually forms a loop, bends forward, and opens by an +anus (Fig. 220 _a_), not directly outwards, but first into the mantle +cavity; from this the excrements are ejected by a common outlet (_a_′) +together with the used-up water and the sexual products. The outlet is +sometimes called the branchial pore, and sometimes the cloaca or +ejection-aperture. In many of the Ascidiæ a glandular mass opens into +the gut, and this represents the liver. In some there is another gland +besides the liver, and this is taken to represent the kidneys. The +body-cavity proper, or cœloma, which is filled with blood and encloses +the hepatic gut, is very narrow in the Ascidia, as in the Amphioxus, +and is here also usually confounded with the wide atrium, or +peribranchial cavity, full of water. + + +Organisation of an Ascidia (as in Fig. 220, seen from the left). Fig. +221—Organisation of an Ascidia (as in Fig. 220, seen from the left). +_sb_ branchial sac, _v_ stomach, _i_ small intestine, _c_ heart, _t_ +testicle, _vd_ sperm-duct, _o_ ovary, _ o_′ ripe ova in the branchial +cavity. The two small arrows indicate the entrance and exit of the +water through the openings of the mantle. (From _Milne-Edwards._) + + +There is no trace in the fully-developed Ascidia of a chorda dorsalis, +or internal axial skeleton. It is the more interesting that the young +animal that emerges from the ovum _has_ a chorda, and that there is a +rudimentary medullary tube above it. The latter is wholly atrophied in +the developed Ascidia, and looks like a small nerve-ganglion in front +above the gill-crate. It corresponds to the upper “gullet-ganglion” or +“primitive brain” in other vermalia. Special sense-organs are either +wanting altogether or are only found in a very rudimentary form, as +simple optic spots and touch-corpuscles or tentacles that surround the +mouth. The muscular system is very slightly and irregularly developed. +Immediately under the thin corium, and closely connected with it, we +find a thin muscle tube, as in the worms. On the other hand, the +Ascidia has a centralised heart, and in this respect it seems to be +more advanced than the Amphioxus. On the ventral side of the gut, some +distance behind the gill-crate, there is a spindle-shaped heart. It +retains permanently the simple tubular form that we find temporarily as +the first structure of the heart in the vertebrates. This simple heart +of the Ascidia has, however, a remarkable peculiarity. It contracts in +alternate directions. In all other animals the beat of the heart is +always in the same direction (generally from rear to front); it changes +in the Ascidia to the reverse direction. The heart contracts first from +the rear to the front, stands still for a minute, and then begins to +beat the opposite way, now driving the blood from front to rear; the +two large vessels that start from either end of the heart act +alternately as arteries and veins. This feature is found in the +Tunicates alone. + +Of the other chief organs we have still to mention the sexual glands, +which lie right behind in the body-cavity. All the Ascidiæ are +hermaphrodites. Each individual has a male and a female gland, and so +is able to fertilise itself. The ripe ova (Fig. 221 _o_′) fall directly +from the ovary (_o_) into the mantle-cavity. The male sperm is +conducted into this cavity from the testicle (_t_) by a special duct +(_vd_). Fertilisation is accomplished here, and in many of the Ascidiæ +developed embryos are found. These are then ejected +with the breathing-water through the cloaca (_q_), and so “born alive.” + +If we now glance at the entire structure of the simple Ascidia +(especially _Phallusia, Cynthia,_ etc.) and compare it with that of the +Amphioxus, we shall find that the two have few points of contact. It is +true that the fully-developed Ascidia resembles the Amphioxus in +several important features of its internal structure, and especially in +the peculiar character of the gill-crate and gut. But in most other +features of organisation it is so far removed from it, and is so unlike +it in external appearance, that the really close relationship of the +two was not discovered until their embryology was studied. We will now +compare the embryonic development of the two animals, and find to our +great astonishment that the same embryonic form develops from the ovum +of the Amphioxus as from that of the Ascidia—a typical _ chordula._ + + + + +Chapter XVII. +EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT + + +The structural features that distinguish the vertebrates from the +invertebrates are so prominent that there was the greatest difficulty +in the earlier stages of classification in determining the affinity of +these two great groups. When scientists began to speak of the affinity +of the various animal groups in more than a figurative—in a +genealogical—sense, this question came at once to the front, and seemed +to constitute one of the chief obstacles to the carrying-out of the +evolutionary theory. Even earlier, when they had studied the relations +of the chief groups, without any idea of real genealogical connection, +they believed they had found here and there among the invertebrates +points of contact with the vertebrates: some of the worms, especially, +seemed to approach the vertebrates in structure, such as the marine +arrow-worm (_Sagitta_). But on closer study the analogies proved +untenable. When Darwin gave an impulse to the construction of a real +stem-history of the animal kingdom by his reform of the theory of +evolution, the solution of this problem was found to be particularly +difficult. When I made the first attempt in my _General Morphology_ +(1866) to work out the theory and apply it to classification, I found +no problem of phylogeny that gave me so much trouble as the linking of +the vertebrates with the invertebrates. + +But just at this time the true link was discovered, and at a point +where it was least expected. Towards the end of 1866 two works of the +Russian zoologist, Kowalevsky, who had lived for some time at Naples, +and studied the embryology of the lower animals, were issued in the +publications of the St. Petersburg Academy. A fortunate accident had +directed the attention of this able observer almost simultaneously to +the embryology of the lowest vertebrate, the Amphioxus, and that of an +invertebrate, the close affinity of which to the Amphioxus had been +least suspected, the Ascidia. To the extreme astonishment of all +zoologists who were interested in this important question, there turned +out to be the utmost resemblance in structure from the commencement of +development between these two very different animals—the lowest +vertebrate and the mis-shaped, sessile invertebrate. With this +undeniable identity of ontogenesis, which can be demonstrated to an +astounding extent, we had, in virtue of the biogenetic law, discovered +the long-sought genealogical link, and definitely identified the +invertebrate group that represents the nearest blood-relatives of the +vertebrates. +The discovery was confirmed by other zoologists, and there can no +longer be any doubt that of all the classes of invertebrates that of +the Tunicates is most closely related to the vertebrates, and of the +Tunicates the nearest are the Ascidiæ. We cannot say that the +vertebrates are descended from the Ascidiæ—and still less the +reverse—but we can say that of all the invertebrates it is the +Tunicates, and, within this group, the Ascidiæ, that are the nearest +blood-relatives of the ancient stem-form of the vertebrates. We must +assume as the common ancestral group of both stems an extinct family of +the extensive vermalia-stem, the _Prochordonia_ or _Prochordata_ +(“primitive chorda-animals”). + +In order to appreciate fully this remarkable fact, and especially to +secure the sound basis we seek for the genealogical tree of the +vertebrates, it is necessary to study thoroughly the embryology of both +these animals, and compare the individual development of the Amphioxus +step by step with that of the Ascidia. We begin with the ontogeny of +the Amphioxus. + +From the concordant observations of Kowalevsky at Naples and Hatschek +at Messina, it follows, firstly, that the ovum-segmentation and +gastrulation of the Amphioxus are of the simplest character. They take +place in the same way as we find them in many of the lower animals of +different invertebrate stems, which we have already described as +original or primordial; the development of the Ascidia is of the same +type. Sexually mature specimens of the Amphioxus, which are found in +great quantities at Messina from April or May onwards, begin as a rule +to eject their sexual products in the evening; if you catch them about +the middle of a warm night and put them in a glass vessel with +seawater, they immediately eject through the mouth their accumulated +sexual products, in consequence of the disturbance. The males give out +masses of sperm, and the females discharge ova in such quantity that +many of them stick to the fibrils about their mouths. Both kinds of +cells pass first into the mantle-cavity after the opening of the +gonads, proceed through the gill-clefts into the branchial gut, and are +discharged from this through the mouth. + +The ova are simply round cells. They are only 1/250 of an inch in +diameter, and thus are only half the size of the mammal ova, and have +no distinctive features. The clear protoplasm of the mature ovum is +made so turbid by the numbers of dark granules of food-yelk or +deutoplasm scattered in it that it is difficult to follow the process +of fecundation and the behaviour of the two nuclei during it (p. 51). +The active elements of the male sperm, the cone-shaped spermatozoa, are +similar to those of most other animals (cf. Fig. 20). Fecundation takes +place when these lively ciliated cells of the sperm approach the ovum, +and seek to penetrate into the yelk-matter or the cellular substance of +the ovum with their head-part—the thicker part of the cell that +encloses the nucleus. Only one spermatozoon can bore its way into the +yelk at one pole of the ovum-axis; its head or nucleus coalesces with +the female nucleus, which remains after the extrusion of the directive +bodies from the germinal vesicle. Thus is formed the “stem-nucleus,” or +the nucleus of the “stem-cell” (cytula, Fig. 2). This now undergoes +total segmentation, dividing into two, four, eight, sixteen, thirty-two +cells, and so on. In this way we get the spherical, mulberry-shaped +body, which we call the _ morula._ + +The segmentation of the Amphioxus is not entirely regular, as was +supposed after the first observations of Kowalevsky (1866). It is not +completely equal, but a little unequal. As Hatschek afterwards found +(1879), the segmentation-cells only remain equal up to the +morula-stage, the spherical body of which consists of thirty-two cells. +Then, as always happens in unequal segmentation, the more sluggish +vegetal cells are outstripped in the cleavage. At the lower or vegetal +pole of the ovum a crown of eight large entodermic cells remains for a +long time unchanged, while the other cells divide, owing to the +formation of a series of horizontal circles, into an increasing number +of crowns of sixteen cells each. Afterwards the segmentation-cells get +more or less irregularly displaced, while the segmentation-cavity +enlarges in the centre of the morula; in the end the former all lie on +the surface of the latter, so that the fœtus attains the familiar +blastula shape and forms a hollow ball, the wall of which consists of a +single stratum of cells (Fig. 38 _ A–C_). This layer is the blastoderm, +the simple epithelium from the cells of which all the tissues of the +body proceed. + + +These important early embryonic processes take place so quickly in the +Amphioxus that four or five hours after fecundation, or about midnight, +the spherical blastula is completed. A pit-like depression is then +formed at the vegetal pole of it, and in consequence of this the hollow +sphere doubles on itself (Fig. 38 _D_). This pit becomes deeper and +deeper (Fig. 38 _E, F_); at last the invagination (or doubling) is +complete, and the inner or folded part of the blastula-wall lies on the +inside of the outer wall. We thus get a hollow hemisphere, the thin +wall of which is made up of two layers of cells (Fig. 38 _E_). From +hemispherical the body soon becomes almost spherical once more, and +then oval, the internal cavity enlarging considerably and its mouth +growing narrower (Fig. 213). The form which the Amphioxus-embryo has +thus reached is a real “cup-larva” or _gastrula,_ of the original +simple type that we have previously described as the “bell-gastrula” or +_archigastrula_ (Figs. 29–35). + +As in all the other animals that form an archigastrula, the whole body +is nothing but a simple gastric sac or stomach; its internal cavity is +the primitive gut (_progaster_ or _ archenteron,_ Fig. 38 _g,_ 35 _d_), +and its aperture the primitive mouth (_prostoma_ or _blastoporus, o_). +The wall is at once gut-wall and body-wall. It is composed of two +simple cell-layers, the familiar primary germinal layers. The inner +layer or the invaginated part of the blastoderm, which immediately +encloses the gut-cavity is the entoderm, the inner or vegetal +germ-layer, from which develop the wall of the alimentary canal and all +its appendages, the cœlom-pouches, etc. (Figs. 35, 36 _ i_). The outer +stratum of cells, or the non-invaginated part of the blastoderm, is the +ectoderm, the outer or animal germ-layer, which provides the outer skin +(epidermis) and the nervous system (_e_). The cells of the entoderm are +much larger, darker, and more fatty than those of the ectoderm, which +are clearer and less rich in fatty particles. Hence before and during +invagination there is an increasing differentiation of the inner from +the outer layer. The animal cells of the outer layer soon develop +vibratory hairs; the vegetal cells of the inner layer do so much later. +A thread-like process grows out of each cell, and effects continuous +vibratory movements. By the vibrations of these slender hairs the +gastrula of the Amphioxus swims about in the sea, when it has pierced +the thin ovolemma, like the gastrula of many other animals (Fig. 36). +As in many other lower animals, the cells have only one whip-like hair +each, and so are called _flagellate_ (whip) cells (in contrast with the +_ciliated_ cells, which have a number of short lashes or cilia). + +In the further course of its rapid development the roundish +bell-gastrula becomes elongated, and begins to flatten on one side, +parallel to the long axis. The flattened side is the subsequent dorsal +side; the opposite or ventral side remains curved. The latter grows +more quickly than the former, with the result that the primitive mouth +is forced to the dorsal side (Fig. 39). In the middle of the dorsal +surface a shallow longitudinal groove or furrow is formed (Fig. 79), +and the edges of the body rise up on each side of this groove in the +shape of two parallel swellings. This groove is, of course, the dorsal +furrow, and the swellings are the dorsal or medullary swellings; they +form the first structure of the central nervous system, the medullary +tube. The medullary swellings now rise higher; the groove between them +becomes deeper and deeper. The edges of the parallel swellings curve +towards each other, and at last unite, and the medullary tube is formed +(Figs. 83 _m,_ 84 _m_). Hence the formation of a medullary tube out of +the outer skin takes place in the naked dorsal surface of the +free-swimming larva of the Amphioxus in just the same way as we have +found in the embryo of man and the higher animals within the fœtal +membranes. + +Simultaneously with the construction of the medullary tube we have in +the Amphioxus-embryo the formation of the chorda, the cœlom-pouches, +and the mesoderm proceeding from their wall. These processes also take +place with characteristic simplicity and clearness, so that they are +very instructive to compare with the vermalia on the one hand and with +the higher vertebrates on the other. While the medullary groove is +sinking in the middle line of the flat dorsal side of the oval embryo, +and its parallel edges unite to form the ectodermic neural tube, the +single chorda is formed directly underneath them, and on each side of +this a parallel longitudinal fold, from the dorsal wall of the +primitive gut. These longitudinal folds of the entoderm proceed from +the primitive mouth, or from its lower +and hinder edge. Here we see at an early stage a couple of large +entodermic cells, which are distinguished from all the others by their +great size, round form, and fine-grained protoplasm; they are the two +promesoblasts, or polar cells of the mesoderm (Fig. 83 _p_). They +indicate the original starting-point of the two cœlom-pouches, which +grow from this spot between the inner and outer germinal layers, sever +themselves from the primitive gut, and provide the cellular material +for the middle layer. + +Immediately after their formation the two cœlom-pouches of the +Amphioxus are divided into several parts by longitudinal and transverse +folds. Each of the primary pouches is divided into an upper dorsal and +a lower ventral section by a couple of lateral longitudinal folds (Fig. +82). But these are again divided by several parallel transverse folds +into a number of successive sacs, the primitive segments or somites +(formerly called by the unsuitable name of “primitive vertebræ”). They +have a different future above and below. The upper or dorsal segments, +the _episomites,_ lose their cavity later on, and form with their cells +the muscular plates of the trunk. The lower or ventral segments, the +_hyposomites,_ corresponding to the lateral plates of the +craniote-embryo, fuse together in the upper part owing to the +disappearance of their lateral walls, and thus form the later +body-cavity (metacœl); in the lower part they remain separate, and +afterwards form the segmental gonads. + +In the middle, between the two lateral cœlom-folds of the primitive +gut, a single central organ detaches from this at an early stage in the +middle line of its dorsal wall. This is the dorsal chorda (Figs. 83, 84 +_ch_). This axial rod, which is the first foundation of the later +vertebral column in all the vertebrates, and is the only representative +of it in the Amphioxus, originates from the entoderm. + +In consequence of these important folding-processes in the primitive +gut, the simple entodermic tube divides into four different sections:— +I, underneath, at the ventral side, the permanent alimentary canal or +permanent gut; II, above, at the dorsal side, the axial rod or chorda; +and III, the two cœlom-sacs, which immediately sub-divide into two +structures:—IIIA, above, on the dorsal side, the _episomites,_ the +double row of primitive or muscular segments; and IIIB, below, on each +side of the gut, the _hyposomites,_ the two lateral plates that give +rise to the sex-glands, and the cavities of which partly unite to form +the body-cavity. At the same time, the neural or medullary tube is +formed above the chorda, on the dorsal surface, by the closing of the +parallel medullary swellings. All these processes, which outline the +typical structure of the vertebrate, take place with astonishing +rapidity in the embryo of the Amphioxus; in the afternoon of the first +day, or twenty-four hours after fertilisation, the young vertebrate, +the typical embryo, is formed; it then has, as a rule, six to eight +somites. + +The chief occurrence on the second day of development is the +construction of the two permanent openings of the gut—the mouth and +anus. In the earlier stages the alimentary tube is found to be entirely +closed, after the closing of the primitive mouth; it only communicates +behind by the neurenteric canal with the medullary tube. The permanent +mouth is a secondary formation, at the opposite end. Here, at the end +of the second day, we find a pit-like depression in the outer skin, +which penetrates inwards into the closed gut. The anus is formed behind +in the same way a few hours later (in the vicinity of the additional +gastrula-mouth). In man and the higher vertebrates also the mouth and +anus are formed, as we have seen, as flat pits in the outer skin; they +then penetrate inwards, gradually becoming connected with the blind +ends of the closed gut-tube. During the second day the Amphioxus-embryo +undergoes few other changes. The number of primitive segments +increases, and generally amounts to fourteen, some forty-eight to fifty +hours after impregnation. + +Almost simultaneously with the formation of the mouth the first +gill-cleft breaks through in the fore section of the Amphioxus-embryo +(generally forty hours after the commencement of development). It now +begins to nourish itself independently, as the food material stored up +in the ovum is completely used up. The further development of the free +larvæ takes place very slowly, and extends over several months. The +body becomes much longer, and is compressed at the sides, the head-end +being broadened in a sort of triangle. Two rudimentary sense-organs are +developed in it. Inside we find the first blood-vessels, an upper or +dorsal vessel, corresponding to the aorta, between the gut and the +dorsal cord, and a lower or ventral +vessel, corresponding to the subintestinal vein, at the lower border of +the gut. Now, the gills or respiratory organs also are formed at the +fore-end of the alimentary canal. The whole of the anterior or +respiratory section of the gut is converted into a gill-crate, which is +pierced trellis-wise by numbers of branchial-holes, as in the ascidia. +This is done by the foremost part of the gut-wall joining star-wise +with the outer skin, and the formation of clefts at the point of +connection, piercing the wall and leading into the gut from without. At +first there are very few of these branchial clefts; but there are soon +a number of them—first in one, then in two, rows. The foremost +gill-cleft is the oldest. In the end we have a sort of lattice work of +fine gill-clefts, supported on a number of stiff branchial rods; these +are connected in pairs by transverse rods. + + +Figs. 222-224. Transverse sections of young Amphioxus-larvae. Figs. +222–224—Transverse sections of young Amphioxus-larvæ (diagrammatic, +from _ Ralph._) (Cf. also Fig. 216.) In Fig. 222 there is free +communication from without with the gut-cavity (_D_) through the +gill-clefts (_K_). In Fig. 223 the lateral folds of the body-wall, or +the gill-covers, which grow downwards, are formed. In Fig. 224 these +lateral folds have united underneath and joined their edges in the +middle line of the ventral side (_R_ seam). The respiratory water now +passes from the gut-cavity (_D_) into the mantle-cavity (_A_). The +letters have the same meaning throughout: _N_ medullary tube, _Ch_ +chorda, _ M_ lateral muscles, _Lh_ body-cavity, _G_ part of the +body-cavity in which the sexual organs are subsequently formed. _ D_ +gut-cavity, clothed with the gut-gland layer (_a_). A mantle-cavity, +_K_ gill-clefts, _b_=_E_ epidermis, _E_1 the same as visceral +epithelium of the mantle-cavity, _E_2 as parietal epithelium of the +mantle-cavity. + + +At an early stage of embryonic development the structure of the +Amphioxus-larva is substantially the same as the ideal picture we have +previously formed of the “Primitive Vertebrate” (Figs. 98–102). But the +body afterwards undergoes various modifications, especially in the +fore-part. These modifications do not concern us, as they depend on +special adaptations, and do not affect the hereditary vertebrate type. +When the free-swimming Amphioxus-larva is three months old, it abandons +its pelagic habits and changes into the young animal that lives in the +sand. In spite of its smallness (one-eighth of an inch), it has +substantially the same structure as the adult. As regards the remaining +organs of the Amphioxus, we need only mention that the gonads or sexual +glands are developed very late, immediately out of the inner cell-layer +of the +body-cavity. Although we can find afterwards no continuation of the +body-cavity (Fig. 216 _U_) in the lateral walls of the mantle-cavity, +in the gill-covers or mantle-folds (Fig. 224 _U_), there is one present +in the beginning (Fig. 224 _Lh_). The sexual cells are formed below, at +the bottom of this continuation (Fig. 224 _S_). For the rest, the +subsequent development into the adult Amphioxus of the larva we have +followed is so simple that we need not go further into it here. + +We may now turn to the embryology of the Ascidia, an animal that seems +to stand so much lower and to be so much more simply organised, +remaining for the greater part of its life attached to the bottom of +the sea like a shapeless lump. It was a fortunate accident that +Kowalevsky first examined just those larger specimens of the Ascidiæ +that show most clearly the relationship of the vertebrates to the +invertebrates, and the larvæ of which behave exactly like those of the +Amphioxus in the first stages of development. This resemblance is so +close in the main features that we have only to repeat what we have +already said of the ontogenesis of the Amphioxus. + +The ovum of the larger Ascidia (_Phallusia, Cynthia,_ etc.) is a simple +round cell of 1/250 to 1/125 of an inch in diameter. In the thick +fine-grained yelk we find a clear round germinal vesicle of about 1/750 +of an inch in diameter, and this encloses a small embryonic spot or +nucleolus. Inside the membrane that surrounds the ovum, the stem-cell +of the Ascidia, after fecundation, passes through just the same +metamorphoses as the stem-cell of the Amphioxus. It undergoes total +segmentation; it divides into two, four, eight, sixteen, thirty-two +cells, and so on. By continued total cleavage the morula, or +mulberry-shaped cluster of cells, is formed. Fluid gathers inside it, +and thus we get once more a globular vesicle (the blastula); the wall +of this is a single stratum of cells, the blastoderm. A real gastrula +(a simple bell-gastrula) is formed from the blastula by invagination, +in the same way as in the amphioxus. + +Up to this there is no definite ground in the embryology of the Ascidiæ +for bringing them into close relationship with the Vertebrates; the +same gastrula is formed in the same way in many other animals of +different stems. But we now find an embryonic process that is peculiar +to the Vertebrates, and that proves irrefragably the affinity of the +Ascidiæ to the Vertebrates. From the epidermis of the gastrula a +_medullary tube_ is formed on the dorsal side, and, between this and +the primitive gut, a _chorda_; these are the organs that are otherwise +only found in Vertebrates. The formation of these very important organs +takes place in the Ascidia-gastrula in precisely the same way as in +that of the Amphioxus. In the Ascidia (as in the other case) the oval +gastrula is first flattened on one side—the subsequent dorsal side. A +groove or furrow (the medullary groove) is sunk in the middle line of +the flat surface, and two parallel longitudinal swellings arise on +either side from the skin layer. These medullary swellings join +together over the furrow, and form a tube; in this case, again, the +neural or medullary tube is at first open in front, and connected with +the primitive gut behind by the neurenteric canal. Further, in the +Ascidia-larva also the two permanent apertures of the alimentary canal +only appear later, as independent and new formations. The permanent +mouth does not develop from the primitive mouth of the gastrula; this +primitive mouth closes up, and the later anus is formed near it by +invagination from without, on the hinder end of the body, opposite to +the aperture of the medullary tube. + +During these important processes, that take place in just the same way +in the Amphioxus, a tail-like projection grows out of the posterior end +of the larva-body, and the larva folds itself up within the round +ovolemma in such a way that the dorsal side is curved and the tail is +forced on to the ventral side. In this tail is developed—starting from +the primitive gut—a cylindrical string of cells, the fore end of which +pushes into the body of the larva, between the alimentary canal and the +neural canal, and is no other than the chorda dorsalis. This important +organ had hitherto been found only in the Vertebrates, not a single +trace of it being discoverable in the Invertebrates. At first the +chorda only consists of a single row of large entodermic cells. It is +afterwards composed of several rows of cells. In the Ascidia-larva, +also, the chorda develops from the dorsal middle part of the primitive +gut, while the two cœlom-pouches detach themselves from it on both +sides. The simple body-cavity is formed by the coalescence of the two. + +When the Ascidia-larva has attained +this stage of development it begins to move about in the ovolemma. This +causes the membrane to burst. The larva emerges from it, and swims +about in the sea by means of its oar-like tail. These free-swimming +larvæ of the Ascidia have been known for a long time. They were first +observed by Darwin during his voyage round the world in 1833. They +resemble tadpoles in outward appearance, and use their tails as oars, +as the tadpoles do. However, this lively and highly-developed condition +does not last long. At first there is a progressive development; the +foremost part of the medullary tube enlarges into a brain, and inside +this two single sense-organs are developed, a dorsal auditory vesicle +and a ventral eye. Then a heart is formed on the ventral side of the +animal, or the lower wall of the gut, in the same simple form and at +the same spot at which the heart is developed in man and all the other +vertebrates. In the lower muscular wall of the gut we find a weal-like +thickening, a solid, spindle-shaped string of cells, which becomes +hollow in the centre; it begins to contract in different directions, +now forward and now backward, as is the case with the adult Ascidia. In +this way the sanguineous fluid accumulated in the hollow muscular tube +is driven in alternate directions into the blood-vessels, which develop +at both ends of the cardiac tube. One principal vessel runs along the +dorsal side of the gut, another along its ventral side. The former +corresponds to the aorta and the dorsal vessel in the worms. The other +corresponds to the subintestinal vein and the ventral vessel of the +worms. + + +Fig.225. An Appendicaria (Copelata), seen from the left. Fig. 225—An +Appendicaria (Copelata), seen from the left. _m_ mouth, _k_ branchial +gut, _o_ gullet, _ v_ stomach, _a_ anus, _n_ brain (ganglion above the +gullet), _g_ auditory vesicle, _f_ ciliated groove under the gills, _h_ +heart, _t_ testicles, _e_ ovary, _ c_ chorda, _s_ tail. + + +With the formation of these organs the progressive development of the +Ascidia comes to an end, and degeneration sets in. The free-swimming +larva sinks to the floor of the sea, abandons its locomotive habits, +and attaches itself to stones, marine plants, mussel-shells, corals, +and other objects; this is done with the part of the body that was +foremost in movement. The attachment is effected by a number of +out-growths, usually three, which can be seen even in the free-swimming +larva. The tail is lost, as there is no further use for it. It +undergoes a fatty degeneration, and disappears with the chorda +dorsalis. The tailless body changes into an unshapely tube, and, by the +atrophy of some parts and the modification of others, gradually assumes +the appearance we have already described. + +Among the living Tunicates there is a very interesting group of small +animals that remain throughout life at the stage of development of the +tailed, free Ascidia-larva, and swim about briskly in the sea by means +of their broad oar-tail. These are the remarkable Copelata +(_Appendicaria_ and _Vexillaria,_ Fig. 225). They are the only living +Vertebrates that have throughout life a chorda dorsalis and a neural +string above it; the latter must be regarded as the prolongation of the +cerebral ganglion and the equivalent of the medullary tube. Their +branchial gut also opens directly outwards by a pair of +branchial clefts. These instructive Copelata, comparable to permanent +Ascidia-larvæ, come next to the extinct Prochordonia, those ancient +worms which we must regard as the common ancestors of the Tunicates and +Vertebrates. The chorda of the Appendicaria is a long, cylindrical +string (Fig. 225 _ c_), and serves as an attachment for the muscles +that work the flat oar-tail. + +Among the various modifications which the Ascidia-larva undergoes after +its establishment at the sea-floor, the most interesting (after the +loss of the axial rod) is the atrophy of one of its chief organs, the +medullary tube. In the Amphioxus the spinal marrow continues to +develop, but in the Ascidia the tube soon shrinks into a small and +insignificant nervous ganglion that lies above the mouth and the +gill-crate, and is in accord with the extremely slight mental power of +the animal. This insignificant relic of the medullary tube seems to be +quite beyond comparison with the nervous centre of the vertebrate, yet +it started from the same structure as the spinal cord of the Amphioxus. +The sense-organs that had been developed in the fore part of the neural +tube are also lost; no trace of which can be found in the adult +Ascidia. On the other hand, the alimentary canal becomes a most +extensive organ. It divides presently into two sections—a wide fore or +branchial gut that serves for respiration, and a narrower hind or +hepatic gut that accomplishes digestion. The branchial or head-gut of +the Ascidia is small at first, and opens directly outwards only by a +couple of lateral ducts or gill-clefts—a permanent arrangement in the +Copelata. The gill-clefts are developed in the same way as in the +Amphioxus. As their number greatly increases we get a large gill-crate, +pierced like lattice work. In the middle line of its ventral side we +find the hypobranchial groove. The mantle or cloaca-cavity (the atrium) +that surrounds the gill-crate is also formed in the same way in the +Ascidia as in the Amphioxus. The ejection-opening of this peribranchial +cavity corresponds to the branchial pore of the Amphioxus. In the adult +Ascidia the branchial gut and the heart on its ventral side are almost +the only organs that recall the original affinity with the vertebrates. + +The further development of the Ascidia in detail has no particular +interest for us, and we will not go into it. The chief result that we +obtain from its embryology is the complete agreement with that of the +Amphioxus in the earliest and most important embryonic stages. They do +not begin to diverge until after the medullary tube and alimentary +canal, and the axial rod with the muscles between the two, have been +formed. The Amphioxus continues to advance, and resembles the embryonic +forms of the higher vertebrates; the Ascidia degenerates more and more, +and at last, in its adult condition, has the appearance of a very +imperfect invertebrate. + +If we now look back on all the remarkable features we have encountered +in the structure and the embryonic development of the Amphioxus and the +Ascidia, and compare them with the features of man’s embryonic +development which we have previously studied, it will be clear that I +have not exaggerated the importance of these very interesting animals. +It is evident that the Amphioxus from the vertebrate side and the +Ascidia from the invertebrate form the bridge by which we can span the +deep gulf that separates the two great divisions of the animal kingdom. +The radical agreement of the lancelet and the sea-squirt in the first +and most important stages of development shows something more than +their close anatomic affinity and their proximity in classification; it +shows also their real blood-relationship and their common origin from +one and the same stem-form. In this way, it throws considerable light +on the oldest roots of man’s genealogical tree. + + + + +Chapter XVIII. +DURATION OF THE HISTORY OF OUR STEM + + +Our comparative investigation of the anatomy and ontogeny of the +Amphioxus and Ascidia has given us invaluable assistance. We have, in +the first place, bridged the wide gulf that has existed up to the +present between the Vertebrates and Invertebrates; and, in the second +place, we have discovered in the embryology of the Amphioxus a number +of ancient evolutionary stages that have long since disappeared from +human embryology, and have been lost, in virtue of the law of curtailed +heredity. The chief of these stages are the spherical blastula (in its +simplest primary form), and the succeeding archigastrula, the pure, +original form of the _gastrula_ which the Amphioxus has preserved to +this day, and which we find in the same form in a number of +Invertebrates of various classes. Not less important are the later +embryonic forms of the cœlomula, the chordula, etc. + +Thus the embryology of the Amphioxus and the Ascidia has so much +increased our knowledge of man’s stem-history that, although our +empirical information is still very incomplete, there is now no defect +of any great consequence in it. We may now, therefore, approach our +proper task, and reconstruct the phylogeny of man in its chief lines +with the aid of this evidence of comparative anatomy and ontogeny. In +this the reader will soon see the immense importance of the direct +application of the biogenetic law. But before we enter upon the work it +will be useful to make a few general observations that are necessary to +understand the processes aright. + +We must say a few words with regard to the period in which the human +race was evolved from the animal kingdom. The first thought that occurs +to one in this connection is the vast difference between the duration +of man’s ontogeny and phylogeny. The individual man needs only nine +months for his complete development, from the fecundation of the ovum +to the moment when he leaves the maternal womb. The human embryo runs +its whole course in the brief space of forty weeks (as a rule, 280 +days). In many other mammals the time of the embryonic development is +much the same as in man—for instance, in the cow. In the horse and ass +it takes a little longer, forty-three to forty-five weeks; in the +camel, thirteen months. In the largest mammals, the embryo needs a much +longer period for its development in the womb—a year and a half in the +rhinoceros, and ninety weeks in the elephant. In these cases pregnancy +lasts twice as long as in the case of man, or one and three-quarter +years. In the smaller mammals the embryonic period is much shorter. The +smallest mammals, the dwarf-mice, develop in three weeks; hares in four +weeks, rats and marmots in five weeks, the dog in nine, the pig in +seventeen, the sheep in twenty-one and the goat in thirty-six. Birds +develop still more quickly. The chick only needs, in normal +circumstances, three weeks for its full development. The duck needs +twenty-five days, the turkey twenty-seven, the peacock thirty-one, the +swan forty-two, and the cassowary sixty-five. The smallest bird, the +humming-bird, leaves the egg after twelve days. Hence the duration of +individual development within the fœtal membranes is, in the mammals +and birds, clearly related to the absolute size of the body of the +animal in question. But this is not the only determining feature. There +are a number of other circumstances that have an influence on the +period of embryonic development. In the Amphioxus the earliest and most +important embryonic processes take place so rapidly that the blastula +is formed in four hours, the gastrula in six, and the typical +vertebrate form in twenty-four. + +In every case the duration of ontogeny shrinks into insignificance when +we compare it with the enormous period that has been necessary for +phylogeny, or the gradual development of the ancestral series. This +period is not measured by years or centuries, but by thousands and +millions of years. Many millions of years had to pass before the most +advanced +vertebrate, man, was evolved, step by step, from his ancient +unicellular ancestors. The opponents of evolution, who declare that +this gradual development of the human form from lower animal forms, and +ultimately from a unicellular organism, is an incredible miracle, +forget that the same miracle takes place within the space of mine +months in the embryonic development of every human being. Each of us +has, in the forty weeks—properly speaking, in the first four weeks—of +his development in the womb, passed through the same series of +transformations that our animal ancestors underwent in the course of +millions of years. + +It is impossible to determine even approximately, in hundreds or even +thousands of years, the real and absolute duration of the phylogenetic +period. But for some time now we have, through the research of +geologists, been in a position to assign the relative length of the +various sections of the organic history of the earth. The immediate +data for determining this relative length of the geological periods are +found in the thickness of the sedimentary strata—the strata that have +been formed at the bottom of the sea or in fresh water from the mud or +slime deposited there. These successive layers of limestone, sandstone, +slate, marl, etc., which make up the greater part of the rocks, and are +often several thousand feet thick, give us a standard for computing the +relative length of the various periods. + +To make the point quite clear, I must say a word about the evolution of +the earth in general, and point out briefly the chief features of the +story. In the first place, we encounter the principle that on our +planet organic life began to exist at a definite period. That statement +is no longer disputed by any competent geologist or biologist. The +organic history of the earth could not commence until it was possible +for water to settle on our planet in fluid condition. Every organism, +without exception, needs fluid water as a condition of existence, and +contains a considerable quantity of it. Our own body, when fully +formed, contains sixty to seventy per cent of water in its tissues, and +only thirty to forty per cent of solid matter. There is even more water +in the body of the child, and still more in the embryo. In the earlier +stages of development the human fœtus contains more than ninety per +cent of water, and not ten per cent of solids. In the lower marine +animals, especially certain medusæ, the body consists to the extent of +more than ninety-nine per cent of sea-water, and has not one per cent +of solid matter. No organism can exist or discharge its functions +without water. No water, no life! + +But fluid water, on which the existence of life primarily depends, +could not exist on our planet until the temperature of the surface of +the incandescent sphere had sunk to a certain point. Up to that time it +remained in the form of steam. But as soon as the first fluid water +could be condensed from the envelope of steam, it began its geological +action, and has continued down to the present day to modify the solid +crust of the earth. The final outcome of this incessant action of the +water—wearing down and dissolving the rocks in the form of rain, hail, +snow, and ice, as running stream or boiling surge—is the formation of +mud. As Huxley says in his admirable _Lectures on the Causes of +Phenomena in Organic Nature,_ the chief document as to the past history +of our earth is mud; the question of the history of past ages resolves +itself into a question about the formation of mud. + +As I have said, it is possible to form an approximate idea of the +relative age of the various strata by comparing them at different parts +of the earth’s surface. Geologists have long been agreed that there is +a definite historical succession of the different strata. The various +superimposed layers correspond to successive periods in the organic +history of the earth, in which they were deposited in the form of mud +at the bottom of the sea. The mud was gradually converted into stone. +This was lifted out of the water owing to variations in the earth’s +surface, and formed the mountains. As a rule, four or five great +divisions are distinguished in the organic history of the earth, +corresponding to the larger and smaller groups of the sedimentary +strata. The larger periods are then sub-divided into a series of +smaller ones, which usually number from twelve to fifteen. The +comparative thickness of the groups of strata enables us to make an +approximate calculation of the relative length of these various periods +of time. We cannot say, it is true, “In a century a stratum of a +certain thickness (about two feet) is formed on the average; therefore, +a layer 1000 feet thick must be 500,000 years old.” Different strata of +the same thickness may need very different periods for their formation. +But from +the thickness or size of the stratum we can draw some conclusion as to +the _relative_ length of the period. + +The first and oldest of the four or five chief divisions of the organic +history of the earth is called the primordial, archaic, or archeozoic +period. If we compute the total average thickness of the sedimentary +strata at about 130,000 feet, this first period comprises 70,000 feet, +or the greater part of the whole. For this and other reasons we may at +once conclude that the corresponding primordial or archeolithic period +must have been in itself much longer than the whole of the remaining +periods together, from its close to the present day. It was probably +much longer than the figures I have quoted (7:6) indicate—possibly 9:6. +Of late years the thickness of the archaic rocks has been put at 90,000 +feet. + + SYNOPSIS OF THE PALEONTOLOGICAL FORMATIONS, +OR THE FOSSILIFEROUS STRATA OF THE CRUST + + Groups Systems Formations Synonyms of + Formations + +V. Anthropolithic groups, or anthropozoic (quaternary) +groups of strata. XIV. Recent (alluvium). 38. Present 37. +Recent Upper alluvial Lower alluvial XIII. Pleistocene (diluvium) +36. Post-glacial 35. Glacial Upper diluvial +Lower diluvial + +IV. Cenolithic groups, or cenozoic (tertiary) groups of strata. +XII. Pliocene (neo-tertiary) 34. Arverne 33. Subapennine +Upper pliocene Lower pliocene XI. Miocene (middle tertiary) 32. Falun +31. Limbourg Upper miocene Lower miocene Xb. Oligocene (old +tertiary) 30. Aquitaine 29. Ligurium Upper oligocene +Lower oligocene Xa. Eocene (primitive tertiary) 28. Gypsum 27. +Coarse chalk 26. London clay Upper eocene Middle eocene Lower +eocene + +III. Mesolithic groups, or mesozoic (secondary) groups of strata. +IX. Chalk +(cretaceous) 25. White chalk 24. Green sand +23. Neoconian 22. Wealden Upper cretaceous +Middle cretaceous Lower cretaceous Weald formation VIII. Jurassic +21. Portland 20. Oxford 19. Bath 18. Lias Upper oolithic Middle +oolithic Lower oolithic Liassic VII. Triassic 17. Keuper 16. +Muschelkalk 15. Bunter Upper triassic Middle triassic Lower +triassic + +II. Paleolithic groups, or paleozoic (primary) groups of strata. + VIb. Permian 14. Zechstein 13. Neurot sand Upper permian +Lower permian VIa. Carboniferous coal-measures) 12. Carboniferous + sandstone 11. Carboniferous + limestone Upper carboniferous + Lower carboniferous V. Devonian 10. Pilton + 9. Ilfracombe 8. Linton Upper devonian Middle devonian Lower devonian + IV. Silurian 7. Ludlow 6. Wenlock 5. Llandeilo Upper silurian + Middle silurian +Lower silurian + +I. Archeolithic groups, or archeozoic (primordial) +groups of strata. III. Cambrian 4. Potsdam 3. Longmynd +Upper cambrian Lower cambrian II. Huronian I. Laurentian 2. +Labrador 1. Ottawa Upper laurentian Lower laurentian + +The primordial period falls into three subordinate sections—the +Laurentian, Huronian, and Cambrian, corresponding to the three chief +groups of rocks that comprise the archaic formation. The immense period +during which these rocks were forming in the primitive ocean probably +comprises more than 50,000,000 years. At the commencement of it the +oldest and simplest organisms were formed by spontaneous generation—the +Monera, with which the history of life on our planet opened. From these +were first developed unicellular organisms of the simplest character, +the Protophyta +and Protozoa (paulotomea, amœbæ, rhizopods, infusoria, and other +Protists). During this period the whole of the invertebrate ancestors +of the human race were evolved from the unicellular organisms. We can +deduce this from the fact that we already find remains of fossilised +fishes (Selachii and Ganoids) towards the close of the following +Silurian period. These are much more advanced and much younger than the +lowest vertebrate, the Amphioxus, and the numerous skull-less +vertebrates, related to the Amphioxus, that must have lived at that +time. The whole of the invertebrate ancestors of the human race must +have preceded these. + +The primordial age is followed by a much shorter division, the +_paleozoic_ or Primary age. It is divided into four long periods, the +Silurian, Devonian, Carboniferous, and Permian. The Silurian strata are +particularly interesting because they contain the first fossil traces +of vertebrates—teeth and scales of Selachii ( _Palæodus_) in the lower, +and Ganoids ( _Pteraspis_) in the upper Silurian. During the Devonian +period the “old red sandstone” was formed; during the Carboniferous +period were deposited the vast coal-measures that yield us our chief +combustive material; in the Permian (or the Dyas), in fine, the new red +sandstone, the Zechstein (magnesian limestone), and the Kupferschiefer +(marl-slate) were formed. The collective depth of these strata is put +at 40,000 to 45,000 feet. In any case, the paleozoic age, taken as a +whole, was much shorter than the preceding and much longer than the +subsequent periods. The strata that were deposited during this primary +epoch contain a large number of fossils; besides the invertebrate +species there are a good many vertebrates, and the fishes preponderate. +There were so many fishes, especially primitive fishes (of the shark +type) and plated fishes, during the Devonian, and also during the +Carboniferous and Permian periods, that we may describe the whole +paleozoic period as “the age of fishes.” Among the paleozoic plated +fishes or Ganoids the Crossopterygii and the Ctenodipterina (dipneusts) +are of great importance. + +During this period some of the fishes began to adapt themselves to +living on land, and so gave rise to the class of the amphibia. We find +in the Carboniferous period fossilised remains of five-toed amphibia, +the oldest terrestrial, air-breathing vertebrates. These amphibia +increase in variety in the Permian epoch. Towards the close of it we +find the first Amniotes, the ancestors of the three higher classes of +Vertebrates. These are lizard-like animals; the first to be discovered +was the _Proterosaurus,_ from the marl at Eisenach. The rise of the +earliest Amniotes, among which must have been the common ancestor of +the reptiles, birds, and mammals, is put back towards the close of the +paleozoic age by the discovery of these reptile remains. The ancestors +of our race during this period were at first represented by true +fishes, then by dipneusts and amphibia, and finally by the earliest +Amniotes, or the Protamniotes. + +The third chief section of the organic history of the earth is the +_Mesozoic_ or Secondary period. This again is subdivided into three +divisions Triassic, Jurassic, and Cretaceous. The thickness of the +strata that were deposited in this period, from the beginning of the +Triassic to the end of the Cretaceous period, is altogether about +15,000 feet, or not half as much as the paleozoic deposits. During this +period there was a very brisk and manifold development in all branches +of the animal kingdom. There were especially a number of new and +interesting forms evolved in the vertebrate stem. Bony fishes ( +_Teleostei_) make their first appearance. Reptiles are found in +extraordinary variety and number; the extinct giant-serpents +(dinosauria), the sea-serpents (halisauria), and the flying lizards +(pterosauria) are the most remarkable and best known of these. On +account of this predominance of the reptile-class, the period is called +“the age of reptiles.” But the bird-class was also evolved during this +period; they certainly originated from some division of the lizard-like +reptiles. This is proved by the embryological identity of the birds and +reptiles and their comparative anatomy, and, among other features, from +the circumstance that in this period there were birds with teeth in +their jaws and with tails like lizards (Archeopteryx, Odontornis). + +Finally, the most advanced and (for us) the most important class of the +vertebrates, the mammals, made their appearance during the mesozoic +period. The earliest fossil remains of them were found in the latest +Triassic strata—lower jaws of small ungulates and marsupials. More +numerous remains are found a little later +in the Jurassic, and some in the Cretaceous. All the mammal remains +that we have from this section belong to the lower promammals and +marsupials; among these were most certainly the ancestors of the human +race. On the other hand, we have not found a single indisputable fossil +of any higher mammal (a placental) in the whole of this period. This +division of the mammals, which includes man, was not developed until +later, towards the close of this or in the following period. + +The fourth section of the organic history of the earth, the Tertiary or +_Cenozoic_ age, was much shorter than the preceding. The strata that +were deposited during this period have a collective thickness of only +about 3,000 feet. It is subdivided into four sections—the Eocene, +Oligocene, Miocene, and Pliocene. During these periods there was a very +varied development of higher plant and animal forms; the fauna and +flora of our planet approached nearer and nearer to the character that +they bear to-day. In particular, the most advanced class, the mammals, +began to preponderate. Hence the Tertiary period may be called “the age +of mammals.” The highest section of this class, the placentals, now +made their appearance; to this group the human race belongs. The first +appearance of man, or, to be more precise, the development of man from +some closely-related group of apes, probably falls in either the +miocene or the pliocene period, the middle or the last section of the +Tertiary period. Others believe that man properly so-called—man endowed +with speech—was not evolved from the non-speaking ape-man ( +_Pithecanthropus_) until the following, the anthropozoic, age. + +In this fifth and last section of the organic history of the earth we +have the full development and dispersion of the various races of men, +and so it is called the _Anthropozoic_ as well as the _Quaternary_ +period. In the imperfect condition of paleontological and +ethnographical science we cannot as yet give a confident answer to the +question whether the evolution of the human race from some extinct ape +or lemur took place at the beginning of this or towards the middle or +the end of the Tertiary period. However, this much is certain: the +development of civilisation falls in the anthropozoic age, and this is +merely an insignificant fraction of the vast period of the whole +history of life. When we remember this, it seems ridiculous to restrict +the word “history” to the civilised period. If we divide into a hundred +equal parts the whole period of the history of life, from the +spontaneous generation of the first Monera to the present day, and if +we then represent the relative duration of the five chief sections or +ages, as calculated from the average thickness of the strata they +contain, as percentages of this, we get something like the following +relation:— + + I. +II. III. +IV. V. Archeolithic or archeozoic (primordial) age +Paleolithic or paleozoic (primary) age Mesolithic or mesozoic +(secondary) age Cenolithic or cenozoic (tertiary) age Anthropolithic or +anthropozoic (quaternary) age 53.6 32.1 +11.5 2.3 0.5 ——— 100.0 + +In any case, the “historical period” is an insignificant quantity +compared with the vast length of the preceding ages, in which there was +no question of human existence on our planet. Even the important +Cenozoic or Tertiary period, in which the first placentals or higher +mammals appear, probably amounts to little over two per cent of the +whole organic age. + +Before we approach our proper task, and, with the aid of our +ontogenetic acquirements and the biogenetic law, follow step by step +the paleontological development of our animal ancestors, let us glance +for a moment at another, and apparently quite remote, branch of +science, a general consideration of which will help us in the solving +of a difficult problem. I mean the science of comparative philology. +Since Darwin gave new life to biology by his theory of selection, and +raised the question of evolution on all sides, it has often been +pointed out that there is a remarkable analogy between the development +of languages and the evolution of species. The comparison is perfectly +just and very instructive. We could hardly find a better analogy when +we are dealing with some of the difficult and obscure features of the +evolution of species. In both cases we find the action of the same +natural laws. + +All philologists of any competence in their science now agree that all +human languages have been gradually evolved from very rudimentary +beginnings. The +idea that speech is a gift of the gods—an idea held by distinguished +authorities only fifty years ago—is now generally abandoned, and only +supported by theologians and others who admit no natural development +whatever. Speech has been developed simultaneously with its organs, the +larynx and tongue, and with the functions of the brain. Hence it will +be quite natural to find in the evolution and classification of +languages the same features as in the evolution and classification of +organic species. The various groups of languages that are distinguished +in philology as primitive, fundamental, parent, and daughter languages, +dialects, etc., correspond entirely in their development to the +different categories which we classify in zoology and botany as stems, +classes, orders, families, genera, species, and varieties. The relation +of these groups, partly co-ordinate and partly subordinate, in the +general scheme is just the same in both cases; and the evolution +follows the same lines in both. + +When, with the assistance of this tree, we follow the formation of the +various languages that have been developed from the common root of the +ancient Indo-Germanic tongue, we get a very clear idea of their +phylogeny. We shall see at the same time how analogous this is to the +development of the various groups of vertebrates that have arisen from +the common stem-form of the primitive vertebrate. The ancient +Indo-Germanic root-language divided first into two principal stems—the +Slavo-Germanic and the Aryo-Romanic. The Slavo-Germanic stem then +branches into the ancient Germanic and the ancient Slavo-Lettic +tongues; the Aryo-Romanic into the ancient Aryan and the ancient +Greco-Roman. If we still follow the genealogical tree of these four +Indo-Germanic tongues, we find that the ancient Germanic divides into +three branches—the Scandinavian, the Gothic, and the German. From the +ancient German came the High German and Low German; to the latter +belong the Frisian, Saxon, and modern Low-German dialects. The ancient +Slavo-Lettic divided first into a Baltic and a Slav language. The +Baltic gave rise to the Lett, Lithuanian, and old-Prussian varieties; +the Slav to the Russian and South-Slav in the south-east, and to the +Polish and Czech in the west. + +We find an equally prolific branching of its two chief stems when we +turn to the other division of the Indo-Germanic languages. The +Greco-Roman divided into the Thracian (Albano-Greek) and the +Italo-Celtic. From the latter came the divergent branches of the Italic +(Roman and Latin) in the south, and the Celtic in the north: from the +latter have been developed all the British (ancient British, ancient +Scotch, and Irish) and Gallic varieties. The ancient Aryan gave rise to +the numerous Iranian and Indian languages. + +This “comparative anatomy” and evolution of languages admirably +illustrates the phylogeny of species. It is clear that in structure and +development the primitive languages, mother and daughter languages, and +varieties, correspond exactly to the classes, orders, genera, and +species of the animal world. In both cases the “natural” system is +phylogenetic. As we have been convinced from comparative anatomy and +ontogeny, and from paleontology, that all past and living vertebrates +descend from a common ancestor, so the comparative study of dead and +living Indo-Germanic tongues proves beyond question that they are all +modifications of one primitive language. This view of their origin is +now accepted by all the chief philologists who have worked in this +branch and are unprejudiced. + +But the point to which I desire particularly to draw the reader’s +attention in this comparison of the Indo-Germanic languages with the +branches of the vertebrate stem is, that one must never confuse direct +descendants with collateral branches, nor extinct forms with living. +This confusion is very common, and our opponents often make use of the +erroneous ideas it gives rise to for the purpose of attacking evolution +generally. When, for instance, we say that man descends from the ape, +this from the lemur, and the lemur from the marsupial, many people +imagine that we are speaking of the living species of these orders of +mammals that they find stuffed in our museums. Our opponents then foist +this idea on us, and say, with more astuteness than intelligence, that +it is quite impossible; or they ask us, by way of physiological +experiment, to turn a kangaroo into a lemur, a lemur into a gorilla, +and a gorilla into a man! The demand is childish, and the idea it rests +on erroneous. All these living forms have diverged more or less from +the ancestral form; none of them could engender the +same posterity that the stem-form really produced thousands of years +ago. + +It is certain that man has descended from some extinct mammal; and we +should just as certainly class this in the order of apes if we had it +before us. It is equally certain that this primitive ape descended in +turn from an unknown lemur, and this from an extinct marsupial. But it +is just as clear that all these extinct ancestral forms can only be +claimed as belonging to the living order of mammals in virtue of their +essential internal structure and their resemblance in the decisive +anatomic characteristics of each _order._ In external appearance, in +the characteristics of the _genus_ or _species,_ they would differ more +or less, perhaps very considerably, from all living representatives of +those orders. It is a universal and natural procedure in phylogenetic +development that the stem-forms themselves, with their specific +peculiarities, have been extinct for some time. The forms that approach +nearest to them among the living species are more or less—perhaps very +substantially—different from them. Hence in our phylogenetic inquiry +and in the comparative study of the living, divergent descendants, +there can only be a question of determining the greater or less +remoteness of the latter from the ancestral form. Not a single one of +the older stem-forms has continued unchanged down to our time. + +We find just the same thing in comparing the various dead and living +languages that have developed from a common primitive tongue. If we +examine our genealogical tree of the Indo-Germanic languages in this +light, we see at once that all the older or parent tongues, of which we +regard the living varieties of the stem as divergent daughter or +grand-daughter languages, have been extinct for some time. The +Aryo-Romanic and the Slavo-Germanic tongues have completely +disappeared; so also the Aryan, the Greco-Roman, the Slavo-Lettic, and +the ancient Germanic. Even their daughters and grand-daughters have +been lost; all the living Indo-Germanic languages are only related in +the sense that they are divergent descendants of common stem-forms. +Some forms have diverged more, and some less, from the original +stem-form. + +This easily demonstrable fact illustrates very well the analogous case +of the origin of the vertebrate species. Phylogenetic comparative +philology here yields a strong support to phylogenetic comparative +zoology. But the one can adduce more direct evidence than the other, as +the paleontological material of philology—the old monuments of the +extinct tongue—have been preserved much better than the paleontological +material of zoology, the fossilised bones and imprints of vertebrates. + +We may, however, trace man’s genealogical tree not only as far as the +lower mammals, but much further—to the amphibia, to the shark-like +primitive fishes, and, in fine, to the skull-less vertebrates that +closely resembled the Amphioxus. But this must not be understood in the +sense that the existing Amphioxus, or the sharks or amphibia of to-day, +can give us any idea of the external appearance of these remote +stem-forms. Still less must it be thought that the Amphioxus or any +actual shark, or any living species of amphibia, is a real ancestral +form of the higher vertebrates and man. The statement can only +rationally mean that the living forms I have referred to are +_collateral lines_ that are much more closely related to the extinct +stem-forms, and have retained the resemblance much better, than any +other animals we know. They are still so like them in regard to their +distinctive internal structure that we should put them in the same +class with the extinct forms if we had these before us. But no direct +descendants of these earlier forms have remained unchanged. Hence we +must entirely abandon the idea of finding direct ancestors of the human +race in their characteristic _external form_ among the living species +of animals. The essential and distinctive features that still connect +living forms more or less closely with the extinct common stem-forms +lie in the internal structure, not the external appearance. The latter +has been much modified by adaptation. The former has been more or less +preserved by heredity. + +Comparative anatomy and ontogeny prove beyond question that man is a +true vertebrate, and, therefore, man’s special genealogical tree must +be connected with that of the other Vertebrates, which spring from a +common root with him. But we have also many important grounds in +comparative anatomy and ontogeny for assuming a common origin for all +the Vertebrates. If the general theory of +evolution is correct, all the Vertebrates, including man, come from a +single common ancestor, a long-extinct “Primitive Vertebrate.” Hence +the genealogical tree of the Vertebrates is at the same time that of +the human race. + +Our task, therefore, of constructing man’s genealogy becomes the larger +aim of discovering the genealogy of the entire vertebrate stem. As we +now know from the comparative anatomy and ontogeny of the Amphioxus and +the Ascidia, this is in turn connected with the genealogical tree of +the Invertebrates (directly with that of the Vermalia), but has no +direct connection with the independent stems of the Articulates, +Molluscs, and Echinoderms. If we do thus follow our ancestral tree +through various stages down to the lowest worms, we come inevitably to +the _Gastræa,_ that most instructive form that gives the clearest +possible picture of an animal with two germinal layers. The Gastræa +itself has originated from the simple multicellular vesicle, the +_Blastæa,_ and this in turn must have been evolved from the lowest +circle of unicellular animals, to which we give the name of Protozoa. +We have already considered the most important primitive type of these, +the unicellular _Amœba,_ which is extremely instructive when compared +with the human ovum. With this we reach the lowest of the solid data to +which we are to apply our biogenetic law, and by which we may deduce +the extinct ancestor from the embryonic form. The amœboid nature of the +young ovum and the unicellular condition in which (as stem-cell or +cytula) every human being begins its existence justify us in affirming +that the earliest ancestors of the human race were simple amœboid +coils. + +But the further question now arises: “Whence came these first amœbæ +with which the history of life began at the commencement of the +Laurentian epoch?” There is only one answer to this. The earliest +unicellular organisms can only have been evolved from the simplest +organisms we know, the _Monera._ These are the simplest living things +that we can conceive. Their whole body is nothing but a particle of +plasm, a granule of living albuminous matter, discharging of itself all +the essential vital functions that form the material basis of life. +Thus we come to the last, or, if you prefer, the first, question in +connection with evolution—the question of the origin of the Monera. +This is the real question of the origin of life, or of spontaneous +generation. + +We have neither space nor occasion to go further in this Chapter into +the question of spontaneous generation. For this I must refer the +reader to the fifteenth chapter of the _History of Creation,_ and +especially to the second book of the _General Morphology,_ or to the +essay on “The Monera and Spontaneous Generation” in my _Studies of the +Monera and other Protists._[29] I have given there fully my own view of +this important question. The famous botanist Nägeli afterwards (1884) +developed the same ideas. I will only say a few words here about this +obscure question of the origin of life, in so far as our main subject, +organic evolution in general, is affected by it. Spontaneous +generation, in the definite and restricted sense in which I maintain +it, and claim that it is a necessary hypothesis in explaining the +origin of life, refers solely to the evolution of the Monera from +inorganic carbon-compounds. When living things made their first +appearance on our planet, the very complex nitrogenous compound of +carbon that we call _plasson,_ which is the earliest material +embodiment of vital action, must have been formed in a purely chemical +way from inorganic carbon-compounds. The first Monera were formed in +the sea by spontaneous generation, as crystals are formed in the +mother-water. Our demand for a knowledge of causes compels us to assume +this. If we believe that the whole inorganic history of the earth has +proceeded on mechanical principles without any intervention of a +Creator, and that the history of life also has been determined by the +same mechanical laws; if we see that there is no need to admit creative +action to explain the origin of the various groups of organisms; it is +utterly irrational to assume such creative action in dealing with the +first appearance of organic life on the earth. + + [29] The English reader will find a luminous and up-to-date chapter on + the subject in Haeckel’s recently written and translated _Wonders of + Life._—Translator. + + +This much-disputed question of “spontaneous generation” seems so +obscure, because people have associated with the term a mass of very +different, and often very absurd, ideas, and have attempted to solve +the difficulty by the crudest experiments. The real doctrine of the +spontaneous generation of life cannot possibly be refuted by +experiments. +Every experiment that has a negative result only proves that no +organism has been formed out of inorganic matter in the +conditions—highly artificial conditions—we have established. On the +other hand, it would be exceedingly difficult to prove the theory by +way of experiment; and even if Monera were still formed daily by +spontaneous generation (which is quite possible), it would be very +difficult, if not impossible, to find a solid proof of it. Those who +will not admit the spontaneous generation of the first living things in +our sense must have recourse to a supernatural miracle; and this is, as +a matter of fact, the desperate resource to which our “exact” +scientists are driven, to the complete abdication of reason. + +A famous English physicist, Lord Kelvin (then Sir W. Thomson), +attempted to dispense with the hypothesis of spontaneous generation by +assuming that the organic inhabitants of the earth were developed from +germs that came from the inhabitants of other planets, and that chanced +to fall on our planet on fragments of their original home, or +meteorites. This hypothesis found many supporters, among others the +distinguished German physicist, Helmholtz. However, it was refuted in +1872 by the able physicist, Friedrich Zöllner, of Leipzig, in his work, +_On the Nature of Comets._ He showed clearly how unscientific this +hypothesis is; firstly in point of logic, and secondly in point of +scientific content. At the same time he pointed out that our hypothesis +of spontaneous generation is “a necessary condition for understanding +nature according to the law of causality.” + +I repeat that we must call in the aid of the hypothesis only as regards +the Monera, the structureless “organisms without organs.” Every complex +organism must have been evolved from some lower organism. We must not +assume the spontaneous generation of even the simplest cell, for this +itself consists of at least two parts—the internal, firm nuclear +substance, and the external, softer cellular substance or the +protoplasm of the cell-body. These two parts must have been formed by +differentiation from the indifferent plasson of a moneron, or a cytode. +For this reason the natural history of the Monera is of great interest; +here alone can we find the means to overcome the chief difficulties of +the problem of spontaneous generation. The actual living Monera are +specimens of such organless or structureless organisms, as they must +have boon formed by spontaneous generation at the commencement of the +history of life. + + + + +Chapter XIX. +OUR PROTIST ANCESTORS + + +Under the guidance of the biogenetic law, and on the basis of the +evidence we have obtained, we now turn to the interesting task of +determining the series of man’s animal ancestors. Phylogeny us a whole +is an inductive science. From the totality of the biological processes +in the life of plants, animals, and man we have gathered a confident +inductive idea that the whole organic population of our planet has been +moulded on a harmonious law of evolution. All the interesting phenomena +that we meet in ontogeny and paleontology, comparative anatomy and +dysteleology, the distribution and habits of organisms—all the +important general laws that we abstract from the phenomena of these +sciences, and combine in harmonious unity—are the broad bases of our +great biological induction. + +But when we come to the application of this law, and seek to determine +with its aid the origin of the various species of organisms, we are +compelled to frame +hypotheses that have essentially a _deductive_ character, and are +inferences from the general law to particular cases. But these special +deductions are just as much justified and necessitated by the rigorous +laws of logic as the inductive conclusions on which the whole theory of +evolution is built. The doctrine of the animal ancestry of the human +race is a special deduction of this kind, and follows with logical +necessity from the general inductive law of evolution. + +I must point out at once, however, that the certainty of these +evolutionary hypotheses, which rest on clear special deductions, is not +always equally strong. Some of these inferences are now beyond +question; in the case of others it depends on the knowledge and the +competence of the inquirer what degree of certainty he attributes to +them. In any case, we must distinguish between the _ absolute_ +certainty of the general (inductive) theory of descent and the +_relative_ certainty of special (deductive) evolutionary hypotheses. We +can never determine the whole ancestral series of an organism with the +same confidence with which we hold the general theory of evolution as +the sole scientific explanation of organic modifications. The special +indication of stem-forms in detail will always be more or less +incomplete and hypothetical. This is quite natural. The evidence on +which we build is imperfect, and always will be imperfect; just as in +comparative philology. + +The first of our documents, paleontology, is exceedingly incomplete. We +know that all the fossils yet discovered are only an insignificant +fraction of the plants and animals that have lived on our planet. For +every single species that has been preserved for us in the rocks there +are probably hundreds, perhaps thousands, of extinct species that have +left no trace behind them. This extreme and very unfortunate +incompleteness of the paleontological evidence, which cannot be pointed +out too often, is easily explained. It is absolutely inevitable in the +circumstances of the fossilisation of organisms. It is also due in part +to the incompleteness of our knowledge in this branch. It must be borne +in mind that the great majority of the stratified rocks that compose +the crust of the earth have not yet been opened. We have only a few +specimens of the innumerable fossils that are buried in the vast +mountain ranges of Asia and Africa. Only a part of Europe and North +America has been investigated carefully. The whole of the fossils known +to us certainly do not amount to a hundredth part of the remains that +are really buried in the crust of the earth. We may, therefore, look +forward to a rich harvest in the future as regards this science. +However, our paleontological evidence will (for reasons that I have +fully explained in the sixteenth chapter of the _History of Creation_) +always be defective. + +The second chief source of evidence, ontogeny, is not less incomplete. +It is the most important source of all for special phylogeny; but it +has great defects, and often fails us. We must, above all, clearly +distinguish between palingenetic and cenogenetic phenomena. We must +never forget that the laws of curtailed and disturbed heredity often +make the original course of development almost unrecognisable. The +recapitulation of phylogeny by ontogeny is only fairly complete in a +few cases, and is never wholly complete. As a rule, it is precisely the +earliest and most important embryonic stages that suffer most from +alteration and condensation. The earlier embryonic forms have had to +adapt themselves to new circumstances, and so have been modified. The +struggle for existence has had just as profound an influence on the +freely moving and still immature young forms as on the adult forms. +Hence in the embryology of the higher animals, especially, palingenesis +is much restricted by cenogenesis; it is to-day, as a rule, only a +faded and much altered picture of the original evolution of the +animal’s ancestors. We can only draw conclusions from the embryonic +forms to the stem-history with the greatest caution and discrimination. +Moreover, the embryonic development itself has only been fully studied +in a few species. + +Finally, the third and most valuable source of evidence, comparative +anatomy, is also, unfortunately, very imperfect; for the simple reason +that the whole of the living species of animals are a mere fraction of +the vast population that has dwelt on our planet since the beginning of +life. We may confidently put the total number of these at more than a +million species. The number of animals whose organisation has been +studied up to the present in comparative anatomy is proportionately +very small. Here, again, future research will yield incalculable +treasures. +But, for the present, in view of this patent incompleteness of our +chief sources of evidence, we must naturally be careful not to lay too +much stress in human phylogeny on the particular animals we have +studied, or regard all the various stages of development with equal +confidence as stem-forms. + +In my first efforts to construct the series of man’s ancestors I drew +up a list of, at first ten, afterwards twenty to thirty, forms that may +be regarded more or less certainly as animal ancestors of the human +race, or as stages that in a sense mark off the chief sections in the +long story of evolution from the unicellular organism to man. Of these +twenty to thirty stages, ten to twelve belong to the older group of the +Invertebrates and eighteen to twenty to the younger division of the +Vertebrates. + +In approaching, now, the difficult task of establishing the +evolutionary succession of these thirty ancestors of humanity since the +beginning of life, and in venturing to lift the veil that covers the +earliest secrets of the earth’s history, we must undoubtedly look for +the first living things among the wonderful organisms that we call the +Monera; they are the simplest organisms known to us—in fact, the +simplest we can conceive. Their whole body consists merely of a simple +particle or globule of structureless plasm or plasson. The discoveries +of the last four decades have led us to believe with increasing +certainty that wherever a natural body exhibits the vital processes of +nutrition, reproduction, voluntary movement, and sensation, we have the +action of a nitrogenous carbon-compound of the chemical group of the +albuminoids; this plasm (or protoplasm) is the material basis of all +vital functions. Whether we regarded the function, in the monistic +sense, as the direct action of the material substratum, or whether we +take matter and force to be distinct things in the dualistic sense, it +is certain that we have not as yet found any living organism in which +the exercise of the vital functions is not inseparably bound up with +plasm. + +The soft slimy plasson of the body of the moneron is generally called +“protoplasm,” and identified with the cellular matter of the ordinary +plant and animal cells. But we must, to be accurate, distinguish +between the plasson of the cytodes and the protoplasm of the cells. +This distinction is of the utmost importance for the purposes of +evolution. As I have often said, we must recognise two different stages +of development in these “elementary organisms,” or plastids +(“builders”), that represent the ultimate units of organic +individuality. The earlier and lower stage are the unnucleated cytodes, +the body of which consists of only one kind of albuminous matter—the +homogeneous plasson or “formative matter.” The later and higher stage +are the nucleated cells, in which we find a differentiation of the +original plasson into two different formative substances—the caryoplasm +of the nucleus and the cytoplasm of the body of the cell (cf. pp. 37 +and 42). + + +Fig.226. Chroococcus minor. Fig. 226—Chroococcus minor (_Nägeli_), +magnified. A phytomoneron, the globular plastids of which secrete a +gelatinous structureless membrane. The unnucleated globule of plasm +(bluish-green in colour) increases by simple cleavage (_a–d_). + + +The Monera are permanent cytodes. Their whole body consists of soft, +structureless plasson. However carefully we examine it with our finest +chemical reagents and most powerful microscopes, we can find no +definite parts or no anatomic structure in it. Hence, the Monera are +literally organisms without organs; in fact, from the philosophic point +of view they are not organisms at all, since they have no organs. They +can only be called organisms in the sense that they are capable of the +vital functions of nutrition, reproduction, sensation, and movement. If +we were to try to imagine the simplest possible organism, we should +frame something like the moneron. + +The Monera that we find to-day in various forms fall into two groups +according to the nature of their nutrition—the _ Phytomonera_ and the +_Zoomonera_; from the physiological point of view, the former are the +simplest specimens of the plant (_phyton_) kingdom, and the latter of +the animal (_zoon_) world. The Phytomonera, especially in their +simplest form, the Chromacea (_Phycochromacea_ or _Cyanophycea_), are +the most primitive and the +oldest of living organisms. The typical genus _ Chroococcus_ (Fig. 226) +is represented by several fresh-water species, and often forms a very +delicate bluish-green deposit on stones and wood in ponds and ditches. +It consists of round, light green particles, from 1/7000 to 1/2500 of +an inch in diameter. + + +Fig.227. Aphanocapsa primordialis. Fig. 227—Aphanocapsa primordialis +(_Nägeli_), magnified. A phytomoneron, the round plastids of which +(bluish-green in colour) secrete a shapeless gelatinous mass; in this +the unnucleated cytodes increase continually by simple cleavage. + +The whole life of these homogeneous globules of plasm consists of +simple growth and reproduction by cleavage. When the tiny particle has +reached a certain size by the continuous assimilation of inorganic +matter, it divides into two equal halves, by a constriction in the +middle. The two daughter-monera that are thus formed immediately begin +a similar vital process. It is the same with the brown _Procytella +primordialis_ (formerly called the _Protococcus marinus_); it forms +large masses of floating matter in the arctic seas. The tiny +plasma-globules of this species are of a greenish-brown colour, and +have a diameter of 1/10,000 to 1/5000 of an inch. There is no membrane +discoverable in the simplest _Chroococcacea,_ but we find one in other +members of the same family; in _Aphanocapsa_ (Fig. 227) the enveloping +membranes of the social plastids combine; in _Glœcapsa_ they are +retained through several generations, so that the little +plasma-globules are enfolded in many layers of membrane. + +Next to the Chromacea come the Bacteria, which have been evolved from +them by the remarkable change in nutrition which gives us the simple +explanation of the differentiation of plant and animal in the protist +kingdom. The Chromacea build up their plasm directly from inorganic +matter; the Bacteria feed on organic matter. Hence, if we logically +divide the protist kingdom into plasma-forming Protophyta and +plasma-consuming Protozoa, we must class the Bacteria with the latter; +it is quite illogical to describe them—as is still often done—as +_Schizomycetes,_ and class them with the true fungi. The Bacteria, like +the Chromacea, have no nucleus. As is well-known, they play an +important part in modern biology as the causes of fermentation and +putrefaction, and of tuberculosis, typhus, cholera, and other +infectious diseases, and as parasites, etc. But we cannot linger now to +deal with these very interesting features; the Bacteria have no +relation to man’s genealogical tree. + +We may now turn to consider the remarkable Protamœba, or unnucleated +Amœba. I have, in the first volume, pointed out the great importance of +the ordinary Amœba in connection with several weighty questions of +general biology. The tiny Protamœbæ, which are found both in fresh and +salt water, have the same unshapely form and irregular movements of +their simple naked body as the real Amœbæ; but they differ from them +very materially in having no nucleus in their cell-body. The short, +blunt, finger-like processes that are thrust out at the surface of the +creeping Protamœba serve for getting food as well as for locomotion. +They multiply by simple cleavage (Fig. 228). + +The next stage to the simple cytode-forms of the Monera in the +genealogy of mankind (and all other animals) is the simple cell, or the +most rudimentary form of the cell which we find living independently +to-day as the Amœba. The earliest process of inorganic differentiation +in the structureless body of the Monera led to its division into two +different substances—the caryoplasm and the cytoplasm. The caryoplasm +is the inner and firmer part of the cell, the substance of the nucleus. +The cytoplasm is the outer and softer part, the substance of the body +of the cell. By this important differentiation of the plasson into +nucleus and cell-body, the +organised cell was evolved from the structureless cytode, the nucleated +from the unnucleated plastid. That the first cells to appear on the +earth were formed from the Monera by such a differentiation seems to us +the only possible view in the present condition of science. We have a +direct instance of this earliest process of differentiation to-day in +the ontogeny of many of the lower Protists (such as the Gregarinæ). + + +Fig.228. A moneron (Protamoeba) in the act of reproduction. Fig. 228—A +moneron (Protamœba) in the act of reproduction. _A_ The whole moneron, +moving like an ordinary amœba by thrusting out changeable processes. +_B_ It divides into two halves by a constriction in the middle. _C_ The +two halves separate, and each becomes an independent individual. +(Highly magnified.) + + +The unicellular form that we have in the ovum has already been +described as the reproduction of a corresponding unicellular stem-form, +and to this we have ascribed the organisation of an Amœba (cf. Chapter +VI). The irregular-shaped Amœba, which we find living independently +to-day in our fresh and salt water, is the least definite and the most +primitive of all the unicellular Protozoa (Fig. 16). As the unripe ova +(the _protova_ that we find in the ovaries of animals) cannot be +distinguished from the common Amœbæ, we must regard the Amœba as the +primitive form that is reproduced in the embryonic stage of the amœboid +ovum to-day, in accordance with the biogenetic law. I have already +pointed out, in proof of the striking resemblance of the two cells, +that the ova of many of the sponges were formerly regarded as parasitic +Amœbæ (Figure 1.18). Large unicellular organisms like the Amœbæ were +found creeping about inside the body of the sponge, and were thought to +be parasites. It was afterwards discovered that they were really the +ova of the sponge from which the embryos were developed. As a matter of +fact, these sponge-ova are so much like many of the Amœbæ in size, +shape, the character of their nucleus, and movement of the pseudopodia, +that it is impossible to distinguish them without knowing their +subsequent development. + +Our phylogenetic interpretation of the ovum, and the reduction of it to +some ancient amœboid ancestral form, supply the answer to the old +problem: “Which was first, the egg or the chick?” We can now give a +very plain answer to this riddle, with which our opponents have often +tried to drive us into a corner. The egg came a long time before the +chick. We do not mean, of course, that the egg existed from the first +as a bird’s egg, but as an indifferent amœboid cell of the simplest +character. The egg lived for thousands of years as an independent +unicellular organism, the Amœba. The egg, in the modern physiological +sense of the word, did not make its appearance until the descendants of +the unicellular Protozoon had developed into multicellular animals, and +these had undergone sexual differentiation. Even then the egg was first +a gastræa-egg, then a platode-egg, then a vermalia-egg, and +chordonia-egg; later still acrania-egg, then fish-egg, amphibia-egg, +reptile-egg, and finally bird’s egg. The bird’s egg we have experience +of daily is a highly complicated historical product, the result of +countless hereditary processes that have taken place in the course of +millions of years. + +The earliest ancestors of our race were simple Protophyta, and from +these our protozoic ancestors were developed afterwards. From the +morphological point of view both the vegetal and the animal Protists +were simple organisms, individualities of the first order, or plastids. +All our later ancestors are complex organisms, or individualities of a +higher order—social aggregations of a plurality of cells. The earliest +of these, the _ Moræada,_ which represent the third stage in our +genealogy, are very simple associations of homogeneous, indifferent +cells—undifferentiated colonies of social Amœbæ or Infusoria. To +understand the nature and origin of these protozoa-colonies we need +only follow step by step the first embryonic products of the stem-cell. +In all the Metazoa the first embryonic process is the repeated cleavage +of the stem-cell, or first segmentation-cell (Fig. 229). We have +already fully considered this process, and found that all the different +forms of it may be reduced to one type, the original equal or +primordial segmentation (cf. Chapter VIII). In the genealogical tree +of the Vertebrates this palingenetic form of segmentation has been +preserved in the Amphioxus alone, all the other Vertebrates having +cenogenetically modified forms of cleavage. In any case, the latter +were developed from the former, and so the segmentation of the ovum in +the Amphioxus has a great interest for us (cf. Fig. 38). The outcome of +this repeated cleavage is the formation of a round cluster of cells, +composed of homogeneous, indifferent cells of the simplest character +(Fig. 230). This is called the _morula_ (= mulberry-embryo) on account +of its resemblance to a mulberry or blackberry. + + +Fig.229. Original or primordial ovum-cleavage. Fig. 229—Original or +primordial ovum-cleavage. The stem-cell or cytula, formed by +fecundation of the ovum, divides by repeated regular cleavage first +into two (_A_), then four (_B_), then eight (_C_), and finally a large +number of segmentation-cells (_D_). + + +It is clear that this morula reproduces for us to-day the simple +structure of the multicellular animal that succeeded the unicellular +amœboid form in the early Laurentian period. In accordance with the +biogenetic law, the morula recalls the ancestral form of the Moræa, or +simple colony of Protozoa. The first cell-communities to be formed, +which laid the early foundation of the higher multicellular body, must +have consisted of homogeneous and simple amœboid cells. The oldest +Amœbæ lived isolated lives, and even the amœboid cells that were formed +by the segmentation of these unicellular organisms must have continued +to live independently for a long time. But gradually small communities +of Amœbæ arose by the side of these eremitical Protozoa, the +sister-cells produced by cleavage remaining joined together. The +advantages in the struggle for life which these communities had over +the isolated cells favoured their formation and their further +development. We find plenty of these cell-colonies or communities +to-day in both fresh and salt water. They belong to various groups both +of the Protophyta and Protozoa. + + +Fig.230. Morula, or mulberry-shaped embryo. Fig. 230—Morula, or +mulberry-shaped embryo. + + +To have some idea of those ancestors of our race that succeeded +phylogenetically to the Moræada, we have only to follow the further +embryonic development of the morula. We then see that the social cells +of the round cluster secrete a sort of jelly or a watery fluid inside +their globular body, and they themselves rise to the surface of it +(Fig. 29 _F, G_). In this way the solid mulberry-embryo becomes a +hollow sphere, the wall of which is composed of a single layer of +cells. We call this layer the _blastoderm,_ and the sphere itself the +_blastula,_ or embryonic vesicle. + +This interesting blastula is very important. The conversion of the +morula into a hollow ball proceeds on the same lines originally in the +most diverse stems—as, for instance, in many of the zoophytes and +worms, the ascidia, many of the echinoderms and molluscs, and in the +amphioxus. Moreover, in the animals in which we do not find a real +palingenetic blastula the defect is clearly due to cenogenetic causes, +such as the formation of food-yelk and other embryonic adaptations. We +may, therefore, conclude that the ontogenetic blastula is the +reproduction of a very early phylogenetic ancestral form, and that all +the Metazoa are descended from a common stem-form, which was in the +main constructed like the blastula. In many of the lower animals the +blastula is not developed +within the fœtal membranes, but in the open water. In those cases each +blastodermic cell begins at an early stage to thrust out one or more +mobile hair-like processes; the body swims about by the vibratory +movement of these lashes or whips (Fig. 29 _F_). + +We still find, both in the sea and in fresh water, various kinds of +primitive multicellular organisms that substantially resemble the +blastula in structure, and may be regarded in a sense as permanent +blastula-forms—hollow vesicles or gelatinous balls, with a wall +composed of a single layer of ciliated homogeneous cells. There are +“blastæads” of this kind even among the Protophyta—the familiar +Volvocina, formerly classed with the infusoria. The common _Volvox +globator_ is found in the ponds in the spring—a small, green, +gelatinous globule, swimming about by means of the stroke of its +lashes, which rise in pairs from the cells on its surface. In the +similar _ Halosphæra viridis_ also, which we find in the marine +plancton (floating matter), a number of green cells form a simple layer +at the surface of the gelatinous ball; but in this case there are no +cilia. + +Some of the infusoria of the flagellata-class (_Signura, Magosphæra,_ +etc.) are similar in structure to these vegetal clusters, but differ in +their animal nutrition; they form the special group of the +_Catallacta._ In September, 1869, I studied the development of one of +these graceful animals on the island of Gis-Oe, off the coast of Norway +(_Magosphæra planula_), Figures 2.231 and 2.232). The fully-formed body +is a gelatinous ball, with its wall composed of thirty-two to +sixty-four ciliated cells; it swims about freely in the sea. After +reaching maturity the community is dissolved. Each cell then lives +independently for some time, grows, and changes into a creeping amœba. +This afterwards contracts, and clothes itself with a structureless +membrane. The cell then looks just like an ordinary animal ovum. When +it has been in this condition for some time the cell divides into two, +four, eight, sixteen, thirty-two, and sixty-four cells. These arrange +themselves in a round vesicle, thrust out vibratory lashes, burst the +capsule, and swim about in the same magosphæra-form with which we +started. This completes the life-circle of the remarkable and +instructive animal. + +If we compare these permanent blastulæ with the free-swimming ciliated +larvæ or blastulæ, with similar construction, of many of the lower +animals, we can confidently deduce from them that there was a very +early and long-extinct common stem-form of substantially the same +structure as the blastula. We may call it the _Blastæa._ Its body +consisted, when fully formed, of a simple hollow ball, filled with +fluid or structureless jelly, with a wall composed of a single stratum +of ciliated cells. There were probably many genera and species of these +blastæads in the Laurentian period, forming a special class of marine +protists. + +It is an interesting fact that in the plant kingdom also the simple +hollow sphere is found to be an elementary form of the multicellular +organism. At the surface and below the surface (down to a depth of 2000 +yards) of the sea there are green globules swimming about, with a wall +composed of a single layer of chlorophyll-bearing cells. The botanist +Schmitz gave them the name of _Halosphæra viridis_ in 1879. + +The next stage to the _Blastæa,_ and the sixth in our genealogical +tree, is the Gastræa that is developed from it. As we have already +seen, this ancestral form is particularly important. That it once +existed is proved with certainty by the gastrula, which we find +temporarily in the ontogenesis of all the Metazoa (Fig. 29 _J, K_). As +we saw, the original, palingenetic form of the gastrula is a round or +oval uni-axial body, the simple cavity of which (the primitive gut) has +an aperture at one pole of its axis (the primitive mouth). The wall of +the gut consists of two strata of cells, and these are the primary +germinal layers, the animal skin-layer (ectoderm) and vegetal gut-layer +(entoderm). + +The actual ontogenetic development of the gastrula from the blastula +furnishes sound evidence as to the phylogenetic origin of the _Gastræa_ +from the _Blastæa._ A pit-shaped depression appears at one side of the +spherical blastula (Fig. 29 _H_). In the end this invagination goes so +far that the outer or invaginated part of the blastoderm lies close on +the inner or non-invaginated part (Fig. 29 _J_). In explaining the +phylogenetic origin of the gastræa in the light of this ontogenetic +process, we may assume that the one-layered cell-community of the +blastæa began to take in food more largely at one particular part of +its surface. Natural selection would gradually lead to +the formation of a depression or pit at this alimentary spot on the +surface of the ball. The depression would grow deeper and deeper. In +time the vegetal function of taking in and digesting food would be +confined to the cells that lined this hole; the other cells would see +to the animal functions of locomotion, sensation, and protection. This +was the first division of labour among the originally homogeneous cells +of the blastæa. + + +Fig.231. The Norwegian Magosphaera planula, swimming about by means of +the lashes or cilia at its surface. Fig. 232. Section of same, showing +how the pear-shaped cells in the centre of the gelatinous ball are +connected by a fibrous process. Fig. 231—The Norwegian Magosphæra +planula, swimming about by means of the lashes or cilia at its surface. +Fig. 232—Section of same, showing how the pear-shaped cells in the +centre of the gelatinous ball are connected by a fibrous process. Each +cell has a contractile vacuole as well as a nucleus. + + +The effect, then, of this earliest histological differentiation was to +produce two different kinds of cells—nutritive cells in the depression +and locomotive cells on the surface outside. But this involved the +severance of the two primary germinal layers—a most important process. +When we remember that even man’s body, with all its various parts, and +the body of all the other higher animals, are built up originally out +of these two simple layers, we cannot lay too much stress on the +phylogenetic significance of this gastrulation. In the simple primitive +gut or gastric cavity of the gastrula and its rudimentary mouth we have +the first real organ of the animal frame in the morphological sense; +all the other organs were developed afterwards from these. In reality, +the whole body of the gastrula is merely a “primitive gut.” I have +shown already (Chapters VIII and XIX) that the two-layered embryos of +all the Metazoa can be reduced to this typical gastrula. This important +fact justifies us in concluding, in accordance with the biogenetic law, +that their ancestors also were phylogenetically developed from a +similar stem-form. This ancient stem-form is the gastræa. + +The gastræa probably lived in the sea during the Laurentian period, +swimming about in the water by means of its ciliary coat much as free +ciliated gastrulæ do to-day. Probably it differed from the existing +gastrula only in one essential point, though extinct millions of years +ago. We have reason, from comparative anatomy and ontogeny, to believe +that it multiplied by sexual generation, not merely asexually (by +cleavage, gemmation, and spores), as was no doubt the case with the +earlier ancestors. Some of the cells of the primary germ-layers +probably became ova and others fertilising sperm. We base these +hypotheses on the fact that we do to-day find the simplest form of +sexual reproduction in some of the living gastræads and other lower +animals, especially the sponges. + +The fact that there are still in existence various kinds of gastræads, +or lower Metazoa with an organisation little higher than that of the +hypothetical gastræa, is a strong point in favour of our theory. There +are not very many species of these living gastræads; but their +morphological and phylogenetic interest is so great, and their +intermediate position between the Protozoa and Metazoa so instructive, +that I proposed long ago (1876) to make a special class of them. I +distinguished three orders in this class—the Gastremaria, Physemaria, +and Cyemaria (or Dicyemida). +But we might also regard these three orders as so many independent +classes in a primitive gastræad stem. + +The Gastremaria and Cyemaria, the chief of these living gastræads, are +small Metazoa that live parasitically inside other Metazoa, and are, as +a rule, 1/50 to 1/25 of an inch long, often much less (Fig. 233, 1–15). +Their soft body, devoid of skeleton, consists of two simple strata of +cells, the primary germinal layers; the outer of these is thickly +clothed with long hair-like lashes, by which the parasites swim about +in the various cavities of their host. The inner germinal layer +furnishes the sexual products. The pure type of the original gastrula +(or _ archigastrula,_ Fig. 29 _ I_) is seen in the _Pemmatodiscus +gastrulaceus,_ which Monticelli discovered in the umbrella of a large +medusa (_Pilema pulmo_) in 1895; the convex surface of this gelatinous +umbrella was covered with numbers of clear vesicles, of 1/25 to 1/8 +inch in diameter, in the fluid contents of which the little parasites +were swimming. The cup-shaped body of the _Pemmatodiscus_ (Fig. 233, +_1_) is sometimes rather flat, and shaped like a hat or cone, at other +times almost curved into a semi-circle. The simple hollow of the cup, +the primitive gut (_g_), has a narrow opening (_o_). The skin layer +(_e_) consists of long slender cylindrical cells, which bear long +vibratory hairs; it is separated by a thin structureless, gelatinous +plate (_f_) from the visceral or gut layer (_i_), the prismatic cells +of which are much smaller and have no cilia. Pemmatodiscus propagates +asexually, by simple longitudinal cleavage; on this account it has +recently been regarded as the representative of a special order of +gastræads (_Mesogastria_). + +Probably a near relative of the _Pemmatodiscus_ is the _ Kunstleria +Gruveli_ (Fig. 233, _2_). It lives in the body-cavity of Vermalia +(Sipunculida), and differs from the former in having no lashes either +on the large ectodermic cells (_e_) or the small entodermic (_i_); the +germinal layers are separated by a thick, cup-shaped, gelatinous mass, +which has been called the “clear vesicle” (_f_). The primitive mouth is +surrounded by a dark ring that bears very strong and long vibratory +lashes, and effects the swimming movements. + +_Pemmatodiscus_ and _Kunstleria_ may be included in the family of the +Gastremaria. To these gastræads with open gut are closely related the +Orthonectida (_Rhopalura,_ Fig. 233, _3–5_). They live parasitically in +the body-cavity of echinoderms (Ophiura) and vermalia; they are +distinguished by the fact that their primitive gut-cavity is not empty, +but filled with entodermic cells, from which the sexual cells are +developed. These gastræads are of both sexes, the male (Fig. 3) being +smaller and of a somewhat different shape from the oval female (Fig. +4). + +The somewhat similar _Dicyemida_ (Fig. 6) are distinguished from the +preceding by the fact that their primitive gut-cavity is occupied by a +single large entodermic cell instead of a crowded group of sexual +cells. This cell does not yield sexual products, but afterwards divides +into a number of cells (spores), each of which, without being +impregnated, grows into a small embryo. The Dicyemida live +parasitically in the body-cavity, especially the renal cavities, of the +cuttle-fishes. They fall in several genera, some of which are +characterised by the possession of special polar cells; the body is +sometimes roundish, oval, or club-shaped, at other times long and +cylindrical. The genus _Conocyema_ (Figs. 7–15) differs from the +ordinary _Dicyema_ in having four polar pimples in the form of a cross, +which may be incipient tentacles. + +The classification of the Cyemaria is much disputed; sometimes they are +held to be parasitic infusoria (like the _Opalina_), sometimes platodes +or vermalia, related to the suctorial worms or rotifers, but having +degenerated through parasitism. I adhere to the phylogenetically +important theory that I advanced in 1876, that we have here real +gastræads, primitive survivors of the common stem-group of all the +Metazoa. In the struggle for life they have found shelter in the +body-cavity of other animals. + +The small Cœlenteria attached to the floor of the sea that I have +called the Physemaria (_Haliphysema_ and _ Gastrophysema_) probably +form a third order (or class) of the living gastræads. The genus +_Haliphysema_ (Figs. 234, 235) is externally very similar to a large +rhizopod (described by the same name in 1862) of the family of the +_Rhabdamminida,_ which was at first taken for a sponge. In order to +avoid confusion with these, I afterwards gave them the name of +Prophysema. The whole mature body of the _Prophysema_ is a simple +cylindrical or oval tube, with a two-layered wall. The hollow of the +tube is the gastric cavity, and the upper opening of it the mouth (Fig. +235 _m_). + + +Fig.233. Modern gastræads. Fig. 1. Pemmatodiscus gastrulaceus +(Monticelli), in longitudinal section. Fig. 2. Kunstleria gruveli +(Delage), in longitudinal section. (From Kunstler and Gruvel.) Figs. +3-5. Rhopalura Giardi (Julin): Fig. 3 male, Fig. 4 female, Fig. 5 +planula. Fig. 6. Dicyema macrocephala (Van Beneden). Fig. 7-15. +Conocyema polymorpha (Van Beneden): Fig. 7 the mature gastræad, Fig. +8-15 its gastrulation. Fig. 233—Modern gastræads. Fig. 1. Pemmatodiscus +gastrulaceus (_Monticelli_), in longitudinal section. Fig. 2. +Kunstleria gruveli (_Delage_), in longitudinal section. (From _ +Kunstler_ and _Gruvel._) Figs. 3–5. Rhopalura Giardi (_Julin_): Fig. 3 +male, Fig. 4 female, Fig. 5 planula. Fig. 6. Dicyema macrocephala (_Van +Beneden_). Figs. 7–15. Conocyema polymorpha (_Van Beneden_): Fig. 7 the +mature gastræad, Figs. 8–15 its gastrulation. _d_ primitive gut, _o_ +primitive mouth, _ e_ ectoderm, _i_ entoderm, _f_ gelatinous plate +between _e_ and _i_ (supporting plate, blastocœl). + + +The two strata of cells that form the wall of the tube are the primary +germinal layers. These rudimentary zoophytes differ from the swimming +gastræads chiefly in being attached at one end (the end opposite to the +mouth) to the floor of the sea. + + +Figs. 234 and 235. Prophysema primordiale, a living gastraead. Figs. +234 and 235—Prophysema primordiale, a living gastræad. Fig. 234. The +whole of the spindle-shaped animal (attached below to the floor of the +sea. Fig. 235. The same in longitudinal section. The primitive gut +(_d_) opens above at the primitive mouth (_m_). Between the ciliated +cells (_g_) are the amœboid ova (_e_). The skin-layer (_h_) is +encrusted with grains of sand below and sponge-spicules above. + + +In _Prophysema_ the primitive gut is a simple oval cavity, but in the +closely related _Gastrophysema_ it is divided into two chambers by a +transverse constriction; the hind and smaller chamber above furnishes +the sexual products, the anterior one being for digestion. + + +Figs. 236-237. Ascula of gastrophysema, attached to the floor of the +sea. Figs. 236–237—Ascula of gastrophysema, attached to the floor of +the sea. Fig. 236 external view, 237 longitudinal section. _g_ +primitive gut, _o_ primitive mouth, _i_ visceral layer, _e_ cutaneous +layer. (Diagram.) + +The simplest sponges (_Olynthus,_ Fig. 238) have the same organisation +as the Physemaria. The only material difference between them is that in +the sponge the thin two-layered body-wall is pierced by numbers of +pores. When these are closed they resemble the Physemaria. Possibly the +gastræads that we call Physemaria are only olynthi with the pores +closed. The _ Ammoconida,_ or the simple tubular sand-sponges of the +deep-sea (_Ammolynthus,_ etc.), do not differ from the gastræads in any +important point when the pores are closed. In my _ Monograph on the +Sponges_ (with sixty plates) I endeavoured to prove analytically that +all the species of this class can be traced phylogenetically to a +common stem-form (_Calcolynthus_). + +The lowest form of the Cnidaria is also not far removed from the +gastræads. In the interesting common fresh-water polyp (_Hydra_) the +whole body is simply an oval tube with a double wall; only in this case +the mouth has a crown of tentacles. Before these develop the hydra +resembles an ascula (Figs. 236, 237). Afterwards there are slight +histological differentiations in its ectoderm, though the entoderm +remains +a single stratum of cells. We find the first differentiation of +epithelial and stinging cells, or of muscular and neural cells, in the +thick ectoderm of the hydra. + + +Fig.238. Olynthus, a very rudimentary sponge. Fig. 238—Olynthus, a very +rudimentary sponge. A piece cut away in front. + +In all these rudimentary living cœlenteria the sexual cells of both +kinds—ova and sperm cells—are formed by the same individual; it is +possible that the oldest gastræads were hermaphroditic. It is clear +from comparative anatomy that hermaphrodism—the combination of both +kinds of sexual cells in one individual—is the earliest form of sexual +differentiation; the separation of the sexes (gonochorism) was a much +later phenomenon. The sexual cells originally proceeded from the edge +of the primitive mouth of the gastræad. + + + + +Chapter XX. +OUR WORM-LIKE ANCESTORS + + +The gastræa theory has now convinced us that all the Metazoa or +multicellular animals can be traced to a common stem-form, the Gastræa. +In accordance with the biogenetic law, we find solid proof of this in +the fact that the two-layered embryos of all the Metazoa can be reduced +to a primitive common type, the gastrula. Just as the countless species +of the Metazoa do actually develop in the individual from the simple +embryonic form of the gastrula, so they have all descended in past time +from the common stem-form of the Gastræa. In this fact, and the fact we +have already established that the Gastræa has been evolved from the +hollow vesicle of the one-layered Blastæa, and this again from the +original unicellular stem-form, we have obtained a solid basis for our +study of evolution. The clear path from the stem-cell to the gastrula +represents the first section of our human stem-history (Chapters VIII, +IX, and XIX). + +The second section, that leads from the Gastræa to the Prochordonia, is +much more difficult and obscure. By the Prochordonia we mean the +ancient and long-extinct animals which the important embryonic form of +the chordula proves to have once existed (cf. Figs. 83–86). The nearest +of living animals to this embryonic structure are the lowest Tunicates, +the Copelata ( _Appendicaria_) and the larvæ of the Ascidia. As both +the Tunicates and the Vertebrates develop from the same chordula, we +may infer that there was a corresponding common ancestor of both stems. +We may call this the _Chordæa,_ and the corresponding stem-group the +_Prochordonia_ or _Prochordata._ + +From this important stem-group of the unarticulated Prochordonia (or +“primitive chorda-animals”) the stems of the Tunicates and Vertebrates +have been divergently evolved. We shall see presently how this +conclusion is justified in the present condition of morphological +science. + +We have first to answer the difficult and much-discussed question of +the development of the Chordæa from the Gastræa; in other words, “How +and by what transformations were the characteristic animals, resembling +the embryonic chordula, which we regard as the common stem-forms of all +the Chordonia, both +Tunicates and Vertebrates, evolved from the simplest two-layered +Metazoa?” + +The descent of the Vertebrates from the Articulates has been maintained +by a number of zoologists during the last thirty years with more zeal +than discernment; and, as a vast amount has been written on the +subject, we must deal with it to some extent. All three classes of +Articulates in succession have been awarded the honour of being +considered the “real ancestors” of the Vertebrates: first, the Annelids +(earth-worms, leeches, and the like), then the Crustacea (crabs, etc.), +and, finally, the Tracheata (spiders, insects, etc.). The most popular +of these hypotheses was the annelid theory, which derived the +Vertebrates from the Worms. It was almost simultaneously (1875) +formulated by Carl Semper, of Würtzburg, and Anton Dohrn, of Naples. +The latter advanced this theory originally in favour of the failing +degeneration theory, with which I dealt in my work, _Aims and Methods +of Modern Embryology._ + +This interesting degeneration theory—much discussed at that time, but +almost forgotten now—was formed in 1875 with the aim of harmonising the +results of evolution and ever-advancing Darwinism with religious +belief. The spirited struggle that Darwin had occasioned by the +reformation of the theory of descent in 1859, and that lasted for a +decade with varying fortunes in every branch of biology, was drawing to +a close in 1870–1872, and soon ended in the complete victory of +transformism. To most of the disputants the chief point was not the +general question of evolution, but the particular one of “man’s place +in nature”—“the question of questions,” as Huxley rightly called it. It +was soon evident to every clear-headed thinker that this question could +only be answered in the sense of our anthropogeny, by admitting that +man had descended from a long series of Vertebrates by gradual +modification and improvement. + +In this way the real affinity of man and the Vertebrates came to be +admitted on all hands. Comparative anatomy and ontogeny spoke too +clearly for their testimony to be ignored any longer. But in order +still to save man’s unique position, and especially the dogma of +personal immortality, a number of natural philosophers and theologians +discovered an admirable way of escape in the “theory of degeneration.” +Granting the affinity, they turned the whole evolutionary theory upside +down, and boldly contended that “man is not the most highly developed +animal, but the animals are degenerate men.” It is true that man is +closely related to the ape, and belongs to the vertebrate stem; but the +chain of his ancestry goes upward instead of downward. In the beginning +“God created man in his own image,” as the prototype of the perfect +vertebrate; but, in consequence of original sin, the human race sank so +low that the apes branched off from it, and afterwards the lower +Vertebrates. When this theory of degeneration was consistently +developed, its supporters were bound to hold that the entire animal +kingdom was descended from the debased children of men. + +This theory was most strenuously defended by the Catholic priest and +natural philosopher, Michelis, in his _Hæckelogony: An Academic Protest +against Hæckel’s Anthropogeny_ (1875). In still more “academic” and +somewhat mystic form the theory was advanced by a natural philosopher +of the older Jena school—the mathematician and physicist, Carl Snell. +But it received its chief support on the zoological side from Anton +Dohrn, who maintained the anthropocentric ideas of Snell with +particular ability. The Amphioxus, which modern science now almost +unanimously regards as the real Primitive Vertebrate, the ancient model +of the original vertebrate structure, is, according to Dohrn, a late, +degenerate descendant of the stem, the “prodigal son” of the vertebrate +family. It has descended from the Cyclostoma by a profound +degeneration, and these in turn from the fishes; even the Ascidia and +the whole of the Tunicates are merely degenerate fishes! Following out +this curious theory, Dohrn came to contest the general belief that the +Cœlenterata and Worms are “lower animals”; he even declared that the +unicellular Protozoa were degenerate Cœlenterata. In his opinion +“degeneration is the great principle that explains the existence of all +the lower forms.” + +If this Michelis-Dohrn theory were true, and all animals were really +degenerate descendants of an originally perfect humanity, man would +assuredly be the true centre and goal of all terrestrial life; his +anthropocentric position and his immortality would be saved. +Unfortunately, this trustful theory is in such +flagrant contradiction to all the known facts of paleontology and +embryology that it is no longer worth serious scientific consideration. + +But the case is no better for the much-discussed descent of the +Vertebrates from the Annelids, which Dohrn afterwards maintained with +great zeal. Of late years this hypothesis, which raised so much dust +and controversy, has been entirely abandoned by most competent +zoologists, even those who once supported it. Its chief supporter, +Dohrn, admitted in 1890 that it is “dead and buried,” and made a +blushing retraction at the end of his _Studies of the Early History of +the Vertebrate._ + +Now that the annelid-hypothesis is “dead and buried,” and other +attempts to derive the Vertebrates from Medusæ, Echinoderms, or +Molluscs, have been equally unsuccessful, there is only one hypothesis +left to answer the question of the origin of the Vertebrates—the +hypothesis that I advanced thirty-six years ago and called the +“chordonia-hypothesis.” In view of its sound establishment and its +profound significance, it may very well claim to be a _theory,_ and so +should be described as the chordonia or chordæa theory. + +I first advanced this theory in a series of university lectures in +1867, from which the _History of Creation_ was composed. In the first +edition of this work (1868) I endeavoured to prove, on the strength of +Kowalevsky’s epoch-making discoveries, that “of all the animals known +to us the Tunicates are undoubtedly the nearest blood-relatives of the +Vertebrates; they are the most closely related to the Vermalia, from +which the Vertebrates have been evolved. Naturally, I do not mean that +the Vertebrates have descended from the Tunicates, but that the two +groups have sprung from a common root. It is clear that the real +Vertebrates (primarily the Acrania) were evolved in very early times +from a group of Worms, from which the degenerate Tunicates also +descended in another and retrogressive direction.” This common extinct +stem-group are the Prochordonia; we still have a silhouette of them in +the chordula-embryo of the Vertebrates and Tunicates; and they still +exist independently, in very modified form, in the class of the +Copelata ( _Appendicaria,_ Fig. 225). + +The chordæa-theory received the most valuable and competent support +from Carl Gegenbaur. This able comparative morphologist defended it in +1870, in the second edition of his _Elements of Comparative Anatomy_ ; +at the same time he drew attention to the important relations of the +Tunicates to a curious worm, _Balanoglossus_ : he rightly regards this +as the representative of a special class of worms, which he called +“gut-breathers” ( _Enteropneusta_). Gegenbaur referred on many other +occasions to the close blood-relationship of the Tunicates and +Vertebrates, and luminously explained the reasons that justify us in +framing the hypothesis of the descent of the two stems from a common +ancestor, an unsegmented worm-like animal with an axial chorda between +the dorsal nerve-tube and the ventral gut-tube. + +The theory afterwards received a good deal of support from the research +made by a number of distinguished zoologists and anatomists, especially +C. Kupffer, B. Hatschek, F. Balfour, E. Van Beneden, and Julin. Since +Hatschek’s _Studies of the Development of the Amphioxus_ gave us full +information as to the embryology of this lowest vertebrate, it has +become so important for our purpose that we must consider it a document +of the first rank for answering the question we are dealing with. + +The ontogenetic facts that we gather from this sole survivor of the +Acrania are the more valuable for phylogenetic purposes, as +paleontology, unfortunately, throws no light whatever on the origin of +the Vertebrates. Their invertebrate ancestors were soft organisms +without skeleton, and thus incapable of fossilisation, as is still the +case with the lowest vertebrates—the Acrania and Cyclostoma. The same +applies to the greater part of the Vermalia or worm-like animals, the +various classes and orders of which differ so much in structure. The +isolated groups of this rich stem are living branches of a huge tree, +the greater part of which has long been dead, and we have no fossil +evidence as to its earlier form. Nevertheless, some of the surviving +groups are very instructive, and give us clear indications of the way +in which the Chordonia were developed from the Vermalia, and these from +the Cœlenteria. + +While we seek the most important of these palingenetic forms among the +groups of Cœlenteria and Vermalia, it is understood that not a single +one of them +must be regarded as an unchanged, or even little changed, copy of the +extinct stem-form. One group has retained one feature, another a +different feature, of the original organisation, and other organs have +been further developed and characteristically modified. Hence here, +more than in any other part of our genealogical tree, we have to keep +before our mind the _full picture_ of development, and separate the +unessential secondary phenomena from the essential and primary. It will +be useful first to point out the chief advances in organisation by +which the simple Gastræa gradually became the more developed Chordæa. + +We find our first solid datum in the gastrula of the Amphioxus (Figure +1.38). Its bilateral and tri-axial type indicates that the +Gastræads—the common ancestors of all the Metazoa—divided at an early +stage into two divergent groups. The uni-axial Gastræa became sessile, +and gave rise to two stems, the Sponges and the Cnidaria (the latter +all reducible to simple polyps like the hydra). But the tri-axial +Gastræa assumed a certain pose or direction of the body on account of +its swimming or creeping movement, and in order to sustain this it was +a great advantage to share the burden equally between the two halves of +the body (right and left). Thus arose the typical bilateral form, which +has three axes. The same bilateral type is found in all our artificial +means of locomotion—carts, ships, etc.; it is by far the best for the +movement of the body in a certain direction and steady position. Hence +natural selection early developed this bilateral type in a section of +the Gastræads, and thus produced the stem-forms of all the bilateral +animals. + +The _Gastræa bilateralis,_ of which we may conceive the bilateral +gastrula of the amphioxus to be a palingenetic reproduction, +represented the two-sided organism of the earliest Metazoa in its +simplest form. The vegetal entoderm that lined their simple gut-cavity +served for nutrition; the ciliated ectoderm that formed the external +skin attended to locomotion and sensation; finally, the two primitive +mesodermic cells, that lay to the right and left at the ventral border +of the primitive mouth, were sexual cells, and effected reproduction. +In order to understand the further development of the gastræa, we must +pay particular attention to: (1) the careful study of the embryonic +stages of the amphioxus that lie between the gastrula and the chordula; +(2) the morphological study of the simplest Platodes ( _Platodaria_ and +_Turbellaria_) and several groups of unarticulated Vermalia ( +_Gastrotricha, Nemertina, Enteropneusta_). + +We have to consider the Platodes first, because they are on the border +between the two principal groups of the Metazoa, the Cœlenteria and the +Cœlomaria. With the former they share the lack of body-cavity, anus, +and vascular system; with the latter they have in common the bilateral +type, the possession of a pair of nephridia or renal canals, and the +formation of a vertical brain or cerebral ganglion. It is now usual to +distinguish four classes of Platodes: the two free-living classes of +the primitive worms ( _Platodaria_) and the coiled-worms ( +_Turbellaria_), and the two parasitic classes of the suctorial worms ( +_Trematoda_) and the tape-worms ( _Cestoda_). We have only to consider +the first two of these classes; the other two are parasites, and have +descended from the former by adaptation to parasitic habits and +consequent degeneration. + +The primitive worms ( _Platodaria_) are very small flat worms of simple +construction, but of great morphological and phylogenetic interest. +They have been hitherto, as a rule, regarded as a special order of the +_Turbellaria,_ and associated with the _Rhabdocœla_ ; but they differ +considerably from these and all the other Platodes (flat worms) in the +absence of renal canals and a special central nervous system; the +structure of their tissue is also simpler than in the other Platodes. +Most of the Platodes of this group ( _Aphanostomum, Amphichœrus, +Convoluta, Schizoprora,_ etc.) are very soft and delicate animals, +swimming about in the sea by means of a ciliary coat, and very small +(1/10 to 1/20 inch long). Their oval body, without appendages, is +sometimes spindle-shaped or cylindrical, sometimes flat and +leaf-shaped. Their skin is merely a layer of ciliated ectodermic cells. +Under this is a soft medullary substance, which consists of entodermic +cells with vacuoles. The food passes through the mouth directly into +this digestive medullary substance, in which we do not generally see +any permanent gut-cavity (it may have entirely collapsed); hence these +primitive Platodes have been called _Acœla_ (without gut-cavity or +cœlom), or, more correctly, _Cryptocœla,_ or _Pseudocœla._ The sexual +organs of these hermaphroditic +Platodaria are very simple—two pairs of strings of cells, the inner of +which (the ovaries, Fig. 239 _o_) produce ova, and the outer (the +spermaria, _s_) sperm-cells. These gonads are not yet independent +sexual glands, but sexually differentiated cell-groups in the medullary +substance, or, in other words, parts of the gut-wall. Their products, +the sex-cells, are conveyed out behind by two pairs of short canals; +the male opening ( _m_) lies just behind the female ( _f_). Most of the +Platodaria have not the muscular pharynx, which is very advanced in the +_Turbellaria_ and _Trematoda._ On the other hand, they have, as a rule, +before or behind the mouth, a bulbous sense-organ (auditory vesicle or +organ of equilibrium, _g_), and many of them have also a couple of +simple optic spots. The cell-pit of the ectoderm that lies underneath +is rather thick, and represents the first rudiment of a neural ganglion +(vertical brain or acroganglion). + + +Fig.239. Aphanostomum Langii (Haekel), a primitive worm of the +platodaria class, of the order of Cryptocoela or Acoela. Fig. +239—Aphanostomum Langii ( _Haeckel_), a primitive worm of the +platodaria class, of the order of _Cryptocoela_ or _Acoela._ This new +species of the genus Aphanostomum, named after Professor Arnold Lang of +Zurich, was found in September, 1899, at Ajaccio in Corsica (creeping +between fucoidea). It is one-twelfth of an inch long, one-twenty-fifth +of an inch broad, and violet in colour. _a_ mouth, _g_ auditory +vesicle, _e_ ectoderm, _i_ entoderm, _o_ ovaries, _a_ spermaries, _f_ +female aperture, _m_ male aperture. + + +The _Turbellaria,_ with which the similar _Platodaria_ were formerly +classed, differ materially from them in the more advanced structure of +their organs, and especially in having a central nervous system +(vertical brain) and excretory renal canals (nephridia); both originate +from the ectoderm. But between the two germinal layers a mesoderm is +developed, a soft mass of connective tissue, in which the organs are +embedded. The _Turbellaria_ are still represented by a number of +different forms, in both fresh and sea-water. The oldest of these are +the very rudimentary and tiny forms that are known as _Rhabdocœla_ on +account of the simple construction of their gut; they are, as a rule, +less than a quarter of an inch long and of a simple oval or lancet +shape (Fig. 240). The surface is covered with ciliated epithelium, a +stratum of ectodermic cells. The digestive gut is still the simple +primitive gut of the gastræa ( _d_), with a single aperture that is +both mouth and anus ( _m_). There is, however, an invagination of the +ectoderm at the mouth, which has given rise to a muscular pharynx ( +_sd_). It is noteworthy that the mouth of the Turbellaria (like the +primitive mouth of the Gastræa) may, in this class, change its position +considerably in the middle line of the ventral surface; sometimes it +lies behind ( _Opisthostomum_), sometimes in the middle ( +_Mesostomum_), sometimes in front ( _Prosostomum_). This displacement +of the mouth from front to rear is very interesting, because it +corresponds to a phylogenetic displacement of the mouth. This probably +occurred in the Platode ancestors of most (or all?) of the Cœlomaria; +in these the permanent mouth ( _metastoma_) lies at the fore end (oral +pole), whereas the primitive mouth ( _prostoma_) lay at the hind end of +the bilateral body. + +In most of the Turbellaria there is a narrow cavity, containing a +number of secondary organs, between the two primary germinal layers, +the outer or animal layer of which forms the epidermis and the inner +vegetal layer the visceral epithelium. The earliest of these organs are +the sexual organs; they are very variously constructed in the +Platode-class; in the simplest case there are merely two pairs of +gonads or sexual glands—a pair of testicles (Fig. 241 _h_) +and a pair of ovaries ( _e_). They open externally, sometimes by a +common aperture ( _Monogonopora_), sometimes by separate ones, the +female behind the male ( _Digonopora,_ Fig. 241). The sexual glands +develop originally from the two promesoblasts or primitive mesodermic +cells (Fig. 83 _p_). As these earliest mesodermic structures extended, +and became spacious sexual pouches in the later descendants of the +Platodes, probably the two cœlom-pouches were formed from them, the +first trace of the real body-cavity of the higher Metazoa ( +_Enterocœla_). + +The gonads are among the oldest organs, the few other organs that we +find in the Platodes between the gut-wall and body-wall being later +evolutionary products. One of the oldest and most important of these +are the kidneys or _nephridia,_ which remove unusable matter from the +body (Fig. 240 _nc_). These urinary or excretory organs were originally +enlarged skin-glands—a couple of canals that run the length of the +body, and have a separate or common external aperture ( _nm_). They +often have a number of branches. These special excretory organs are not +found in the other Cœlenteria (Gastræads, Sponges, Cnidaria) or the +Cryptocœla. They are first met in the _Turbellaria,_ and have been +transmitted direct from these to the Vermalia, and from these to the +higher stems. + + +Fig.240. A simple turbellarian (Rhabdocoelum). Fig. 241. The same, +showing the other organs. Fig. 240—A simple turbellarian ( +_Rhabdocœlum_). _m_ mouth, _sd_ gullet epithelium, _sm_ gullet muscles, +_d_ gastric gut, _nc_ renal canals, _nm_ renal aperture, _au_ eye, _na_ +olfactory pit. (Diagram.) + +Fig. 241—The same, showing the other organs. _g_ brain, _au_ eye, _na_ +olfactory pit, _n_ nerves, _h_ testicles, _ma_ male aperture, _fa_ +female aperture, _e_ ovary, _f_ ciliated epiderm. (Diagram.) + + +Finally, there is a very important new organ in the Turbellaria, which +we do not find in the _Cryptocœla_ (Fig. 239) and their gastræad +ancestors—the rudimentary nervous system. It consists of a couple of +simple cerebral ganglia (Fig. 241 _g_) and fine nervous fibres that +radiate from them; these are partly voluntary nerves (or motor fibres) +that go to the thin muscular layer developing under the skin; and +partly sensory nerves that proceed to the sense-cells of the ciliated +epiderm ( _f_). Many of the Turbellaria have also special sense-organs; +a couple of ciliated smell pits ( _na_), rudimentary eyes ( _au_), and, +less frequently, auditory vesicles. + +On these principles I assume that the oldest and simplest Turbellaria +arose from Platodaria, and these directly from bilateral Gastræads. The +chief advances were the formation of gonads and nephridia, and of the +rudimentary brain. On this hypothesis, which I advanced in 1872 in the +first sketch of the gastræa-theory ( _Monograph on the Sponges_), there +is no direct affinity between the Platodes and the Cnidaria. + +Next to the ancient stem-group of the Turbellaria come a number of more +recent chordonia ancestors, which we class with the _Vermalia_ or +_Helminthes,_ the unarticulated worms. These true worms ( _Vermes,_ +lately also called _Scolecida_) are the difficulty or the lumber-room +of the zoological classifier, because the various classes have very +complicated relations to the lower Platodes on the one hand and the +more advanced animals on the other. But if we exclude the Platodes and +the Annelids from this stem, we find a fairly satisfactory unity of +organisation +in the remaining classes. Among these worms we find some important +forms that show considerable advance in organisation from the platode +to the chordonia stage. Three of these phenomena are particularly +instructive: (1) The formation of a true (secondary) body-cavity +(cœloma); (2) the formation of a second aperture of the gut, the anus; +and (3) the formation of a vascular system. The great majority of the +Vermalia have these three features, and they are all wanting in the +Platodes; in the rest of the worms at least one or two of them are +developed. + + +Figs. 242 and 243. Chaetonotus, a rudimentary vermalian, of the group +of Gastrotricha. Figs. 242 and 243—Chætonotus, a rudimentary vermalian, +of the group of Gastrotricha. _m_ mouth, _s_ gullet, _d_ gut, _a_ anus, +_g_ brain, _n_ nerves, _ss_ sensory hairs, _au_ eye, _ms_ muscular +cells, _h_ skin, _f_ ciliated bands of the ventral surface, _nc_ +nephridia, _nm_ their aperture, _e_ ovaries. + + +Next and very close to the Platodes we have the Ichthydina ( +_Gastrotricha_), little marine and fresh-water worms, about 1/250 to +1/1000 inch long. Zoologists differ as to their position in +classification. In my opinion, they approach very close to the +Rhabdocœla (Figs. 240, 241), and differ from them chiefly in the +possession of an anus at the posterior end (Fig. 242 _a_). Further, the +cilia that cover the whole surface of the Turbellaria are confined in +the Gastrotricha to two ciliated bands ( _f_) on the ventral surface of +the oval body, the dorsal surface having bristles. Otherwise the +organisation of the two classes is the same. In both the gut consists +of a muscular gullet ( _s_) and a glandular primitive gut ( _d_). Over +the gullet is a double brain (acroganglion, _g_). At the side of the +gut are two serpentine prorenal canals (water-vessels or pronephridia, +_nc_), which open on the ventral side ( _nm_). Behind are a pair of +simple sexual glands or gonads (Fig. 243 _e_). + +While the Ichthydina are thus closely related to the Platodes, we have +to go farther away for the two classes of Vermalia which we unite in +the group of the “snout-worms” ( _Frontonia_). These are the +_Nemertina_ and the _Enteropneusta._ +Both classes have a complete ciliary coat on the epidermis, a heritage +from the Turbellaria and the Gastræads; also, both have two openings of +the gut, the mouth and anus, like the Gastrotricha. But we find also an +important organ that is wanting in the preceding forms—the vascular +system. In their more advanced mesoderm we find a few contractile +longitudinal canals which force the blood through the body by their +contractions; these are the first blood-vessels. + + +Fig.244. A simple Nemertine. Fig. 244—A simple Nemertine. _m_ mouth, +_d_ gut, _a_ anus, _g_ brain, _n_ nerves, _h_ ciliary coat, _ss_ +sensory pits (head-clefts), _au_ eyes, _r_ dorsal vessel, _l_ lateral +vessels. (Diagram.) + + +Fig. 245. A young Enteropneust. Fig. 245—A young Enteropneust ( +_Balanaglossus_). (From _Alexander Agassiz._) _r_ acorn-shaped snout, +_h_ neck, _k_ gill-clefts and gill-arches of the fore-gut, in long rows +on each side, _d_ digestive hind-gut, filling the greater part of the +body-cavity, _v_ intestinal vein or ventral vessel, lying between the +parallel folds of the skin, _a_ anus. + + +The Nemertina were formerly classed with the much less advanced +Turbellaria. But they differ essentially from them in having an anus +and blood-vessels, and several other marks of higher organisation. They +have generally long and narrow bodies, like a more or less flattened +cord; there are, besides several small species, giant-forms with a +width of 1/5 to 2/5 inch and a length of several yards (even ten to +fifteen). Most of them live in the sea, but some in fresh water and +moist earth. In their internal structure they approach the Turbellaria +on the one hand and the higher Vermalia (especially the Enteropneusta) +on the other. They have a good deal of interest as the lowest and +oldest of all animals with blood. In them we find blood-vessels for the +first time, distributing real blood through the body. The blood is red, +and the red colouring-matter is hæmoglobin, connected with elliptic +discoid blood-cells, as in the Vertebrates. Most of them have two or +three parallel blood-canals, which run the whole length of the body, +and are connected in front and behind by loops, and often by a number +of ring-shaped pieces. The chief of these primitive blood-vessels is +the one that lies above the gut in the middle line of the back (Fig. +244 _r_); it may be compared to either the dorsal vessel of the +Articulates or the aorta of the Vertebrates. To the right and left are +the two serpentine lateral vessels (Fig. 244 _l_). + + +Fig.246. Transverse section of the branchial gut. Fig. 246—Transverse +section of the branchial gut. _A_ of Balanoglossus, _B_ of Ascidia. _r_ +branchial gut, _n_ pharyngeal groove, * ventral folds between the two. +Diagrammatic illustration from _Gegenbaur,_ to show the relation of the +dorsal branchial-gut cavity ( _r_) to the pharyngeal or hypobranchial +groove ( _n_). + + +After the Nemertina, I take (as distant relatives) the _Enteropneusta_ +; they may be classed together with them as _Frontonia_ or _Rhyncocœla_ +(snout-worms). There is now only one genus of this class, with several +species ( _Balanoglossus_); but it is very remarkable, and may be +regarded as the last survivor of an ancient and long-extinct class of +Vermalia. They are related, on the one hand, to the Nemertina and their +immediate ancestors, the Platodes, and to the lowest and oldest forms +of the Chordonia on the other. + +The Enteropneusta (Fig. 245) live in the sea sand, and are long worms +of very simple shape, like the Nemertina. From the latter they have +inherited: (1) The bilateral type, with incomplete segmentation; (2) +the ciliary coat of the soft epidermis; (3) the double rows of gastric +pouches, alternating with a single or double row of gonads; (4) +separation of the sexes (the Platode ancestors were hermaphroditic); +(5) the ventral mouth, underneath a protruding snout; (6) the anus +terminating the simple gut-tube; and (7) several parallel blood-canals, +running the length of the body, a dorsal and a ventral principal stem. + +On the other hand, the Enteropneusta differ from their Nemertine +ancestors in several features, some of which are important, that we may +attribute to adaptation. The chief of these is the branchial gut (Fig. +245 _k_). The anterior section of the gut is converted into a +respiratory organ, and pierced by two rows of gill-clefts; between +these there is a branchial (gill) skeleton, formed of rods and plates +of chitine. The water that enters at the mouth makes its exit by these +clefts. They lie in the dorsal half of the fore-gut, and this is +completely separated from the ventral half by two longitudinal folds +(Fig. 246 _A*_). This ventral half, the glandular walls of which are +clothed with ciliary epithelium and secrete mucus, corresponds to the +pharyngeal or hypo-branchial groove of the Chordonia ( _Bn_), the +important organ from which the later thyroid gland is developed in the +Craniota (cf. p. 184). The agreement in the structure of the branchial +gut of the Enteropneusts, Tunicates, and Vertebrates was first +recognised by Gegenbaur (1878); it is the more significant as at first +we find only a couple of gill-clefts in the young animals of all three +groups; the number gradually increases. We can infer from this the +common descent of the three groups with all the more +confidence when we find the _Balanoglossus_ approaching the Chordonia +in other respects. Thus, for instance, the chief part of the central +nervous system is a long dorsal neural string that runs above the gut +and corresponds to the medullary tube of the Chordonia. Bateson +believes he has detected a rudimentary chorda between the two. + +Of all extant invertebrate animals the Enteropneusts come nearest to +the Chordonia in virtue of these peculiar characters; hence we may +regard them as the survivors of the ancient gut-breathing Vermalia from +which the Chordonia also have descended. Again, of all the +chorda-animals the Copelata (Fig. 225) and the tailed larvæ of the +ascidia approach nearest to the young _Balanoglossus._ Both are, on the +other hand, very closely related to the _Amphioxus,_ the Primitive +Vertebrate of which we have considered the importance (Chapters XVI and +XVII). As we saw there, the unarticulated Tunicates and the articulated +Vertebrates must be regarded as two independent stems, that have +developed in divergent directions. But the common root of the two +stems, the extinct group of the Prochordonia, must be sought in the +vermalia stem; and of all the living Vermalia those we have considered +give us the safest clue to their origin. It is true that the actual +representatives of the important groups of the Copelata, Balanoglossi, +Nemertina, Icthydina, etc., have more or less departed from the +primitive model owing to adaptation to special environment. But we may +just as confidently affirm that the main features of their organisation +have been preserved by heredity. + +We must grant, however, that in the whole stem-history of the +Vertebrates the long stretch from the Gastræads and Platodes up to the +oldest Chordonia remains by far the most obscure section. We might +frame another hypothesis to raise the difficulty—namely, that there was +a long series of very different and totally extinct forms between the +Gastræa and the Chordæa. Even in this modified chordæa-theory the six +fundamental organs of the chordula would retain their great value. The +medullary tube would be originally a chemical sensory organ, a dorsal +olfactory tube, taking in respiratory-water and food by the neuroporus +in front and conveying them by the neurenteric canal into the primitive +gut. This olfactory tube would afterwards become the nervous centre, +while the expanding gonads (lying to right and left of the primitive +mouth) would form the cœloma. The chorda may have been originally a +digestive glandular groove in the dorsal middle line of the primitive +gut. The two secondary gut-openings, mouth and anus, may have arisen in +various ways by change of functions. In any case, we should ascribe the +same high value to the chordula as we did before to the gastrula. + +In order to explain more fully the chief stages in the advance of our +race, I add the hypothetical sketch of man’s ancestry that I published +in my _Last Link_ [a translation by Dr. Gadow of the paper read at the +International Zoological Congress at Cambridge in 1898]:— + + +A.—Man’s Genealogical Tree, First Half: +EARLIER SERIES OF ANCESTORS, WITHOUT FOSSIL EVIDENCE. + + + Chief Stages Ancestral Stem-groups Living Relatives of + Ancestors Stages 1–5: +Protist ancestors Unicellular organisms. +1–2: + +Prototypes 3–5: Protozoa 1. Monera + +Without nucleus + 2. Algaria Unicellular algæ + 1. Chromacea _ (Chroococcus) Phycochromacea _ 2. Paulotomea _ + Palmellacea Eremosphæra _ 3. Lobosa Unicellular (amœbina) +rhizopods 4. Infusoria Unicellular + 5. Blastæades Multicellular hollow spheres 3. Amœbina _Amœba + Leucocyta_ + + +4. Flagellata _ Euflagellata Zoomonades _ 5. Catallacta _ Magosphæra, +Volvocina, Blastula _ Stages 6–11: +Invertebrate metazoa ancestors 6–8: +Cœlenteria without anus and body-cavity 9–11: +Vermalia, with anus and body-cavity 6. Gastræades With two +germ-layers + 7 Platodes I _Platodaria_ (without nephridia) 8. Platodes II + _Platodinia_ (with nephridia) 6. Gastrula _ Hydra, Olynthus, + Gastremaria _ 7. Cryptocœla _Convoluta, Porporus_ + 8. Rhabdocœla _Vortex, Monolus_ 9. Provermalia (Primitive worms) +_Rotatoria_ 10. Frontonia _(Rhynchelminthes)_ + Snout-worms 11. Prochordonia Chorda-worms + 9. Gastrotricha _Trochozoa, Trochophora_ + 10. Enteropneusta _ Balanglossus Cephalodiscus _ 11. Copelata +_Appendicaria_ Chordula-larvæ Stages 12–15: Monorhina + ancestors Oldest vertebrates without jaws or pairs of limbs, single + nose 12. Acrania I + (Prospondylia) 13. Acrania II More recent 14. Cyclostoma I + (Archicrania) 15. Cyclostoma II +More recent 12. Amphioxus larvæ + 13. Leptocardia Amphioxus 14. Petromyzonta larvæ + + 15. Marsipobranchia Petromyzonta + + + + +B.—Man’s Genealogical Tree, Second Half: +LATER ANCESTORS, WITH FOSSIL EVIDENCE. + + + Geological Periods Ancestral Stem-groups Living Relatives + of Ancestors Silurian 16. Selachii Primitive fishes _Proselachii_ + 16. Natidanides Chlamydoselachius Heptanchus Silurian 17. + Ganoids Plated-fishes _Proganoids_ 17. Accipenserides + (Sturgeons) Polypterus Devonian 18. Dipneusta _Paladipneusta_ + 18. Neodipneusta Ceratodus Proptopterus Carboniferous 19. + Amphibia _Stegocephala_ 19. Phanerobranchia Salamandrina +(Proteus, triton) Permian 20. Reptilia _Proreptilia_ 20. +Rhynchocephalia Primitive lizards +Hatteria Triassic 21. Monotrema _Promammalia_ 21. +Ornithodelphia _ Echidna +Ornithorhyncus _ Jurassic 22. Marsupialia _Prodidelphia_ 22. +Didelphia _ Didelphys Perameles _ Cretaceous 23. Mallotheria +_Prochoriata_ 23. Insectivora Erinaceida (Ictopsia +) Older Eocene + 24. Lemuravida Older lemurs Dentition 3. 1. 4. 3. 24. +Pachylemures _ (Hyopsodus +) + (Adapis +) _ Neo-Eocene 25. Lemurogona Later lemurs Dentition 2. 1. 4. + 3. 25. Autolemures _ Eulemur Stenops _ Oligocene 26. Dysmopitheca + Western apes Dentition 2. 1. 3. 3. 26. Platyrrhinæ _ (Anthropops + +) +(Homunculus +) _ Older Miocene 27. Cynopitheca Dog-faced apes +(tailed) 27. Papiomorpha _Cynocephalus_ Neo-Miocene 28. +Anthropoides Man-like apes (tail-less) 28. Hylobatida Hylobates +Satyrus Pliocene 29. Pithecanthropi Ape-men (alali, speechless) +29. Anthropitheca Chimpanzee +Gorilla Pleistocene 30. Homines Men with speech 30. Weddahs +Australian negroes + + + + +Chapter XXI. +OUR FISH-LIKE ANCESTORS + + +Our task of detecting the extinct ancestors of our race among the vast +numbers of animals known to us encounters very different difficulties +in the various sections of man’s stem-history. These were very great in +the series of our invertebrate ancestors; they are much slighter in the +subsequent series of our vertebrate ancestors. Within the vertebrate +stem there is, as we have already seen, so complete an agreement in +structure and embryology that it is impossible to doubt their +phylogenetic unity. In this case the evidence is much clearer and more +abundant. + +The characteristics that distinguish the Vertebrates as a whole from +the Invertebrates have already been discussed in our description of the +hypothetical Primitive Vertebrate (Chapter XI, Figs. 98–102). The chief +of these are: (1) The evolution of the primitive brain into a dorsal +medullary tube; (2) the formation of the chorda between the medullary +tube and the gut; (3) the division of the gut into branchial (gill) and +hepatic (liver) gut; and (4) the internal articulation or metamerism. +The first three features are shared by the Vertebrates with the +ascidia-larvæ and the Prochordonia; the fourth is peculiar to them. +Thus the chief advantage in organisation by which the earliest +Vertebrates took precedence of the unsegmented Chordonia consisted in +the development of internal segmentation. + +The whole vertebrate stem divides first into the two chief sections of +Acrania and Craniota. The Amphioxus is the only surviving +representative of the older and lower section, the Acrania +(“skull-less”). All the other vertebrates belong to the second +division, the Craniota (“skull-animals”). The Craniota descend directly +from the Acrania, and these from the primitive Chordonia. The +exhaustive study that we made of the comparative anatomy and ontogeny +of the Ascidia and the Amphioxus has proved these relations for us. +(See Chapters XVI and XVII.) The Amphioxus, the lowest Vertebrate, and +the Ascidia, the nearest related Invertebrate, descend from a common +extinct stem-form, the Chordæa; and this must have had, substantially, +the organisation of the chordula. + +However, the Amphioxus is important not merely because it fills the +deep gulf between the Invertebrates and Vertebrates, but also because +it shows us to-day the typical vertebrate in all its simplicity. We owe +to it the most important data that we proceed on in reconstructing the +gradual historical development of the whole stem. All the Craniota +descend from a common stem-form, and this was substantially identical +in structure with the Amphioxus. This stem-form, the Primitive +Vertebrate (_Prospondylus,_ Figs. 98–102), had the characteristics of +the vertebrate as such, but not the important features that distinguish +the Craniota from the Acrania. Though the Amphioxus has many +peculiarities of structure and has much degenerated, and though it +cannot be regarded as an unchanged descendant of the Primitive +Vertebrate, it must have inherited from it the specific characters we +enumerated above. We may not say that “Amphioxus is the ancestor of the +Vertebrates”; but we can say: “Amphioxus is the nearest relation to the +ancestor of all the animals we know.” Both belong to the same small +family, or lowest class of the Vertebrates, that we call the Acrania. +In our genealogical tree this group forms the twelfth stage, or the +first stage among the vertebrate ancestors (p. 228). From this group of +Acrania both the Amphioxus and the Craniota were evolved. + +The vast division of the Craniota embraces all the Vertebrates known to +us, with the exception of the Amphioxus. All of them have a head +clearly differentiated from the trunk, and a skull enclosing a brain. +The head has also three pairs of higher sense-organs (nose, eyes, and +ears). The brain is very rudimentary at first, a mere bulbous +enlargement of the +fore end of the medullary tube. But it is soon divided by a number of +transverse constrictions into, first three, then five successive +cerebral vesicles. In this formation of the head, skull, and brain, +with further development of the higher sense-organs, we have the +advance that the Craniota made beyond their skull-less ancestors. Other +organs also attained a higher development; they acquired a compact +centralised heart with valves and a more advanced liver and kidneys, +and made progress in other important respects. + + +Fig.247. The large marine lamprey (Petromyzon marinus). Fig. 247—The +large marine lamprey _(Petromyzon marinus),_ much reduced. Behind the +eye there is a row of seven gill-clefts visible on the left, in front +the round suctorial mouth. + + +We may divide the Craniota generally into _Cyclostoma_ +(“round-mouthed”) and _Gnathostoma_ (“jaw-mouthed”). There are only a +few groups of the former in existence now, but they are very +interesting, because in their whole structure they stand midway between +the Acrania and the Gnathostoma. They are much more advanced than the +Acrania, much less so than the fishes, and thus form a very welcome +connecting-link between the two groups. We may therefore consider them +a special intermediate group, the fourteenth and fifteenth stages in +the series of our ancestors. + +The few surviving species of the Cyclostoma are divided into two +orders—the _Myxinoides_ and the _Petromyzontes._ The former, the +hag-fishes, have a long, cylindrical, worm-like body. They were classed +by Linné with the worms, and by later zoologists, with the fishes, or +the amphibia, or the molluscs. They live in the sea, usually as +parasites of fishes, into the skin of which they bore with their round +suctorial mouths and their tongues, armed with horny teeth. They are +sometimes found alive in the body cavity of fishes (such as the torsk +or sturgeon); in these cases they have passed through the skin into the +interior. The second order consists of the Petromyzontes or lampreys; +the small river lamprey (_Petromyzon fluviatilis_) and the large marine +lamprey (_Petromyzon marinus,_ Fig. 247). They also have a round +suctorial mouth, with horny teeth inside it; by means of this they +attach themselves by sucking to fishes, stones, and other objects +(hence the name _Petromyzon_ = stone-sucker). It seems that this habit +was very widespread among the earlier Vertebrates; the larvæ of many of +the Ganoids and frogs have suctorial disks near the mouth. + +The class that is formed of the Myxinoides and Petromyzontes is called +the Cyclostoma (round-mouthed), because their mouth has a circular or +semi-circular aperture. The jaws (upper and lower) that we find in all +the higher Vertebrates are completely wanting in the Cyclostoma, as in +the Amphioxus. Hence the other Vertebrates are collectively opposed to +them as Gnathostoma (jaw-mouthed). The Cyclostoma might also be called +_Monorhina_ (single-nosed), because they have only a single nasal +passage, while all the Gnathostoma have two nostrils (_Amphirhina_ = +double-nosed). But apart from these peculiarities the Cyclostoma differ +more widely from the fishes in other special features of their +structure than the fishes do from man. Hence they are obviously the +last survivors of a very ancient class of Vertebrates, that was far +from attaining the advanced organisation of the true fish. To mention +only the chief points, the Cyclostoma show no trace of pairs of limbs. +Their mucous skin is quite naked and smooth and devoid of scales. There +is no bony skeleton. A very rudimentary skull is developed at the +foremost end of their chorda. At this point a soft membranous (partly +turning into cartilage), small skull-capsule is formed, and encloses +the brain. + + +Fig.248. Fossil Permian primitive fish (Pleuracanthus Dechenii), from +the red sandstone of Saarbrücken. Fig. 248—Fossil Permian primitive +fish _(Pleuracanthus Dechenii),_ from the red sandstone of Saarbrücken. +(From _Döderlein._) _I_ Skull and branchial skeleton: _o_ eye-region, +_pq_ palatoquadratum, _nd_ lower jaw, _hm_ hyomandibular, _hy_ +tongue-bone, _k_ gill-radii, _kb_ gill-arches, _z_ jaw-teeth, _sz_ +gullet-teeth, _st_ neck-spine. _II_ Vertebral column: _ob_ upper +arches, _ub_ lower arches, _hc_ intercentra, _r_ ribs. _III_ Single +fins: _d_ dorsal fin, _c_ tail-fin (tail-end wanting), _an_ anus-fin, +_ft_ supporter of fin-rays. _IV_ Breast-fin: _sg_ shoulder-zone, _ax_ +fin-axis, _ss_ double lines of fin-rays, _bs_ additional rays, _sch_ +plates. _V_ Ventral fin: _p_ pelvis, _ax_ fin-axis, _ss_ single row of +fin-rays, _bs_ additional rays, _sch_ scales, _cop_ penis. + + +The brain of the Cyclostoma is merely a very small and comparatively +insignificant swelling of the spinal marrow, a simple vesicle at first. +It afterwards divides into five successive cerebral vesicles, like the +brain of the Gnathostoma. These five primitive cerebral vesicles, that +are found in the embryos of all the higher vertebrates from the fishes +to man, and grow into very complex structures, remain at a very +rudimentary stage in the Cyclostoma. The histological structure of the +nerves is also less advanced than in the rest of the vertebrates. In +these the auscultory organ always contains three circular canals, but +in the lampreys there are only two, and in the hag-fishes only one. In +most other respects the organisation of the Cyclostoma is much +simpler—for instance, in the structure of the heart, circulation, and +kidneys. We must especially note the absence of a very important organ +that we find in the fishes, the floating-bladder, from which the lungs +of the higher Vertebrates have been developed. + +When we consider all these peculiarities in the structure of the +Cyclostoma, we may formulate the following thesis: Two divergent lines +proceeded from the earliest Craniota, or the primitive Craniota +(_Archicrania_). One of these lines is preserved in a greatly modified +condition: these are the Cyclostoma, a very backward and partly +degenerate side-line. The other, the chief line of the Vertebrate stem, +advanced straight to the fishes, and by fresh adaptations acquired a +number of important improvements. + +The Cyclostoma are almost always classified by zoologists among the +fishes; but the incorrectness of this may be judged from the fact that +in all the chief and distinctive features of organisation they are +further removed from the fishes than the fishes are from the Mammals, +and even man. With the fishes we enter upon the vast division of the +jaw-mouthed +or double-nosed Vertebrates (_Gnathostoma_ or _Amphirhina_). We have to +consider the fishes carefully as the class which, on the evidence of +palæontology, comparative anatomy, and ontogeny, may be regarded with +absolute certainty as the stem-class of all the higher Vertebrates or +Gnathostomes. Naturally, none of the actual fishes can be considered +the direct ancestor of the higher Vertebrates. But it is certain that +all the Vertebrates or Gnathostomes, from the fishes to man, descend +from a common, extinct, fish-like ancestor. If we had this ancient +stem-form before us, we would undoubtedly class it as a true fish. +Fortunately the comparative anatomy and classification of the fishes +are now so far advanced that we can get a very clear idea of these +interesting and instructive features. + + +Fig.249. Embryo of a shark (Scymnus lichia), seen from the ventral +side. Fig. 249—Embryo of a shark (_Scymnus lichia_), seen from the +ventral side. _v_ breast-fins (in front five pairs of gill-clefts), _h_ +belly-fins, _a_ anus, _s_ tail-fin, _k_ external gill-tuft, _d_ +yelk-sac (removed for most part), _g_ eye, _n_ nose, _m_ mouth-cleft. + + +In order to understand properly the genealogical tree of our race +within the vertebrate stem, it is important to bear in mind the +characteristics that separate the whole of the Gnathostomes from the +Cyclostomes and Craniota. In these respects the fishes agree entirely +with all the other Gnathostomes up to man, and it is on this that we +base our claim of relationship to the fishes. The following +characteristics of the Gnathostomes are anatomic features of this kind: +(1) The internal gill-arch apparatus with the jaw arches; (2) the pair +of nostrils; (3) the floating bladder or lungs; and (4) the two pairs +of limbs. + +The peculiar formation of the frame work of the branchial (gill) arches +and the connected maxillary (jaw) apparatus is of importance in the +whole group of the Gnathostomes. It is inherited in rudimentary form by +all of them, from the earliest fishes to man. It is true that the +primitive transformation (which we find even in the Ascidia) of the +fore gut into the branchial gut can be traced in all the Vertebrates to +the same simple type; in this respect the gill-clefts, which pierce the +walls of the branchial gut in all the Vertebrates and in the Ascidia, +are very characteristic. But the _external,_ superficial branchial +skeleton that supports the gill-crate in the Cyclostoma is replaced in +the Gnathostomes by an _internal_ branchial skeleton. It consists of a +number of successive cartilaginous arches, which lie in the wall of the +gullet between the gill-clefts, and run round the gullet from both +sides. The foremost pair of gill-arches become the maxillary arches, +from which we get our upper and lower jaws. + +The olfactory organs are at first found in the same form in all the +Gnathostomes, as a pair of depressions in the fore part of the skin of +the head, above the mouth; hence, they are also called the Amphirhina +(“double-nosed”). The Cyclostoma are “one-nosed” (_Monorhina_); their +nose is a single passage in the middle of the frontal surface. But as +the olfactory nerve is double in both cases, it is possible that the +peculiar form of the nose in the actual Cyclostomes is a secondary +acquisition (by adaptation to suctorial habits). + + +Fig.250. Fully-developed man-eating shark (Carcharias melanopterus), +left view. Fig. 250—Fully developed man-eating shark (_Carcharias +melanopterus_), left view. _r1_ first, _r2_ second dorsal fin, _s_ +tail-fin, _a_ anus-fin, _v_ breast-fins, _h_ belly-fins.) + + +A third essential character of the Gnathostomes, that distinguishes +them very conspicuously from the lower vertebrates we have dealt with, +is the formation of a blind sac by invagination from the fore part of +the gut, which becomes in the fishes the air-filled floating-bladder. +This organ acts as a hydrostatic apparatus, increasing or reducing the +specific gravity of the fish by compressing or altering the quantity of +air in it. The fish can rise or sink in the water by means of it. This +is the organ from which the lungs of the higher vertebrates are +developed. + +Finally, the fourth character of the Gnathostomes in their simple +embryonic form is the two pairs of extremities or limbs—a pair of fore +legs (breast-fins in the fish, Fig. 250 _v_) and a pair of hind legs +(ventral fins in the fish, Fig. 250 _h_). The comparative anatomy of +these fins is very interesting, because they contain the rudiments of +all the skeletal parts that form the framework of the fore and hind +legs in all the higher vertebrates right up to man. There is no trace +of these pairs of limbs in the Acrania and Cyclostomes. + +Turning, now, to a closer inspection of the fish class, we may first +divide it into three groups or sub-classes, the genealogy of which is +well known to us. The first and oldest group is the sub-class of the +_Selachii_ or primitive fishes; the best-known representatives of which +to-day are the orders of the sharks and rays (Figs. 248–252). Next to +this is the more advanced sub-class of the plated fishes or _Ganoids_ +(Figs. 253–5). It has been long extinct for the most part, and has very +few living representatives, such as the sturgeon and the bony pike; but +we can form some idea of the earlier extent of this interesting group +from the large numbers of fossils. From these plated fishes the +sub-class of the bony fishes +or _Teleostei_ was developed, to which the great majority of living +fishes belong (especially nearly all our river fishes). Comparative +anatomy and ontogeny show clearly that the Ganoids descended from the +Selachii, and the Teleostei from the Ganoids. On the other hand, a +collateral line, or rather the advancing chief line of the vertebrate +stem, was developed from the earlier Ganoids, and this leads us through +the group of the Dipneusta to the important division of the Amphibia. + + +Fig.251. Fossil angel-shark (Squatina alifera) from the upper Jurassic +at Eichstätt. Fig. 251—Fossil angel-shark (_Squatina alifera_), from +the upper Jurassic at Eichstätt. (From _Zittel._) The cartilaginous +skull is clearly seen in the broad head, and the gill-arches behind. +The wide breast-fin and the narrower belly-fin have a number of radii; +between these and the vertebral column are a number of ribs. + + +The earliest fossil remains of Vertebrates that we know were found in +the Upper Silurian (p. 201), and belong to two groups—the Selachii and +the Ganoids. The most primitive of all known representatives of the +earliest fishes are probably the remarkable _Pleuracanthida,_ the +genera _Pleuracanthus, Xenacanthus, Orthocanthus,_ etc. (Fig. 248). +These ancient cartilaginous fishes agree in most points of structure +with the real sharks (Figs. 249, 250); but in other respects they seem +to be so much simpler in organisation that many palæontologists +separate them altogether, and regard them as _Proselachii_; they are +probably closely related to the extinct ancestors of the Gnathostomes. +We find well-preserved remains of them in the Permian period. +Well-preserved impressions of other sharks are found in the Jurassic +schist, such as of the angel-fish (_Squatina,_ Fig. 251). Among the +extinct earlier sharks of the Tertiary period there were some twice as +large as the biggest living fishes; _Carcharodon_ was more than 100 +feet long. The sole surviving species of this genus (_C. Rondeleti_) is +eleven yards long, and has teeth two inches long; but among the fossil +species we find teeth six inches long (Fig. 252). + +From the primitive fishes or Selachii, the earliest Gnathostomes, was +developed the legion of the Ganoids. There are very few genera now of +this interesting and varied group—the ancient sturgeons (_Accipenser_), +the eggs of which are eaten as caviare, and the stratified pikes +(_Polypterus,_ Fig. 255) in African rivers, and bony pikes +(_Lepidosteus_) in the rivers of North America. On the other hand, we +have a great variety of specimens of this group in the fossil state, +from the Upper Silurian onward. Some of these fossil Ganoids approach +closely to the Selachii; others are nearer to the Dipneusts; others +again represent a transition to the Teleostei. For our genealogical +purposes the most interesting are the intermediate forms between the +Selachii and the Dipneusts. Huxley, to whom we owe particularly +important works on the fossil Ganoids, classed them in the order of the +_Crossopterygii._ Many genera and species of this order are found in +the Devonian and Carboniferous strata (Fig. 253); a single, greatly +modified survivor of the group is still found in the large rivers of +Africa (_Polypterus,_ Fig. 255, and the closely related +_Calamichthys_). In many impressions of the Crossopterygii the floating +bladder seems to be ossified, +and therefore well preserved—for instance, in the _Undina_ (Fig. 254, +immediately behind the head). + +Part of these Crossopterygii approach very closely in their chief +anatomic features to the Dipneusts, and thus represent phylogenetically +the transition from the Devonian Ganoids to the earliest air-breathing +vertebrates. This important advance was made in the Devonian period. +The numerous fossils that we have from the first two geological +sections, the Laurentian and Cambrian periods, belong exclusively to +aquatic plants and animals. From this paleontological fact, in +conjunction with important geological and biological indications, we +may infer with some confidence that there were no terrestrial animals +at that time. During the whole of the vast archeozoic period—many +millions of years—the living population of our planet consisted almost +exclusively of aquatic organisms; this is a very remarkable fact, when +we remember that this period embraces the larger half of the whole +history of life. The lower animal-stems are wholly (or with very few +exceptions) aquatic. But the higher stems also remained in the water +during the primordial epoch. It was only towards its close that some of +them came to live on land. We find isolated fossil remains of +terrestrial animals first in the Upper Silurian, and in larger numbers +in the Devonian strata, which were deposited at the beginning of the +second chief section of geology (the paleozoic age). The number +increases considerably in the Carboniferous and Permian deposits. We +find many species both of the articulate and the vertebrate stem that +lived on land and breathed the atmosphere; their aquatic ancestors of +the Silurian period only breathed water. This important change in +respiration is the chief modification that the animal organism +underwent in passing from the water to the solid land. The first +consequence was the formation of lungs for breathing air; up to that +time the gills alone had served for respiration. But there was at the +same time a great change in the circulation and its organs; these are +always very closely correlated to the respiratory organs. Moreover, the +limbs and other organs were also more or less modified, either in +consequence of remote correlation to the preceding or owing to new +adaptations. + + +Fig.252. Tooth of a gigantic shark (Carcharodon megalodon), from the +Pliocene at Malta. Fig. 252—Tooth of a gigantic shark (_Carcharodon +megalodon_), from the Pliocene at Malta. (From _Zittel._) + + +In the vertebrate stem it was unquestionably a branch of the fishes—in +fact, of the Ganoids—that made the first fortunate experiment during +the Devonian period of adapting themselves to terrestrial life and +breathing the atmosphere. This led to a modification of the heart and +the nose. The true fishes have merely a pair of blind olfactory pits on +the surface of the head; but a connection of these with the cavity of +the mouth was now formed. A canal made its appearance on each side, and +led directly from the nasal depression into the mouth-cavity, thus +conveying atmospheric air to the lungs even when the mouth was closed. +Further, in all true fishes the heart has only two sections—an atrium +that receives the venous blood from the veins, and a ventricle that +propels it through a conical artery to the gills; the atrium was now +divided into two halves, or right and left auricles, by an incomplete +partition. The right auricle alone now received the venous blood from +the body, while the left auricle received the venous blood that flowed +from the lungs and gills to the heart. Thus the double circulation of +the higher vertebrates was evolved from the simple +circulation of the true fishes, and, in accordance with the laws of +correlation, this advance led to others in the structure of other +organs. + + +Fig.253. A Devonian Crossopterygius (Holoptychius nobilissimus), from +the Scotch old red sandstone. Fig. 254. A Jurassic Crossopterygius +(Undina penicillata), from the upper Jurassic at Eichstätt. Fig. 255. A +living Crossopterygius, from the Upper Nile. Fig. 253—A Devonian +Crossopterygius (_Holoptychius nobilissimus_), from the Scotch old red +sandstone. (From _Huxley._) Fig. 254.—A Jurassic Crossopterygius +(_Undina penicillata_), from the upper Jurassic at Eichstätt. (From +_Zittel._) _j_ jugular plates, _b_ three ribbed scales. +Fig. 255—A living Crossopterygius, from the Upper Nile ((Polypterus +bichir). + + +The vertebrate class, that thus adapted itself to breathing the +atmosphere, and was developed from a branch of the Ganoids, takes the +name of the _Dipneusts_ or _Dipnoa_ (“double-breathers”), because they +retained the earlier gill-respiration along with the new pulmonary +(lung) respiration, like the lowest amphibia. This class was +represented during the paleozoic age (or the Devonian, Carboniferous, +and Permian periods) by a number of different genera. There are only +three genera of the class living to-day: _Protopterus annectens_ in the +rivers +of tropical Africa (the White Nile, the Niger, Quelliman, etc.), +_Lepidosiren paradoxa_ in tropical South America (in the tributaries of +the Amazon), and _Ceratodus Forsteri_ in the rivers of East Australia. +This wide distribution of the three isolated survivors proves that they +represent a group that was formerly very large. In their whole +structure they form a transition from the fishes to the amphibia. The +transitional formation between the two classes is so pronounced in the +whole organisation of these remarkable animals that zoologists had a +lively controversy over the question whether they were really fishes or +amphibia. Several distinguished zoologists classed them with the +amphibia, though most now associate them with the fishes. As a matter +of fact, the characters of the two classes are so far united in the +Dipneusts that the answer to the question depends entirely on the +definition we give of “fish” and “amphibian.” In habits they are true +amphibia. During the tropical winter, in the rainy season, they swim in +the water like the fishes, and breathe water by gills. During the dry +season they bury themselves in the dry mud, and breathe the atmosphere +through lungs, like the amphibia and the higher vertebrates. In this +double respiration they resemble the lower amphibia, and have the same +characteristic formation of the heart; in this they are much superior +to the fishes. But in most other features +they approach nearer to the fishes, and are inferior to the amphibia. +Externally they are entirely fish-like. + + +Fig.256. Fossil Dipneust (Dipterus Valenciennesi), from the old red +sandstone (Devon). Fig. 257. The Australian Dipneust (Ceratodus +Forsteri). Fig. 256—Fossil Dipneust (_Dipterus Valenciennesi_), from +the old red sandstone (Devon). (From _Pander._) Fig. 257—The Australian +Dipneust (_Ceratodus Forsteri_). _B_ view from the right, _A_ lower +side of the skull, _C_ lower jaw. (From _Gunther._) _Qu_ quadrate bone, +_Psph_ parasphenoid, _Pt P_ pterygopalatinum, _Vo_ vomer, _d_ teeth, +_na_ nostrils, _Br_ branchial cavity, _C_ first rib. _D_ lower-jaw +teeth of the fossil _Ceratodus Kaupi_ (from the Triassic). + + +In the Dipneusts the head is not marked off from the trunk. The skin is +covered with large scales. The skeleton is soft, cartilaginous, and at +a low stage of development, as in the lower Selachii and the earliest +Ganoids. The chorda is completely retained, and surrounded by an +unsegmented sheath. The two pairs of limbs are very simple fins of a +primitive type, like those of the lowest Selachii. The formation of the +brain, the gut, and the sexual organs is also the same as in the +Selachii. Thus the Dipneusts have preserved by heredity many of the +less advanced features of our primitive fish-like ancestors, and at the +same time have made a great step forward in adaptation to air-breathing +by means of lungs and the correlative improvement of the heart. + + +Fig.258. Young ceratodus, shortly after issuing from the egg. Fig. 259. +Young ceratodus six weeks after issuing from the egg. Fig. 258—Young +ceratodus, shortly after issuing from the egg, magnified. _k_ +gill-cover, _l_ liver. (From _Richard Semon._) Fig. 259—Young ceratodus +six weeks after issuing from the egg. _s_ spiral fold of gut, _b_ +rudimentary belly-fin. (From _Richard Semon._) + + +Ceratodus is particularly interesting on account of the primitive build +of its skeleton; the cartilaginous skeleton of its two pairs of fins, +for instance, has still the original form of a bi-serial or feathered +leaf, and was on that account described by Gegenbaur as a “primitive +fin-skeleton.” On the other hand, the skeleton of the pairs of fins is +greatly reduced in the African dipneust (_Protopterus_) and the +American (_Lepidosiren_). Further, the lungs are double in these modern +dipneusts, as in all the other air-breathing vertebrates; they have on +that account been called “double-lunged” (_Dipneumones_) in contrast to +the Ceratodus; the latter has only a single lung (_Monopneumones_). At +the same time the gills also are developed as water-breathing organs in +all these lung-fishes. Protopterus has external as well as internal +gills. + +The paleozoic Dipneusts that are in the direct line of our ancestry, +and form the connecting-bridge between the Ganoids and the Amphibia, +differ in many respects +from their living descendants, but agree with them in the above +essential features. This is confirmed by a number of interesting facts +that have lately come to our knowledge in connection with the embryonic +development of the Ceratodus and Lepidosiren; they give us important +information as to the stem-history of the lower Vertebrates, and +therefore of our early ancestors of the paleozoic age. + + + + +Chapter XXII. +OUR FIVE-TOED ANCESTORS + + +With the phylogenetic study of the four higher classes of Vertebrates, +which must now engage our attention, we reach much firmer ground and +more light in the construction of our genealogy than we have, perhaps, +enjoyed up to the present. In the first place, we owe a number of very +valuable data to the very interesting class of Vertebrates that come +next to the Dipneusts and have been developed from them—the Amphibia. +To this group belong the salamander, the frog, and the toad. In earlier +days all the reptiles were, on the example of Linne, classed with the +Amphibia (lizards, serpents, crocodiles, and tortoises). But the +reptiles are much more advanced than the Amphibia, and are nearer to +the birds in the chief points of their structure. The true Amphibia are +nearer to the Dipneusta and the fishes; they are also much older than +the reptiles. There were plenty of highly-developed (and sometimes +large) Amphibia during the Carboniferous period; but the earliest +reptiles are only found in the Permian period. It is probable that the +Amphibia were evolved even earlier—during the Devonian period—from the +Dipneusta. The extinct Amphibia of which we have fossil remains from +that remote period (very numerous especially in the Triassic strata) +were distinguished for a graceful scaly coat or a powerful bony armour +on the skin (like the crocodile), whereas the living amphibia have +usually a smooth and slippery skin. + +The earliest of these armoured Amphibia (_Phractamphibia_) form the +order of _Stegocephala_ (“roof-headed”) (Fig. 260). It is among these, +and not among the actual Amphibia, that we must look for the forms that +are directly related to the genealogy of our race, and are the +ancestors of the three higher classes of Vertebrates. But even the +existing Amphibia have such important relations to us in their anatomic +structure, and especially their embryonic development, that we may say: +Between the Dipneusts and the Amniotes there was a series of extinct +intermediate forms which we should certainly class with the Amphibia if +we had them before us. In their whole organisation even the actual +Amphibia seem to be an instructive transitional group. In the important +respects of respiration and circulation they approach very closely to +the Dipneusta, though in other respects they are far superior to them. + +This is particularly true of the development of their limbs or +extremities. In them we find these for the first time as five-toed +feet. The thorough investigations of Gegenbaur have shown that the +fish’s fins, of which very erroneous opinions were formerly held, are +many-toed feet. The various cartilaginous or bony radii that are found +in large numbers in each fin correspond to the fingers or toes of the +higher Vertebrates. The several joints of each fin-radius correspond to +the various parts of the toe. Even in the Dipneusta the fin is of the +same construction as in the fishes; it was afterwards gradually evolved +into the five-toed form, which we first encounter in the Amphibia. This +reduction of the number of the toes to six, and then to five, probably +took place in the second half of the Devonian period—at the latest, in +the subsequent Carboniferous period—in those Dipneusta which we regard +as the ancestors of the Amphibia. We have several fossil remains of +five-toed Amphibia from this period. There are numbers of fossil +impressions of them in the Triassic of Thuringia (_Chirotherium_). + + +The fact that the toes number five is of great importance, because they +have clearly been transmitted from the Amphibia to all the higher +Vertebrates. Man entirely resembles his amphibian ancestors in this +respect, and indeed in the whole structure of the bony skeleton of his +five-toed extremities. A careful comparison of the skeleton of the frog +with our own is enough to show this. It is well known that this +hereditary number of the toes has assumed a very great practical +importance from remote times; on it our whole system of enumeration +(the decimal system applied to measurement of time, mass, weight, etc.) +is based. There is absolutely no reason why there should be five toes +in the fore and hind feet in the lowest Amphibia, the reptiles, and the +higher Vertebrates, unless we +ascribe it to inheritance from a common stem-form. Heredity alone can +explain it. It is true that we find less than five toes in many of the +Amphibia and of the higher Vertebrates. But in all these cases we can +prove that some of the toes atrophied, and were in time lost +altogether. + + +Fig.260. Fossil amphibian from the Permian, found in the Plauen terrain +near Dresden (Branchiosaurus amblystomus). Fig. 260—Fossil amphibian +from the Permian, found in the Plauen terrain near Dresden +(_Branchiosaurus amblystomus_). (From _Credner._) _A_ skeleton of a +young larva. _B_ larva, restored, with gills. _C_ the adult form.) + + +The causes of this evolution of the five-toed foot from the many-toed +fin in the amphibian ancestor must be sought in adaptation to the +entire change of function that the limbs experienced in passing from an +exclusively aquatic to a partly terrestrial life. The many-toed fin had +been used almost solely for motion in the water; it had now also to +support the body in creeping on the solid ground. This led to a +modification both of the skeleton and the muscles of the limbs. The +number of the fin-radii was gradually reduced, and sank finally to +five. But these five remaining radii became much stronger. The soft +cartilaginous radii became bony rods. The rest of the skeleton was +similarly strengthened. Thus from the one-armed lever of the many-toed +fish-fin arose the improved many-armed lever system of the five-toed +amphibian limbs. The movements of the body gained in variety as well as +in strength. The various parts of the skeletal system and correlated +muscular system began to differentiate more and more. In view of the +close correlation of the muscular and nervous systems, this also made +great advance in structure and function. Hence we find, as a matter of +fact, that the brain is much more developed in the higher Amphibia than +in the fishes, the Dipneusta, and the lower Amphibia. + + +Fig.261. Larva of the Spotted Salamander (Salamandra maculata), seen +from the ventral side. Fig. 261—Larva of the Spotted Salamander +(_Salamandra maculata_), seen from the ventral side. In the centre a +yelk-sac still hangs from the gut. The external gills are gracefully +ramified. The two pairs of legs are still very small. + + +The first advance in organisation that was occasioned by the adoption +of life on land was naturally the construction of an organ for +breathing air—a lung. This was formed directly from the +floating-bladder inherited from the fishes. At first its function was +insignificant beside that of the gills, the older organ for +water-respiration. Hence we find in the lowest Amphibia, the gilled +Amphibia, that, like the Dipneusta, they pass the greater part of their +life in the water, and breathe water through gills. They only come to +the surface at brief intervals, or creep on to the land, and then +breathe air by their lungs. But some of the tailed Amphibia—the +salamanders—remain entirely in the water when they are young, and +afterwards spend most of their time on land. In the adult state they +only breathe air through lungs. The same applies to the most advanced +of the Amphibia, the Batrachia (frogs and toads); some of them have +entirely lost the gill-bearing larva form.[30] This is also the case +with certain small, serpentine Amphibia, the Cæcilia (which live in the +ground like earth-worms). + + [30] The tree-frog of Martinique (_Hylades martinicensis_) loses the + gills on the seventh, and the tail and yelk-sac on the eighth, day of + fœtal life. On the ninth or tenth day after fecundation the frog + emerges from the egg. + + +The great interest of the natural history of the Amphibia consists +especially in their intermediate position between the lower and higher +Vertebrates. The lower Amphibia approach very closely to the Dipneusta +in their whole organisation, live mainly in the water, and breathe by +gills; but the higher Amphibia are just as close to the Amniotes, live +mainly on land, and breathe by lungs. But in their younger state the +latter resemble the former, and only reach the higher stage by a +complete metamorphosis. The embryonic development of most of the +higher Amphibia still faithfully reproduces the stem-history of the +whole class, and the various stages of the advance that was made by the +lower Vertebrates in passing from aquatic to terrestrial life during +the Devonian or the Carboniferous period are repeated in the spring by +every frog that develops from an egg in our ponds. + + +Fig.262. Larva of the common grass-frog (Rana temporaria), or +“tadpole.” Fig. 262—Larva of the common grass-frog (_Rana temporaria_), +or “tadpole.” _m_ mouth, _n_ a pair of suckers for fastening on to +stones, _d_ skin-fold from which the gill-cover develops; behind it the +gill-clefts, from which the branching gills (_k_) protrude, _s_ +tail-muscles, _f_ cutaneous fin-fringe of the tail. + + +The common frog leaves the egg in the shape of a larva, like the tailed +salamander (Fig. 261), and this is altogether different from the mature +frog (Fig. 262). The short trunk ends in a long tail, with the form and +structure of a fish’s tail (_s_). There are no limbs at first. The +respiration is exclusively branchial, first through external (_k_) and +then internal gills. In harmony with this the heart has the same +structure as in the fish, and consists of two sections—an atrium that +receives the venous blood from the body, and a ventricle that forces it +through the arteries into the gills. + +We find the larvæ of the frog (or tadpoles, _Gyrini_) in great numbers +in our ponds every spring in this fish-form, using their muscular tails +in swimming, just like the fishes and young Ascidia. When they have +reached a certain size, the remarkable metamorphosis from the fish-form +to the frog begins. A blind sac grows out of the gullet, and expands +into a couple of spacious sacs: these are the lungs. The simple chamber +of the heart is divided into two sections by the development of a +partition, and there are at the same time considerable changes in the +structure of the chief arteries. Previously all the blood went from the +auricle through the aortic arches into the gills, but now only part of +it goes to the gills, the other part passing to the lungs through the +new-formed pulmonary artery. From this point arterial blood returns to +the left auricle of the heart, while the venous blood gathers in the +right auricle. As both auricles open into a single ventricle, this +contains mixed blood. The dipneust form has now succeeded to the +fish-form. In the further course of the metamorphosis the gills and the +branchial vessels entirely disappear, and the respiration becomes +exclusively pulmonary. Later, the long swimming tail is lost, and the +frog now hops to the land with the legs that have grown meantime. + +This remarkable metamorphosis of the Amphibia is very instructive in +connection with our human genealogy, and is particularly interesting +from the fact that the various groups of actual Amphibia have remained +at different stages of their stem-history, in harmony with the +biogenetic law. We have first of all a very low order of Amphibia—the +_Sozobranchia_ (“gilled-amphibia”), which retain their gills throughout +life, like the fishes. In a second order of the salamanders the gills +are lost in the metamorphosis, and when fully grown they have only +pulmonary respiration. Some of the tailed Amphibia still retain the +gill-clefts in the side of the neck, though they have lost the gills +themselves (_Menopoma_). If we force the larvæ of our salamanders (Fig. +261) and tritons to remain in the water, and prevent them from reaching +the land, we can in favourable circumstances make them retain their +gills. In this fish-like condition they reach sexual maturity, and +remain throughout life at the lower stage of the gilled Amphibia. + + +fish-like axolotl (_Siredon pisciformis_). It was formerly regarded as +a permanent gilled amphibian persisting throughout life at the +fish-stage. But some of the hundreds of these animals that are kept in +the Botanical Garden at Paris got on to the land for some reason or +other, lost their gills, and changed into a form closely resembling the +salamander (_Amblystoma_). Other species of the genus became sexually +mature for the first time in this condition. This has been regarded as +an astounding phenomenon, although every common frog and salamander +repeats the metamorphosis in the spring. The whole change from the +aquatic and gill-breathing animal to the terrestrial lung-breathing +form may be followed step by step in this case. But what we see here in +the development of the individual has happened to the whole class in +the course of its stem-history. + + +Fig.263. Fossil mailed amphibian, from the Bohemian Carboniferous +(Seeleya). Fig. 263—Fossil mailed amphibian, from the Bohemian +Carboniferous (_Seeleya_). (From _Fritsch._) The scaly coat is retained +on the left. + + +The metamorphosis goes farther in a third order of Amphibia, the +_Batrachia_ or _Anura,_ than in the salamander. To this belong the +various kinds of toads, ringed snakes, water-frogs, tree-frogs, etc. +These lose, not only the gills, but also (sooner or later) the tail, +during metamorphosis. + +The ontogenetic loss of the gills and the tail in the frog and toad can +only be explained on the assumption that they are descended from +long-tailed Amphibia of the salamander type. This is also clear from +the comparative anatomy of the two groups. This remarkable +metamorphosis is, however, also interesting because it throws a certain +light on the phylogeny of the tail-less apes and man. Their ancestors +also had long tails and gills like the gilled Amphibia, as the tail and +the gill-arches of the human embryo clearly show. + +For comparative anatomical and ontogenetic reasons, we must not seek +these amphibian ancestors of ours—as one would be inclined to do, +perhaps—among the tail-less Batrachia, but among the tailed lower +Amphibia. + +The vertebrate form that comes next to the Amphibia in the series of +our ancestors is a lizard-like animal, the earlier existence of which +can be confidently deduced from the facts of comparative anatomy and +ontogeny. The living _Hatteria_ of New Zealand (Fig. 264) and the +extinct _Rhyncocephala_ of the Permian period (Fig. 265) are closely +related to this important stem-form; we may call them the +_Protamniotes,_ or Primitive Amniotes. All the Vertebrates above the +Amphibia—or the three classes of reptiles, birds, and mammals—differ so +much in their whole organisation from all the lower Vertebrates we have +yet considered, and have so great a resemblance to each other, that we +put them all together in a single group with the title of _Amniotes._ +In these three classes alone we find the remarkable embryonic membrane, +already mentioned, which we called the _amnion_; a cenogenetic +adaptation that we may regard as a result of the sinking of the growing +embryo into the yelk-sac. + +All the Amniotes known to us—all reptiles, birds, and mammals +(including man)—agree in so many important points of internal structure +and development that their descent from a common ancestor can be +affirmed with tolerable certainty. If the evidence of comparative +anatomy and ontogeny is ever entirely beyond suspicion, it is certainly +the case here. All the peculiarities that accompany and follow the +formation of the amnion, and that we have learned in our consideration +of human embryology; all the peculiarities in the development of the +organs which we will presently follow in detail; finally, all the +principal special features of the internal structure of the full-grown +Amniotes—prove so clearly the common origin of all the Amniotes from +single extinct stem-form that it is difficult to entertain the idea of +their evolution from several independent stems. This unknown common +stem-form is our primitive Amniote (_Protamnion_). In outward +appearance it was probably something between the salamander and the +lizard. + +It is very probable that some part of the Permian period was the age of +the origin of the Protamniotes. This follows from the fact that the +Amphibia are not fully developed until the Carboniferous period, and +that the first fossil reptiles (_Palæhatteria, Homœosaurus, +Proterosaurus_) are found towards the close of the Permian period. +Among the important changes of the vertebrate organisation that marked +the rise of the first Amniotes from salamandrine Amphibia during this +period the following three are especially noteworthy: the entire +disappearance of the water-breathing gills and the conversion of the +gill-arches into other organs, the formation of the allantois or +primitive urinary sac, and the development of the amnion. + +One of the most salient characteristics of the Amniotes is the complete +loss of the gills. All Amniotes, even if living in water (such as +sea-serpents and whales), breathe air through lungs, never water +through gills. All the Amphibia (with very rare exceptions) retain +their gills for some time when young, and have for a time (if not +permanently) branchial respiration; but after these there is no +question of branchial respiration. The Protamniote itself must have +entirely abandoned water-breathing. Nevertheless, the gill-arches are +preserved by heredity, and develop into totally different (in part +rudimentary) organs—various parts of the bone of the tongue, the frame +of the jaws, the organ of hearing, etc. But we do not find in the +embryos of the Amniotes any trace of gill-leaves, or of real +respiratory organs on the gill-arches. + +With this complete abandonment of the gills is probably connected the +formation of another organ, to which we have already referred in +embryology—namely, the allantois or primitive urinary sac (cf. p. 166). +It is very probable that the urinary bladder of the Dipneusts is the +first structure of the allantois. We find in these a urinary bladder +that proceeds from the lower wall of the hind end of the gut, and +serves as receptacle for the renal secretions. This organ has been +transmitted to the Amphibia, as we can see in the frog. + +The formation of the amnion and the allantois and the complete +disappearance of the gills are the chief characteristics that +distinguish the Amniotes from the lower Vertebrates we have hitherto +considered. To these we may add several subordinate features that are +transmitted to all the Amniotes, and are found in these only. One +striking embryonic character of the Amniotes is the great curve of the +head and neck in the embryo. We also find an advance in the structure +of several of the internal organs of the Amniotes which raises them +above the highest of the anamnia. In particular, a partition is formed +in the simple ventricle of the heart, dividing into right and left +chambers. In connection with the complete metamorphosis of the +gill-arches we find a further development of the auscultory organs. +Also, there is a great advance in the structure of the brain, skeleton, +muscular system, and other parts. Finally, one of the most important +changes is the reconstruction of the kidneys. In all the earlier +Vertebrates we have found the primitive kidneys as excretory organs, +and these appear at an early stage in the embryos of all the higher +Vertebrates up to man. But in the Amniotes these primitive kidneys +cease to act at an early stage of embryonic life, and their function is +taken up by the permanent or secondary kidneys, which develop from the +terminal section of the prorenal ducts. + +Taking all these peculiarities of the Amniotes together, it is +impossible to doubt that all the animals of this group—all reptiles, +birds, and mammals—have a common origin, and form a single +blood-related stem. Our own race belongs to this stem. Man is, in every +feature of his organisation and embryonic development, a true Amniote, +and has descended from the Protamniote with all the other +Amniotes. Though they appeared at the end (possibly even in the middle) +of the Paleozoic age, the Amniotes only reached their full development +during the Mesozoic age. The birds and mammals made their first +appearance during this period. Even the reptiles show their greatest +growth at this time, so that it is called “the reptile age.” The +extinct Protamniote, the ancestor of the whole group, belongs in its +whole organisation to the reptile class. + + +Fig.264. The lizard (Hatteria punctata = Sphenodon punctatus) of New +Zealand. Fig. 264—The lizard (_Hatteria punctata = Sphenodon +punctatus_) of New Zealand. The sole surviving proreptile. (From +_Brehm._) + + +The genealogical tree of the amniote group is clearly indicated in its +chief lines by their paleontology, comparative anatomy, and ontogeny. +The group succeeding the Protamniote divided into two branches. The +branch that will claim our whole interest is the class of the Mammals. +The other branch, which developed in a totally different direction, and +only comes in contact with the Mammals at its root, is the combined +group of the reptiles and birds; these two classes may, with Huxley, be +conveniently grouped together as the _Sauropsida._ Their common +stem-form is an extinct lizard-like reptile of the order of the +Rhyncocephalia. From this have been developed in various directions the +serpents, crocodiles, tortoises, etc.—in a word, all the members of the +reptile class. But the remarkable class of the birds has also been +evolved directly from a branch of the reptile group, as is now +established beyond question. The embryos of the reptiles and birds are +identical until a very late stage, and have an astonishing resemblance +even later. Their whole structure agrees so much that no anatomist now +questions the descent of the birds from the reptiles. On the other +hand, the mammal line has descended from the group of the +Sauromammalia, a different branch of the Proreptilia. It is connected +at its deepest roots with the reptile line, but it then diverges +completely from it and follows a distinctive development. Man is the +highest outcome of this class, the “crown of creation.” The hypothesis +that the three higher Vertebrate classes represent a single +Amniote-stem, and that the common root of this stem is to be found in +the amphibian class, is now generally admitted. + + +Fig.265. Homoeosaurus pulchellus, a Jurassic proreptile from Kehlheim. +Fig. 265—Homœosaurus pulchellus, a Jurassic proreptile from Kehlheim. +(From _Zittel._) + + +The instructive group of the Permian Tocosauria, the common root from +which the divergent stems of the Sauropsids and mammals have issued, +merits our particular attention as the stem-group of all the Amniotes. +Fortunately a living representative of this extinct ancestral group has +been preserved to our day; this is the remarkable lizard of New +Zealand, _Hatteria punctata_ (Fig. 264). Externally it differs little +from the ordinary lizard; but in many important points of internal +structure, especially in the primitive construction of the vertebral +column, the skull, and the limbs, it occupies a much lower position, +and approaches its amphibian ancestors, the Stegocephala. Hence +Hatteria is the phylogenetically oldest of all living reptiles, an +isolated survivor from the Permian period, closely resembling the +common ancestor of the Amniotes. It must differ so little from this +extinct form, our hypothetical Protamniote, that we put it next to the +Proreptilia. The remarkable Permian _Palæhatteria,_ that Credner +discovered in the Plauen terrain at Dresden in 1888, belongs to the +same group (Fig. 266). The Jurassic genus _Homœosaurus_ (Fig. 265), of +which well-preserved skeletons are found in the Solenhofen schists, is +perhaps still more closely related to them. + +Unfortunately, the numerous fossil remains of Permian and Triassic +Tocosauria that we have found in the last two decades are, for the most +part, very imperfectly preserved. Very often we can make only +precarious inferences from these skeletal fragments as to the anatomic +characters of the soft parts that went with the bony skeleton of the +extinct Tocosauria. Hence it has not yet been possible to arrange these +important fossils with any confidence in the ancestral series that +descend from the Protamniotes to the Sauropsids on the one side and the +Mammals on the other. Opinions are particularly divided as to the place +in classification and the phylogenetic significance of the remarkable +_Theromorpha._ Cope gives this name to a very interesting and extensive +group of extinct terrestrial reptiles, of which we have only fossil +remains from the Permian and Triassic strata. Forty years ago some of +these Therosauria (fresh-water animals) were described by Owen as +_Anomodontia._ But during the last twenty years the distinguished +American paleontologists, Cope and Osborn, have greatly increased our +knowledge of them, and have claimed that the stem-forms of the Mammals +must be sought in this order. As a matter of fact, the Theromorpha are +nearer to the Mammals in the chief points of structure than any other +reptiles. This is especially true of the Thereodontia, to which the +_Pureosauria_ and _Pelycosauria_ belong (Fig. 267). The whole structure +of their pelvis and hind-feet has attained the same form as in the +Monotremes, the lowest Mammals. The formation of the +scapula and the quadrate bone shows an approach to the Mammals such as +we find in no other group of reptiles. The teeth also are already +divided into incisors, canines, and molars. Nevertheless, it is very +doubtful whether the Theromorpha really are in the ancestral line of +the Sauromammals, or lead direct from the Tocosauria to the earliest +Mammals. Other experts on this group believe that it is an independent +legion of the reptiles, connected, perhaps, at its lowest root, with +the Sauromammals, but developed quite independently of the +Mammals—though parallel to them in many ways. + +One of the most important of the zoological facts that we rely on in +our investigation of the genealogy of the human race is the position of +man in the Mammal class. However different the views of zoologists may +have been as to this position in detail, and as to his relations to the +apes, no scientist has ever doubted that man is a true mammal in his +whole organisation and development. Linné drew attention to this fact +in the first edition of his famous _Systema Naturæ_ (1735). As will be +seen in any museum of anatomy or any manual of comparative anatomy; the +human frame has all the characteristics that are common to the Mammals +and distinguish them conspicuously from all other animals. + + +Fig.266. Skull of a Permian lizard (Palaehatteria longicaudata). Fig. +266—Skull of a Permian lizard (_Palæhatteria longicaudata_). (From +_Credner._) _n_ nasal bone, _pf_ frontal bone, _l_ lachrymal bone, _po_ +postorbital bone, _sq_ covering bone, _i_ cheek-bone, _vo_ vomer, _im_ +inter-maxillary. + + +If we examine this undoubted fact from the point of view of phylogeny, +in the light of the theory of descent, it follows at once that man is +of a common stem with all the other Mammals, and comes from the same +root as they. But the various features in which the Mammals agree and +by which they are distinguished are of such a character as to make a +polyphyletic hypothesis quite inadmissible. It is impossible to +entertain the idea that all the living and extinct Mammals come from a +number of separate roots. If we accept the general theory of evolution, +we are bound to admit the monophyletic hypothesis of the descent of all +the Mammals (including man) from a single mammalian stem-form. We may +call this long-extinct root-form and its earliest descendants (a few +genera of one family) “primitive mammals” or “stem-mammals” +(_Promammalia_). As we have already seen, this root-form developed from +the primitive Proreptile stem in a totally different direction from the +birds, and soon separated from the main stem of the reptiles. The +differences between the Mammals and the reptiles and birds are so +important and characteristic that we can assume with complete +confidence this division of the vertebrate stem at the commencement of +the development of the Amniotes. The reptiles and birds, which we group +together as the _Sauropsids,_ generally agree in the characteristic +structure of the skull and brain, and this is notably different from +that of the Mammals. In most of the reptiles and birds the skull is +connected with the first cervical vertebra (the _atlas_) by a single, +and in the Mammals (and Amphibia) by a double, condyle at the back of +the head. In the former the lower jaw is composed of several pieces, +and connected with the skull so that it can move by a special maxillary +bone (the _quadratum_); in the Mammals the lower jaw consists of one +pair of bony pieces, which articulate directly with the temporal bone. +Further, in the Sauropsids the skin is clothed with scales or feathers; +in the Mammals with hair. The red blood-cells of the former have a +nucleus; those of the latter have not. In fine, two quite +characteristic features of the Mammals, which distinguish them not only +from the birds and reptiles, but from all other animals, are the +possession of a +complete diaphragm and of mammary glands that produce the milk for the +nutrition of the young. It is only in the Mammals that the diaphragm +forms a transverse partition of the body-cavity, completely separating +the pectoral from the abdominal cavity. It is only in the mammals that +the mother suckles its young, and this rightly gives the name to the +whole class (_mamma_ = breast). + + +Fig.267. Skull of a Triassic theromorphum (Galesaurus planiceps), from +the Karoo formation in South Africa. Fig. 267—Skull of a Triassic +theromorphum (_Galesaurus planiceps_), from the Karoo formation in +South Africa. (From _Owen._) a from the right, _b_ from below, _c_ from +above, _d_ tricuspid tooth. _N_ nostrils, _Na_ nasal bone, _Mx_ upper +jaw, _Prf_ prefrontal, _Fr_ frontal bone, _A_ eye-pits, _S_ +temple-pits. _Pa_ Parietal eye, _Bo_ joint at back of head, _Pt_ +pterygoid-bone, _Md_ lower jaw. + + +From these pregnant facts of comparative anatomy and ontogeny it +follows absolutely that the whole of the Mammals belong to a single +natural stem, which branched off at an early date from the +reptile-root. It follows further with the same absolute certainty that +the human race is also a branch of this stem. Man shares all the +characteristics I have described with all the Mammals, and differs in +them from all other animals. Finally, from these facts we deduce with +the same confidence those advances in the vertebrate organisation by +which one branch of the Sauromammals was converted into the stem-form +of the Mammals. Of these advances the chief were: (1) The +characteristic modification of the skull and the brain; (2) the +development of a hairy coat; (3) the complete formation of the +diaphragm; and (4) the construction of the mammary glands and +adaptation to suckling. Other important changes of structure proceeded +step by step with these. + +The epoch at which these important advances were made, and the +foundation of the Mammal class was laid, may be put with great +probability in the first section of the Mesozoic or secondary age—the +Triassic period. The oldest fossil remains of mammals that we know were +found in strata that belong to the earliest Triassic period—the upper +Kueper. One of the earliest forms is the genus _Dromatherium,_ from the +North American Triassic (Fig. 268). Their teeth still strikingly recall +those of the Pelycosauria. Hence we may assume that this small and +probably insectivorous mammal belonged to the stem-group of the +Promammals. We do not find any positive trace of the third and most +advanced division of the Mammals—the Placentals. These (including man) +are much younger, and we do not find indisputable fossil remains of +them until the Cenozoic age, or the Tertiary period. This +paleontological fact is very important, because it fully harmonises +with the evolutionary succession of the Mammal orders that is deduced +from their comparative anatomy and ontogeny. + +The latter science teaches us that the whole Mammal class divides into +three main groups or sub-classes, which correspond to three successive +phylogenetic stages. These three stages, which also represent three +important stages in our human genealogy, were first distinguished in +1816 by the eminent French zoologist, Blainville, and received the +names of _Ornithodelphia, Didelphia,_ and _Monodelphia,_ according to +the construction of the female organs (_delphys_ = uterus or womb). +Huxley afterwards gave them the names of _Prototheria, Metatheria,_ and +_Epitheria._ But the three sub-classes differ so widely from each +other, not only in the construction of the sexual organs, but in many +other respects also, that we may confidently draw up the following +important phylogenetic thesis: The Monodelphia or Placentals descend +from the Didelphia or Marsupials; and the latter, in turn, are +descended from the Monotremes or Ornithodelphia. + +Thus we must regard as the twenty-first stage in our genealogical tree +the earliest and lowest chief group of the Mammals—the sub-class of the +Monotremes (“cloaca-animals,” Ornithodelphia, or Prototheria, Figs. 269 +and 270). They take their name from the cloaca which they share with +all the lower Vertebrates. This cloaca is the common outlet for the +passage of the excrements, the urine, and the sexual products. The +urinary ducts and sexual canals open into the hindmost part of the gut, +while in all the other Mammals they are separated from the rectum and +anus. The latter have a special uro-genital outlet (_porus +urogenitalis_). The bladder also opens into the cloaca in the +Monotremes, and, indeed, apart from the two urinary ducts; in all the +other Mammals the latter open directly into the bladder. It was proved +by Haacke and Caldwell in 1884 that the Monotremes lay large eggs like +the reptiles, while all the other Mammals are viviparous. In 1894 +Richard Semon further proved that these large eggs, rich in food-yelk, +have a partial segmentation and discoid gastrulation, as I had +hypothetically assumed in 1879; here again they resemble their +reptilian ancestors. The construction of the mammary gland is also +peculiar in the Monotremes. In them the glands have no teats for the +young animal to suck, but there is a special part of the breast pierced +with holes like a sieve, from which the milk issues, and the young +Monotreme must lick it off. Further, the brain of the Monotremes is +very little advanced. It is feebler than that of any of the other +Mammals. The fore-brain or cerebrum, in particular, is so small that it +does not cover the cerebellum. In the skeleton (Fig. 270) the formation +of the scapula among other parts is curious; it is quite different from +that of the other Mammals, and rather agrees with that of the reptiles +and Amphibia. Like these, the Monotremes have a strongly developed +_caracoideum._ From these and other less prominent characteristics it +follows absolutely that the Monotremes occupy the lowest place among +the Mammals, and represent a transitional group between the Tocosauria +and the rest of the Mammals. All these remarkable reptilian characters +must have been possessed by the stem-form of the whole mammal class, +the Promammal of the Triassic period, and have been inherited from the +Proreptiles. + + +Fig.268. Lower jaw of a Primitive Mammal or Promammal (Dromatherium +silvestre) from the North American Triassic. Fig. 268—Lower jaw of a +Primitive Mammal or Promammal (_Dromatherium silvestre_) from the North +American Triassic. _i_ incisors, _c_ canine, _p_ premolars, _m_ molars. +(From _Döderlein._) + + +During the Triassic and Jurassic periods the sub-class of the +Monotremes was represented by a number of different stem-mammals. +Numerous fossil remains of them have lately been discovered in the +Mesozoic strata of Europe, Africa, and America. To-day there are only +two surviving specimens of the group, which we place together in the +family of the duck-bills, _Ornithostoma._ They are confined to +Australia and the neighbouring island of Van Diemen’s Land (or +Tasmania); they become scarcer every year, and will soon, like their +blood-relatives, be counted among the extinct animals. One form lives +in the rivers, and builds subterraneous dwellings on the banks; this is +the _Ornithorhyncus paradoxus,_ with webbed feet, a thick soft fur, and +broad flat jaws, which look very much like the bill of a duck (Figs. +269, 270). The other form, the land duck-bill, or spiny ant-eater +(_Echidna hystrix_), is very much like the anteaters in its habits and +the peculiar construction of its thin snout and very long tongue; it is +covered with needles, and can roll itself up like a hedgehog. A cognate +form (_Parechidna Bruyni_) has lately been found in New Guinea. + +These modern Ornithostoma are the scattered survivors of the vast +Mesozoic group of Monotremes; hence they have the same interest in +connection with the stem history of the Mammals as the living +stem-reptiles (_Hatteria_) for that of the reptiles, and the isolated +Acrania (_Amphioxus_) for the phylogeny of the Vertebrate stem. + +The Australian duck-bills are distinguished externally by a toothless +bird-like +beak or snout. This absence of real bony teeth is a late result of +adaptation, as in the toothless Placentals (_Edentata,_ armadillos and +ant-eaters). The extinct Monotremes, to which the Promammalia belonged, +must have had developed teeth, inherited from the reptiles. Lately +small rudiments of real molars have been discovered in the young of the +Ornithorhyncus, which has horny plates in the jaws instead of real +teeth. + + +Fig. 269. The Ornithorhyncus or Duck-mole. (Ornithorhyncus paradoxus). +Fig. 269—The Ornithorhyncus or Duck-mole. (_Ornithorhyncus paradoxus_). + + +Fig. 270. Skeleton of the Ornithorhyncus. Fig. 270—Skeleton of the +Ornithorhyncus. + + +The living Ornithostoma and the stem-forms of the Marsupials (or +_Didelphia_) must be regarded as two widely diverging lines from the +Promammals. This second sub-class of the Mammals is very interesting as +a perfect intermediate stage between the other two. While the +Marsupials retain a great part of the characteristics of the +Monotremes, they have also acquired some of the chief features of the +Placentals. Some features +are also peculiar to the Marsupials, such as the construction of the +male and female sexual organs and the form of the lower jaw. The +Marsupials are distinguished by a peculiar hook-like bony process that +bends from the corner of the lower jaw and points inwards. As most of +the Placentals have not this process, we can, with some probability, +recognise the Marsupial from this feature alone. Most of the mammal +remains that we have from the Jurassic and Cretaceous deposits are +merely lower jaws, and most of the jaws found in the Jurassic deposits +at Stonesfield and Purbeck have the peculiar hook-like process that +characterises the lower jaw of the Marsupial. On the strength of this +paleontological fact, we may suppose that they belonged to Marsupials. +Placentals do not seem to have existed at the middle of the Mesozoic +age—not until towards its close (in the Cretaceous period). At all +events, we have no fossil remains of indubitable Placentals from that +period. + +The existing Marsupials, of which the plant-eating kangaroo and the +carnivorous opossum (Fig. 272) are the best known, differ a good deal +in structure, shape, and size, and correspond in many respects to the +various orders of Placentals. Most of them live in Australia, and a +small part of the Australian and East Malayan islands. There is now not +a single living Marsupial on the mainland of Europe, Asia, or Africa. +It was very different during the Mesozoic and even during the Cenozoic +age. The sedimentary deposits of these periods contain a great number +and variety of marsupial remains, sometimes of a colossal size, in +various parts of the earth, and even in Europe. We may infer from this +that the existing Marsupials are the remnant of an extensive earlier +group that was distributed all over the earth. It had to give way in +the struggle for life to the more powerful Placentals during the +Tertiary period. The survivors of the group were able to keep alive in +Australia and South America because the one was completely separated +from the other parts of the earth during the whole of the Tertiary +period, and the other during the greater part of it. + + +Fig.271. Lower jaw of a Promammal (Dryolestes priscus), from the +Jurassic of the Felsen strata. Fig. 271—Lower jaw of a Promammal +(_Dryolestes priscus_), from the Jurassic of the Felsen strata. (From +_Marsh._) + + +From the comparative anatomy and ontogeny of the existing Marsupials we +may draw very interesting conclusions as to their intermediate position +between the earlier Monotremes and the later Placentals. The defective +development of the brain (especially the cerebrum), the possession of +marsupial bones, and the simple construction of the allantois (without +any placenta as yet) were inherited by the Marsupials, with many other +features, from the Monotremes, and preserved. On the other hand, they +have lost the independent bone (_caracoideum_) at the shoulder-blade. +But we have a more important advance in the disappearance of the +cloaca; the rectum and anus are separated by a partition from the +uro-genital opening (_sinus urogenitalis_). Moreover, all the +Marsupials have teats on the mammary glands, at which the new-born +animal sucks. The teats pass into the cavity of a pouch or pocket on +the ventral side of the mother, and this is supported by a couple of +marsupial bones. The young are born in a very imperfect condition, and +carried by the mother for some time longer in her pouch, until they are +fully developed (Fig. 272). In the giant kangaroo, which is as tall as +a man, the embryo only develops for a month in the uterus, is then born +in a very imperfect state, and finishes its growth in the mother’s +pouch (_marsupium_); it remains in this about nine months, and at first +hangs continually on to the teat of the mammary gland. + +From these and other characteristics (especially the peculiar +construction of the internal and external sexual organs in male and +female) it is clear that we must conceive the whole sub-class of the +Marsupials as one stem group, which has been developed from the +Promammalia. From one branch of these Marsupials (possibly from more +than one) the stem-forms of the higher Mammals, the Placentals, were +afterwards evolved. Of the existing forms of the Marsupials, +which have undergone various modifications through adaptation to +different environments, the family of the opossums (_Didelphida_ or +_Pedimana_) seems to be the oldest and nearest to the common stem-form +of the whole class. To this family belong the crab-eating opossum of +Brazil (Fig. 272) and the opossum of Virginia, on the embryology of +which Selenka has given us a valuable work (cf. Figs. 63–67 and +131–135). These Didelphida climb trees like the apes, grasping the +branches with their hand-shaped hind feet. We may conclude from this +that the stem-forms of the Primates, which we must regard as the +earliest Lemurs, were evolved directly from the opossum. We must not +forget, however, that the conversion of the five-toed foot into a +prehensile hand is polyphyletic. By the same adaptation to climbing +trees the habit of grasping their branches with the feet has in many +different cases brought about that opposition of the thumb or great toe +to the other toes which makes the hand prehensile. We see this in the +climbing lizards (chameleon), the birds, and the tree-dwelling mammals +of various orders. + + +Fig.272. The crab-eating Opossum (Philander cancrivorus). The female +has three young in the pouch. Fig. 272—The crab-eating Opossum +(_Philander cancrivorus_). The female has three young in the pouch. +(From _Brehm._) + + +Some zoologists have lately advanced the opposite opinion, that the +Marsupials represent a completely independent +sub-class of the Mammals, with no direct relation to the Placentals, +and developing independently of them from the Monotremes. But this +opinion is untenable if we examine carefully the whole organisation of +the three sub-classes, and do not lay the chief stress on incidental +features and secondary adaptations (such as the formation of the +marsupium). It is then clear that the Marsupials—viviparous Mammals +without placenta—are a necessary transition from the oviparous +Monotremes to the higher Placentals with chorion-villi. In this sense +the Marsupial class certainly contains some of man’s ancestors. + + + + +Chapter XXIII. +OUR APE ANCESTORS + + +The long series of animal forms which we must regard as the ancestors +of our race has been confined within narrower and narrower circles as +our phylogenetic inquiry has progressed. The great majority of known +animals do not fall in the line of our ancestry, and even within the +vertebrate stem only a small number are found to do so. In the most +advanced class of the stem, the mammals, there are only a few families +that belong directly to our genealogical tree. The most important of +these are the apes and their predecessors, the half-apes, and the +earliest Placentals (_Prochoriata_). + +The Placentals (also called _Choriata, Monodelphia, Eutheria_ or +_Epitheria_) are distinguished from the lower mammals we have just +considered, the Monotremes and Marsupials, by a number of striking +peculiarities. Man has all these distinctive features; that is a very +significant fact. We may, on the ground of the most careful +comparative-anatomical and ontogenetic research, formulate the thesis: +“Man is in every respect a true Placental.” He has all the +characteristics of structure and development that distinguish the +Placentals from the two lower divisions of the mammals, and, in fact, +from all other animals. Among these characteristics we must especially +notice the more advanced development of the brain. The fore-brain or +cerebrum especially is much more developed in them than in the lower +animals. The _corpus callosum,_ which forms a sort of wide bridge +connecting the two hemispheres of the cerebrum, is only fully formed in +the Placentals; it is very rudimentary in the Marsupials and +Monotremes. It is true that the lowest Placentals are not far removed +from the Marsupials in cerebral development; but within the placental +group we can trace an unbroken gradation of progressive development of +the brain, rising gradually from this lowest stage up to the elaborate +psychic organ of the apes and man. The human soul—a physiological +function of the brain—is in reality only a more advanced ape-soul. + +The mammary glands of the Placentals are provided with teats like those +of the Marsupials; but we never find in the Placentals the pouch in +which the latter carry and suckle their young. Nor have they the +marsupial bones in the ventral wall at the anterior border of the +pelvis, which the Marsupials have in common with the Monotremes, and +which are formed by a partial ossification of the sinews of the inner +oblique abdominal muscle. There are merely a few insignificant remnants +of them in some of the Carnivora. The Placentals are also generally +without the hook-shaped process at the angle of the lower jaw which is +found in the Marsupials. + +However, the feature that characterises the Placentals above all +others, and that has given its name to the whole sub-class, is the +formation of the placenta. We have already considered the formation and +significance of this remarkable embryonic organ when we traced the +development of the chorion and the allantois in the human embryo +(pp.165–9) The urinary sac or the allantois, the +curious vesicle that grows out of the hind part of the gut, has +essentially the same structure and function in the human embryo as in +that of all the other Amniotes (cf. Figs. 194–6). There is a quite +secondary difference, on which great stress has wrongly been laid, in +the fact that in man and the higher apes the original cavity of the +allantois quickly degenerates, and the rudiment of it sticks out as a +solid projection from the primitive gut. The thin wall of the allantois +consists of the same two layers or membranes as the wall of the gut—the +gut-gland layer within and the gut-fibre layer without. In the +gut-fibre layer of the allantois there are large blood-vessels, which +serve for the nutrition, and especially the respiration, of the +embryo—the umbilical vessels (p. 170).In the reptiles and birds the +allantois enlarges into a spacious sac, which encloses the embryo with +the amnion, and does not combine with the outer fœtal membrane (the +chorion). This is the case also with the lowest mammals, the oviparous +Monotremes and most of the Marsupials. It is only in some of the later +Marsupials (_Peramelida_) and all the Placentals that the allantois +develops into the distinctive and remarkable structure that we call the +_placenta._ + + +Fig.273. Foetal membranes of the human embryo (diagrammatic). Fig. +273—Fœtal membranes of the human embryo (diagrammatic). _m_ the thick +muscular wall of the womb. _plu_ placenta [the inner layer (_plu_′) of +which penetrates into the chorion-villi (_chz_) with its processes]. +_chf_ tufted, _chl_ smooth chorion. _a_ amnion, _ah_ amniotic cavity, +_as_ amniotic sheath of the umbilical cord (which passes under into the +navel of the embryo—not given here), _dg_ vitelline duct, _ds_ yelk +sac, _dv, dr_ decidua (vera and reflexa). The uterine cavity (_uh_) +opens below into the vagina and above on the right into an oviduct +(_t_). (From _Kölliker._) + + +The placenta is formed by the branches of the blood-vessels in the wall +of the allantois growing into the hollow ectodermic tufts (villi) of +the chorion, which run into corresponding depressions in the mucous +membrane of the womb. The latter also is richly permeated with +blood-vessels which bring the mother’s blood to the embryo. As the +partition in the villi between the maternal blood-vessels and those of +the fœtus is extremely thin, there is a direct exchange of fluid +between the two, and this is of the greatest importance in the +nutrition of the young mammal. It is true that the maternal vessels do +not entirely pass into the fœtal vessels, so that the two kinds of +blood are simply mixed. But the partition between them is so thin that +the nutritive fluid easily transudes through it. By means of this +transudation or diosmosis the exchange of fluids takes place without +difficulty. The larger the embryo is in the placentals, and the longer +it remains in the womb, the more necessary it is to have special +structures to meet its great consumption of food. + +In this respect there is a very conspicuous difference between the +lower and higher mammals. In the Marsupials, in which the embryo is +only a comparatively short time in the womb and is born in a very +immature condition, the vascular arrangements in the yelk-sac and the +allantois suffice for its nutrition, as we find them in the Monotremes, +birds, and reptiles. But in the Placentals, where gestation lasts a +long time, and the embryo reaches its full development under the +protection of its enveloping membranes, there has to be a new mechanism +for the direct supply of a large quantity of food, and this is +admirably met by the formation of the placenta. + +Branches of the blood-vessels penetrate into the chorion-villi from +within, starting from the gut-fibre layer of the allantois, and +bringing the blood of the fœtus through the umbilical vessels (Fig. 273 +_chz_). On the other hand, a thick network of blood-vessels develops in +the mucous membrane that clothes the inner surface of the womb, +especially in the region of the depressions into which the +chorion-villi penetrate (_plu_). This network of arteries contains +maternal blood, brought by the uterine vessels. As the connective +tissue between the enlarged capillaries of +the uterus disappears, wide cavities filled with maternal blood appear, +and into these the chorion-villi of the embryo penetrate. The sum of +these vessels of both kinds, that are so intimately correlated at this +point, together with the connective and enveloping tissue, is the +_placenta._ The placenta consists, therefore, properly speaking, of two +different though intimately connected parts—the fœtal placenta (Fig. +273 _chz_) within and the maternal or uterine placenta (_plu_) without. +The latter is made up of the mucous coat of the uterus and its +blood-vessels, the former of the tufted chorion and the umbilical +vessels of the embryo (cf. Fig. 196). + + +Fig.274. Skull of a fossil lemur (Adapis parisiensis,), from the +Miocene at Quercy. Fig. 274—Skull of a fossil lemur (_Adapis +parisiensis_), from the Miocene at Quercy. _A_ lateral view from the +right. _B_ lower jaw, _C_ lower molar, _i_ incisors, _c_ canines, _p_ +premolars, _m_ molars. + + +The manner in which these two kinds of vessels combine in the placenta, +and the structure, form, and size of it, differ a good deal in the +various Placentals; to some extent they give us valuable data for the +natural classification, and therefore the phylogeny, of the whole of +this sub-class. On the ground of these differences we divide it into +two principal sections; the lower Placentals or _Indecidua,_ and the +higher Placentals or _ Deciduata._ + +To the Indecidua belong three important groups of mammals: the Lemurs +(_Prosimiæ_), the Ungulates (tapirs, horses, pigs, ruminants, etc.), +and the Cetacea (dolphins and whales). In these Indecidua the villi are +distributed over the whole surface of the chorion (or its greater part) +either singly or in groups. They are only loosely connected with the +mucous coat of the uterus, so that the whole fœtal membrane with its +villi can be easily withdrawn from the uterine depressions like a hand +from a glove. There is no real coalescence of the two placentas at any +part of the surface of contact. Hence at birth the fœtal placenta alone +comes away; the uterine placenta is not torn away with it. + +The formation of the placenta is very different in the second and +higher section of the Placentals, the _Deciduata._ Here again the whole +surface of the chorion is thickly covered with the villi in the +beginning. But they afterwards disappear from one part of the surface, +and grow proportionately thicker on the other part. We thus get a +differentiation between the smooth chorion (_chorion laeve,_ Fig. 273 +_chl_) and the thickly-tufted chorion (_chorion frondosum,_ Fig. 273 +_chf_). The former has only a few small villi or none at all; the +latter is thickly covered with large and well-developed villi; this +alone now constitutes the placenta. In the great majority of the +Deciduata the placenta has the same shape as in man >(Figs. 197 and +200)—namely a thick, circular disk like a cake; so we find in the +Insectivora, Chiroptera, Rodents, and Apes. This _discoplacenta_ lies +on one side of the chorion. But in the Sarcotheria (both the Carnivora +and the seals, _Pinnipedia_) and in the elephant and several other +Deciduates we find a _zonoplacenta_; in these the rich mass of villi +runs like a girdle round the middle of the ellipsoid chorion, the two +poles of it being free from them. + +Still more characteristic of the Deciduates is the peculiar and very +intimate connection between the _chorion frondosum_ and the +corresponding part of the mucous coat of the womb, which we must regard +as a real coalescence of the two. The villi of the chorion push their +branches into the blood-filled tissues of the coat of the uterus, and +the vessels of each loop together so intimately that it is no longer +possible to separate the fœtal +from the maternal placenta; they form henceforth a compact and +apparently simple placenta. In consequence of this coalescence, a whole +piece of the lining of the womb comes away at birth with the fœtal +membrane that is interlaced with it. This piece is called the +“falling-away” membrane (_decidua_). It is also called the serous +(spongy) membrane, because it is pierced like a sieve or sponge. All +the higher Placentals that have this decidua are classed together as +the “Deciduates.” The tearing away of the decidua at birth naturally +causes the mother to lose a quantity of blood, which does not happen in +the Indecidua. The last part of the uterine coat has to be repaired by +a new growth after birth in the Deciduates. (Cf. Figs. 199, 200, pp. +168–70.) + + +Fig.275. The Slender Lori (Stenops gracilis) of Ceylon, a tail-less +lemur. Fig. 275—The Slender Lori (_Stenops gracilis_) of Ceylon, a +tail-less lemur. + + +In the various orders of the Deciduates, the placenta differs +considerably both in outer form and internal structure. The extensive +investigations of the last ten years have shown that there is more +variation in these respects among the higher mammals than was formerly +supposed. The physiological work of this important embryonic organ, the +nutrition of the fœtus during its long sojourn in the womb, is +accomplished in the various groups of the Placentals by very different +and sometimes very elaborate structures. They have lately been fully +described by Hans Strahl. + +The phylogeny of the placenta has become more intelligible from the +fact that we have found a number of transitional forms of it. Some of +the Marsupials (_Perameles_) have the beginning of a placenta. In some +of the Lemurs (_Tarsius_) a discoid placenta with decidua is developed. + +While these important results of comparative embryology have been +throwing further light on the close blood-relationship of man and the +anthropoid apes in the last few years (p. 172), the great advance of +paleontology has at the same time been affording us a deeper insight +into the stem-history of the Placental group. In the seventh chapter of +my _Systematic Phylogeny of the Vertebrates_ I advanced the hypothesis +that the Placentals form a single stem with many branches, which has +been evolved from an older group of the Marsupials (_Prodidelphia_). +The four great legions of the Placentals—Rodents, Ungulates, Carnassia, +and Primates—are sharply separated to-day by important features of +organisation. But if we consider their extinct ancestors of the +Tertiary period, the differences gradually disappear, the deeper we go +in the Cenozoic deposits; in the end we find that they vanish +altogether. +The primitive stem-forms of the Rodents (_Esthonychida_), the Ungulates +(_Chondylarthra_), the Carnassia (_Ictopsida_), and the Primates +(_Lemuravida_) are so closely related at the beginning of the Tertiary +period that we might group them together as different families of one +order, the Proplacentals (_Mallotheria_ or _Prochoriata_). + +Hence the great majority of the Placentals have no direct and close +relationship to man, but only the legion of the _ Primates._ This is +now generally divided into three orders—the half-apes (_Prosimiæ_), +apes (_Simiæ_), and man (_Anthropi_). The lemurs or half-apes are the +stem-group, descending from the older _ Mallotheria_ of the Cretaceous +period. From them the apes were evolved in the Tertiary period, and man +was formed from these towards its close. + +The Lemurs (_Prosimiæ_) have few living representatives. But they are +very interesting, and are the last survivors of a once extensive group. +We find many fossil remains of them in the older Tertiary deposits of +Europe and North America, in the Eocene and Miocene. We distinguish two +sub-orders, the fossil _Lemuravida_ and the modern _Lemurogona._ The +earliest and most primitive forms of the Lemuravida are the Pachylemurs +(_Hypopsodina_); they come next to the earliest Placentals +(_Prochoriata_), and have the typical full dentition, with forty-four +teeth (3.1.4.3. over 3.1.4.3.). The Necrolemurs (_Adapida,_ Fig. 274) +have only forty teeth, and have lost an incisor in each jaw (2.1.4.3. +over 2.1.4.3.). The dentition is still further reduced in the +Lemurogona (_Autolemures_), which usually have only thirty-six teeth +(2.1.3.3. over 2.1.3.3.). These living survivors are scattered far over +the southern part of the Old World. Most of the species live in +Madagascar, some in the Sunda Islands, others on the mainland of Asia +and Africa. They are gloomy and melancholic animals; they live a quiet +life, climbing trees, and eating fruit and insects. They are of +different kinds. Some are closely related to the Marsupials (especially +the opossum). Others (_Macrotarsi_) are nearer to the Insectivora, +others again (_Chiromys_) to the Rodents. Some of the lemurs +(_Brachytarsi_) approach closely to the true apes. The numerous fossil +remains of half-apes and apes that have been recently found in the +Tertiary deposits justify us in thinking that man’s ancestors were +represented by several different species during this long period. Some +of these were almost as big as men, such as the diluvial lemurogonon +_Megaladapis_ of Madagascar. + + +Fig.276. The white-nosed ape (Cercopithecus petaurista). Fig. 276—The +white-nosed ape (_Cercopithecus petaurista_). + + +Next to the lemurs come the true apes (_Simiæ_), the twenty-sixth stage +in our ancestry. It has been beyond question for some time now that the +apes approach nearest to man in every respect of all the animals. Just +as the lowest apes come close to the lemurs, so the highest come next +to man. When we carefully study the comparative anatomy of the apes and +man, we can trace a gradual and uninterrupted advance in the +organisation of the ape up to the purely human frame, and, after +impartial examination of the “ape problem” that has been discussed of +late years with such passionate interest, we come infallibly to the +important conclusion, first formulated by Huxley in 1863: “Whatever +systems of organs we take, the comparison of their modifications in the +series of apes leads to the same result: that the anatomic differences +that separate man from the gorilla and chimpanzee are not as great as +those that separate the gorilla from the lower apes.” Translated into +phylogenetic language, this “pithecometra-law,” formulated in such +masterly fashion by Huxley, is quite equivalent to the popular saying: +“Man is descended from the apes.” + +In the very first exposition of his profound natural classification +(1735) Linné +placed the anthropoid mammals at the head of the animal kingdom, with +three genera: man, the ape, and the sloth. He afterwards called them +the “Primates”—the “lords” of the animal world; he then also separated +the lemur from the true ape, and rejected the sloth. Later zoologists +divided the order of Primates. First the Gottingen anatomist, +Blumenbach, founded a special order for man, which he called _ Bimana_ +(“two-handed”); in a second order he united the apes and lemurs under +the name of _Quadrumana_ (“four-handed”); and a third order was formed +of the distantly-related _Chiroptera_ (bats, etc.). The separation of +the Bimana and Quadrumana was retained by Cuvier and most of the +subsequent zoologists. It seems to be extremely important, but, as a +matter of fact, it is totally wrong. This was first shown in 1863 by +Huxley, in his famous _Man’s Place in Nature._ On the strength of +careful comparative anatomical research he proved that the apes are +just as truly “two-handed” as man; or, if we prefer to reverse it, that +man is as truly four-handed as the ape. He showed convincingly that the +ideas of hand and foot had been wrongly defined, and had been +improperly based on physiological instead of morphological grounds. The +circumstance that we oppose the +thumb to the other four fingers in our hand, and so can grasp things, +seemed to be a special distinction of the hand in contrast to the foot, +in which the corresponding great toe cannot be opposed in this way to +the others. But the apes can grasp with the hind-foot as well as the +fore, and so were regarded as quadrumanous. However, the inability to +grasp that we find in the foot of civilised man is a consequence of the +habit of clothing it with tight coverings for thousands of years. Many +of the bare-footed lower races of men, especially among the negroes, +use the foot very freely in the same way as the hand. As a result of +early habit and continued practice, they can grasp with the foot (in +climbing trees, for instance) just as well as with the hand. Even +new-born infants of our own race can grasp very strongly with the great +toe, and hold a spoon with it as firmly as with the hand. Hence the +physiological distinction between hand and foot can neither be pressed +very far, nor has it a scientific basis. We must look to morphological +characters. + + +Fig.277. The drill-baboon (Cynocephalus leucophaeus) (From Brehm.) Fig. +277—The drill-baboon (_Cynocephalus leucophæus_). (From _Brehm._) + + +As a matter of fact, it is possible to draw such a sharp morphological +distinction—a distinction based on anatomic structure—between the fore +and hind extremity. In the formation both of the bony skeleton and of +the muscles that are connected with the hand and foot before and behind +there are material and constant differences; and these are found both +in man and the ape. For instance, the number and arrangement of the +smaller bones of the hand and foot are quite different. There are +similar constant differences in the muscles. The hind extremity always +has three muscles (a short flexor muscle, a short extensor muscle, and +a long calf-muscle) that are not found in the fore extremity. The +arrangement of the muscles also is different before and behind. These +characteristic differences between the fore and hind extremities are +found in man as well as the ape. There can be no doubt, therefore, that +the ape’s foot deserves that name just as much as the human foot does, +and that all true apes are just as “bimanous” as man. The common +distinction of the apes as “quadrumanous” is altogether wrong +morphologically. + +But it may be asked whether, quite apart from this, we can find any +other features that distinguish man more sharply from the ape than the +various species of apes are distinguished from each other. Huxley gave +so complete and demonstrative a reply to this question that the +opposition still raised on many sides is absolutely without foundation. +On the ground of careful comparative anatomical research, Huxley proved +that in all morphological respects the differences between the highest +and lowest apes are greater than the corresponding differences between +the highest apes and man. He thus restored Linné’s order of the +Primates (excluding the bats), and divided it into three sub-orders, +the first composed of the half-apes (_Lemuridæ_), the second of the +true apes (_Simiadæ_), the third of men (_Anthropidæ_). + +But, as we wish to proceed quite consistently and impartially on the +laws of systematic logic, we may, on the strength of Huxley’s own law, +go a good deal farther in this division. We are justified in going at +least one important step farther, and assigning man his natural place +within one of the sections of the order of apes. All the features that +characterise this group of apes are found in man, and not found in the +other apes. We do not seem to be justified, therefore, in founding for +man a special order distinct from the apes. + +The order of the true apes (_Simiæ_ or _ Pitheca_)—excluding the +lemurs—has long been divided into two principal groups, which also +differ in their geographical distribution. One group (_Hesperopitheca,_ +or western apes) live in America. The other group, to which man +belongs, are the _ Eopitheca_ or eastern apes; they are found in Asia +and Africa, and were formerly in Europe. All the eastern apes agree +with man in the features that are chiefly used in zoological +classification to distinguish between the two simian groups, especially +in the dentition. The objection might be raised that the teeth are too +subordinate an organ physiologically for us to lay stress on them in so +important a question. But there is a good reason for it; it is with +perfect justice that zoologists have for more than a century paid +particular attention to the teeth in the systematic division and +arrangement of the orders of mammals. The number, form, and arrangement +of the teeth are much more faithfully inherited in the various orders +than most other characters. + +Hence the form of dentition in man is very important. In the fully +developed +condition we have thirty-two teeth; of these eight are incisors, four +canine, and twenty molars. The eight incisors, in the middle of the +jaws, have certain characteristic differences above and below. In the +upper jaw the inner incisors are larger than the outer; in the lower +jaw the inner are the smaller. Next to these, at each side of both +jaws, is a canine (or “eye tooth”), which is larger than the incisors. +Sometimes it is very prominent in man, as it is in most apes and many +of the other mammals, and forms a sort of tusk. Next to this there are +five molars above and below on each side, the first two of which (the +“pre-molars”) are small, have only one root, and are included in the +change of teeth; the three back ones are much larger, have two roots, +and only come with the second teeth. The apes of the Old World, or all +the living or fossil apes of Asia, Africa, and Europe, have the same +dentition as man. + + +Figs. 278 to 282. Skeletons of a man and the four anthropoid apes. Fig. +278. Gibbon (Hylobates). Fig. 279. Orang (Satyrus). Fig. 280. +Chimpanzee (Anthropithecus). Fig. 281. Gorilla (Gorilla). Fig. 282. Man +(Homo). Fig. 278–282—Skeletons of a man and the four anthropoid apes. +(Fig. 278, Gibbon; Fig. 279, Orang; Fig. 280, Chimpanzee; Fig. 281, +Gorilla; Fig. 282, Man. (From _Huxley._) Cf. Figs. 203–209. + + +On the other hand, all the American apes have an additional pre-molar +in each half of the jaw. They have six molars above and below on each +side, or thirty-six teeth altogether. This characteristic difference +between the eastern and western apes has been so faithfully inherited +that it is very instructive for us. It is true that there seems to be +an exception in the case of a small family of South American apes. The +small silky apes (_Arctopitheca_ or _Hapalidæ_), which include the +tamarin (_Midas_) and the brush-monkey (_Jacchus_), have only five +molars in each half of the jaw (instead of six), and so seem to be +nearer to the eastern apes. But it is found, on closer examination, +that they have three premolars, like all the western apes, and that +only the last molar has been lost. Hence the apparent exception really +confirms the above distinction. + +Of the other features in which the two groups of apes differ, the +structure of the nose is particularly instructive and conspicuous. All +the eastern apes have the same type of nose as man—a comparatively +narrow partition between the two halves, so that the nostrils run +downwards. In some of them the nose protrudes as far as in man, and has +the same characteristic structure. We have already alluded to the +curious long-nosed apes, which have a long, finely-curved nose. Most of +the eastern apes have, it is true, rather flat noses, like, for +instance, the white-nosed monkey (Fig. 276); but the nasal partition is +thin and narrow in them all. The American apes have a different type of +nose. The partition is very broad and thick at the bottom, and the +wings of the nostrils are not developed, so that they point outwards +instead of downwards. This difference in the form of the nose is so +constantly inherited in both groups that the apes of the New World are +called “flat-nosed” (_Platyrrhinæ_), and those of the Old World +“narrow-nosed” (_Catarrhinæ_). The bony passage of the ear (at the +bottom of which is the tympanum) is short and wide in all the +Platyrrhines, but long and narrow in all the Catarrhines; and in man +this difference also is significant. + +This division of the apes into Platyrrhines and Catarrhines, on the +ground of the above hereditary features, is now generally admitted in +zoology, and receives strong support from the geographical distribution +of the two groups in the east and west. It follows at once, as regards +the phylogeny of the apes, that two divergent lines proceeded from the +common stem-form of the ape-order in the early Tertiary period, one of +which spread over the Old, the other over the New, World. It is certain +that all the Platyrrhines come of one stock, and also all the +Catarrhines; but the former are phylogenetically older, and must be +regarded as the stem-group of the latter. + +What can we deduce from this with regard to our own genealogy? Man has +just the same characters, the same form of dentition, auditory passage, +and nose, as all the Catarrhines; in this he radically differs from the +Platyrrhines. We are thus forced to assign him a position among the +eastern apes in the order of Primates, or at least place him alongside +of them. But it follows that man is a direct blood relative of the apes +of the Old World, and can be traced to a common stem-form together with +all the Catarrhines. In his whole organisation and in his origin man is +a true Catarrhine; he originated in the Old World from an unknown, +extinct group of the eastern apes. The apes of the New World, or the +Platyrrhines, form a divergent branch of our genealogical tree, and +this is only distantly related at its root to the human race. We must +assume, of course, that the earliest Eocene apes had the full dentition +of the Platyrrhines; hence we may regard this stem-group as a special +stage (the twenty-sixth) in our ancestry, and deduce from it (as the +twenty-seventh stage) the earliest Catarrhines. + +We have now reduced the circle of our nearest relatives to the small +and comparatively scanty group that is represented by the sub-order of +the Catarrhines; and we are in a position to answer the question of +man’s place in this sub-order, and say whether we can deduce anything +further from this position as to our immediate ancestors. In answering +this question the comprehensive and able studies that Huxley gives of +the comparative anatomy of man and the various Catarrhines in his +_Man’s Place in Nature_ are of great assistance to us. It is quite +clear from these that the differences between man and the highest +Catarrhines (gorilla, chimpanzee, and orang) are in every respect +slighter than the corresponding differences between the highest and the +lowest Catarrhines (white-nosed monkey, macaco, baboon, etc.). In fact, +within the small group of the tail-less anthropoid apes the differences +between the various genera are not less than the differences between +them and man. This is seen by a glance at the skeletons that Huxley has +put together (Figs. 278–282). Whether we take the skull or the +vertebral column or the ribs or the fore or hind limbs, or whether we +extend the comparison to the muscles, blood-vessels, brain, placenta, +etc., we always reach the same result on impartial examination—that man +is not more different from the other Catarrhines than the extreme forms +of them (for instance, the gorilla and baboon) differ from each other. +We may now, therefore, complete the Huxleian law we have already quoted +with the following thesis: “Whatever system of organs we take, a +comparison of their modifications in the series of Catarrhines always +leads to the same conclusion; the anatomic differences that separate +man from the most advanced Catarrhines (orang, gorilla, chimpanzee) are +not as great as those that separate the latter from the lowest +Catarrhines (white-nosed monkey, macaco, baboon).” + +We must, therefore, consider the descent of man from other Catarrhines +to be fully proved. Whatever further information on the comparative +anatomy and ontogeny of the living Catarrhines we may obtain in the +future, it cannot possibly disturb this conclusion. Naturally, our +Catarrhine ancestors must have passed through a long series of +different forms before the human type was produced. The chief advances +that effected this “creation of man,” or his differentiation from the +nearest related Catarrhines, were: the adoption of the erect posture +and the consequent greater differentiation of the fore and hind limbs, +the evolution of articulate speech and its organ, the larynx, and the +further development of the brain and its function, the soul; sexual +selection had a great influence in this, as Darwin showed in his famous +work. + +With an eye to these advances we can distinguish at least four +important stages in our simian ancestry, which represent prominent +points in the historical process of the making of man. We may take, +after the Lemurs, the earliest and lowest Platyrrhines of South +America, with thirty-six teeth, as the twenty-sixth stage of our +genealogy; they were developed from the Lemurs by a peculiar +modification of the brain, teeth, nose, and fingers. From these Eocene +stem-apes were formed the earliest Catarrhines or eastern apes, with +the human dentition (thirty-two teeth), by modification of the nose, +lengthening of the bony channel of the ear, and the loss of four +pre-molars. These oldest stem-forms of the whole Catarrhine group were +still thickly coated with hair, and had long tails—baboons +(_Cynopitheca_) or tailed apes (_Menocerca,_ Fig. 276). They lived +during the Tertiary period, and are found fossilised in the Miocene. Of +the actual tailed apes perhaps the nearest to them are the _ +Semnopitheci._ + +If we take these Semnopitheci as the twenty-seventh stage in our +ancestry, we may put next to them, as the twenty-eighth, the tail-less +anthropoid apes. This name is given to the most advanced and man-like +of the existing Catarrhines. They were developed from the other +Catarrhines by losing the tail and part of the hair, and by a higher +development of the brain, which found expression in the enormous growth +of the skull. Of this remarkable family there are only a few genera +to-day, and we have already dealt with them (Chapter XV)—the gibbon +(_Hylobates,_ Fig. 203) and orang (_Satyrus,_ Figs. 204, 205) in +South-Eastern Asia and the Archipelago; and the chimpanzee +(_Anthropithecus,_ Figs. 206, 207) and gorilla (_Gorilla,_ Fig. 208) in +Equatorial Africa. + +The great interest that every thoughtful man takes in these nearest +relatives of ours has found expression recently in a fairly large +literature. The most distinguished of these works for impartial +treatment of the question of affinity is Robert Hartmann’s little work +on _The Anthropoid Apes._ Hartmann divides the primate order into two +families: (1) _ Primarii_ (man and the anthropoid apes); and (2) _ +Simianæ_ (true apes, Catarrhines and Platyrrhines). Professor Klaatsch, +of Heidelberg, has advanced a different view in his interesting and +richly illustrated work on _The Origin and Development of the Human_ +_Race._ This is a substantial supplement to my _Anthropogeny,_ in so +far as it gives the chief results of modern research on the early +history of man and civilisation. But when Klaatsch declares the descent +of man from the apes to be “irrational, narrow-minded, and false,” in +the belief that we are thinking of some living species of ape, we must +remind him that no competent scientist has ever held so narrow a view. +All of us look merely—in the sense of Lamarck and Darwin—to the +original unity (admitted by Klaatsch) of the primate stem. This common +descent of all the Primates (men, apes, and lemurs) from one primitive +stem-form, from which the most far-reaching conclusions follow for the +whole of anthropology and philosophy, is admitted by Klaatsch as well +as by myself and all other competent zoologists who accept the theory +of evolution in general. He says explicitly (p. 172): “The three +anthropoid apes—gorilla, chimpanzee, and orang—seem to be branches from +a common root, and this was not far from that of the gibbon and man.” +That is in the main the opinion that I have maintained (especially +against Virchow) in a number of works ever since 1866. The hypothetical +common ancestor of all the Primates, which must have lived in the +earliest Tertiary period (more probably in the Cretaceous), was called +by me _Archiprimus_; Klaatsch now calls it _Primatoid._ Dubois has +proposed the appropriate name of _Prothylobates_ for the common and +much younger stem-form of the anthropomorpha (man and the anthropoid +apes). The actual _ Hylobates_ is nearer to it than the other three +existing anthropoids. None of these can be said to be absolutely the +most man-like. The gorilla comes next to man in the structure of the +hand and foot, the chimpanzee in the chief features of the skull, the +orang in brain development, and the gibbon in the formation of the +chest. None of these existing anthropoid apes is among the direct +ancestors of our race; they are scattered survivors of an ancient +branch of the Catarrhines, from which the human race developed in a +particular direction. + + +Fig.283. Skull of the fossil ape-man of Java (Pithecanthropus erectus), +restored by Eugen Dubois. Fig. 283—Skull of the fossil ape-man of Java +(_Pithecanthropus erectus_), restored by _Eugen Dubois._ + + +Although man is directly connected with this anthropoid family and +originates from it, we may assign an important intermediate form +between the _Prothylobates_ and him (the twenty-ninth stage in our +ancestry), the ape-men (_Pithecanthropi_). I gave this name in the +_History of Creation_ to the “speechless primitive men” (_Alali_), +which were men in the ordinary sense as far as the general structure is +concerned (especially in the differentiation of the limbs), but lacked +one of the chief human characteristics, articulate speech and the +higher intelligence that goes with it, and so had a less developed +brain. The phylogenetic hypothesis of the organisation of this +“ape-man” which I then advanced was brilliantly confirmed twenty-four +years afterwards by the famous discovery of the fossil _Pithecanthropus +erectus_ by Eugen Dubois (then military surgeon in Java, afterwards +professor at Amsterdam). In 1892 he found at Trinil, in the residency +of Madiun in Java, in Pliocene deposits, certain remains of a large and +very man-like ape (roof of the skull, femur, and teeth), which he +described as “an erect ape-man” and a survivor of a “stem-form of man” +(Fig. 283). Naturally, the Pithecanthropus excited the liveliest +interest, as the long-sought transitional form between man and the ape: +we seemed to have found “the missing link.” There were very interesting +scientific discussions of it at the last three International Congresses +of Zoology (Leyden, 1895, Cambridge, 1898, and Berlin, 1901). I took an +active part in the discussion at +Cambridge, and may refer the reader to the paper I read there on “The +Present Position of Our Knowledge of the Origin of Man” (translated by +Dr. Gadow with the title of _ The Last Link_). + +An extensive and valuable literature has grown up in the last ten years +on the Pithecanthropus and the pithecoid theory connected with it. A +number of distinguished anthropologists, anatomists, paleontologists, +and phylogenists have taken part in the controversy, and made use of +the important data furnished by the new science of pre-historic +research. Hermann Klaatsch has given a good summary of them, with many +fine illustrations, in the above-mentioned work. I refer the reader to +it as a valuable supplement to the present work, especially as I cannot +go any further here into these anthropological and pre-historic +questions. I will only repeat that I think he is wrong in the attitude +of hostility that he affects to take up with regard to my own views on +the descent of man from the apes. + +The most powerful opponent of the pithecoid theory—and the theory of +evolution in general—during the last thirty years (until his death in +September, 1902) was the famous Berlin anatomist, Rudolf Virchow. In +the speeches which he delivered every year at various congresses and +meetings on this question, he was never tired of attacking the hated +“ape theory.” His constant categorical position was: “It is quite +certain that man does not descend from the ape or any other animal.” +This has been repeated incessantly by opponents of the theory, +especially theologians and philosophers. In the inaugural speech that +he delivered in 1894 at the Anthropological Congress at Vienna, he said +that “man might just as well have descended from a sheep or an elephant +as from an ape.” Absurd expressions like this only show that the famous +pathological anatomist, who did so much for medicine in the +establishment of cellular pathology, had not the requisite attainments +in comparative anatomy and ontogeny, systematic zoology and +paleontology, for sound judgment in the province of anthropology. The +Strassburg anatomist, Gustav Schwalbe, deserved great praise for having +the moral courage to oppose this dogmatic and ungrounded teaching of +Virchow, and showing its untenability. The recent admirable works of +Schwalbe on the Pithecanthropus, the earliest races of men, and the +Neanderthal skull (1897–1901) will supply any candid and judicious +reader with the empirical material with which he can convince himself +of the baselessness of the erroneous dogmas of Virchow and his clerical +friends (J. Ranke, J. Bumüller, etc.). + +As the Pithecanthropus walked erect, and his brain (judging from the +capacity of his skull, Fig. 283) was midway between the lowest men and +the anthropoid apes, we must assume that the next great step in the +advance from the Pithecanthropus to man was the further development of +human speech and reason. + +Comparative philology has recently shown that human speech is +polyphyletic in origin; that we must distinguish several (probably +many) different primitive tongues that were developed independently. +The evolution of language also teaches us (both from its ontogeny in +the child and its phylogeny in the race) that human speech proper was +only gradually developed after the rest of the body had attained its +characteristic form. It is probable that language was not evolved until +after the dispersal of the various species and races of men, and this +probably took place at the commencement of the Quaternary or Diluvial +period. The speechless ape-men or _Alali_ certainly existed towards the +end of the Tertiary period, during the Pliocene, possibly even the +Miocene, period. + +The third, and last, stage of our animal ancestry is the true or +speaking man (_Homo_), who was gradually evolved from the preceding +stage by the advance of animal language into articulate human speech. +As to the time and place of this real “creation of man” we can only +express tentative opinions. It was probably during the Diluvial period +in the hotter zone of the Old World, either on the mainland in tropical +Africa or Asia or on an earlier continent (Lemuria—now sunk below the +waves of the Indian Ocean), which stretched from East Africa +(Madagascar, Abyssinia) to East Asia (Sunda Islands, Further India). I +have given fully in my _History of Creation,_ (chapter xxviii) the +weighty reasons for claiming this descent of man from the anthropoid +eastern apes, and shown how we may conceive the spread of the various +races from this “Paradise” over the whole earth. I have also dealt +fully with the relations of the various races and species of men to +each other. + +SYNOPSIS OF THE CHIEF SECTIONS OF OUR STEM-HISTORY + +First Stage: The Protists + + +Man’s ancestors are unicellular protozoa, originally unnucleated Monera +like the Chromacea, structureless green particles of plasm; afterwards +real nucleated cells (first plasmodomous _Protophyta,_ like the +Palmella; then plasmophagous _Protozoa,_ like the Amœba). + +Second Stage: The Blastæads + + +Man’s ancestors are round cœnobia or colonies of Protozoa; they consist +of a close association of many homogeneous cells, and thus are +individuals of the second order. They resemble the round +cell-communities of the Magospheræ and Volvocina, equivalent to the +ontogenetic blastula: hollow globules, the wall of which consists of a +single layer of ciliated cells (blastoderm). + +Third Stage: The Gastræads + +Man’s ancestors are Gastræads, like the simplest of the actual Metazoa +(Prophysema, Olynthus, Hydra, Pemmatodiscus). Their body consists +merely of a primitive gut, the wall of which is made up of the two +primary germinal layers. + +Fourth Stage: The Platodes + + +Man’s ancestors have substantially the organisation of simple Platodes +(at first like the cryptocœlic Platodaria, later like the rhabdocœlic +Turbellaria). The leaf-shaped bilateral-symmetrical body has only one +gut-opening, and develops the first trace of a nervous centre from the +ectoderm in the middle line of the back (Figs. 239, 240). + +Fifth Stage: The Vermalia + + +Man’s ancestors have substantially the organisation of unarticulated +Vermalia, at first Gastrotricha (Ichthydina), afterwards Frontonia +(Nemertina, Enteropneusta). Four secondary germinal layers develop, two +middle layers arising between the limiting layers (cœloma). The dorsal +ectoderm forms the vertical plate, acroganglion (Fig. 243). + +Sixth Stage: The Prochordonia + + +Man’s ancestors have substantially the organisation of a simple +unarticulated Chordonium (Copelata and Ascidia-larvæ). The unsegmented +chorda develops between the dorsal medullary tube and the ventral +gut-tube. The simple cœlom-pouches divide by a frontal septum into two +on each side; the dorsal pouch (episomite) forms a muscle-plate; the +ventral pouch (hyposomite) forms a gonad. Head-gut with gill-clefts. + +Seventh Stage: The Acrania + + +Man’s ancestors are skull-less Vertebrates, like the Amphioxus. The +body is a series of metamera, as several of the primitive segments are +developed. The head contains in the ventral half the branchial gut, the +trunk the hepatic gut. The medullary tube is still simple. No skull, +jaws, or limbs. + +Eighth Stage: The Cyclostoma + + +Man’s ancestors are jaw-less Craniotes (like the Myxinoida and +Petromyzonta). The number of metamera increases. The fore-end of the +medullary tube expands into a vesicle and forms the brain, which soon +divides into five cerebral vesicles. In the sides of it appear the +three higher sense-organs: nose, eyes, and auditory vesicles. No jaws, +limbs, or floating bladder. + +Ninth Stage: The Ichthyoda + + +Man’s ancestors are fish-like Craniotes: (1) Primitive fishes +(Selachii); (2) plated fishes (Ganoida); (3) amphibian fishes +(Dipneusta); (4) mailed amphibia (Stegocephala). The ancestors of this +series develop two pairs of limbs: a pair of fore (breast-fins) and of +hind (belly-fins) legs. The gill-arches are formed between the +gill-clefts: the first pair form the maxillary arches (the upper and +lower jaws). The floating bladder (lung) and pancreas grow out of the +gut. + +Tenth Stage: The Amniotes + + +Man’s ancestors are Amniotes or gill-less Vertebrates: (1) Primitive +Amniotes (Proreptilia); (2) Sauromammals; (3) Primitive Mammals +(Monotremes); (4) Marsupials; (5) Lemurs (Prosimiæ); (6) Western apes +(Platyrrhinæ); (7) Eastern apes (Catarrhinæ): at first tailed +Cynopitheca; then tail-less anthropoids; later speechless ape-men +(Alali); finally speaking man. The ancestors of these Amniotes develop +an amnion and allantois, and gradually assume the mammal, and finally +the specifically human, form. + + + + +Chapter XXIV. +EVOLUTION OF THE NERVOUS SYSTEM + + +The previous chapters have taught us how the human body as a whole +develops from the first simple rudiment, a single layer of cells. The +whole human race owes its origin, like the individual man, to a simple +cell. The unicellular stem-form of the race is reproduced daily in the +unicellular embryonic stage of the individual. We have now to consider +in detail the evolution of the various parts that make up the human +frame. I must, naturally, confine myself to the most general and +principal outlines; to make a special study of the evolution of each +organ and tissue is both beyond the scope of this work, and probably +beyond the anatomic capacity of most of my readers to appreciate. In +tracing the evolution of the various organs we shall follow the method +that has hitherto guided us, except that we shall now have to consider +the ontogeny and phylogeny of the organs together. We have seen, in +studying the evolution of the body as a whole, that phylogeny casts a +light over the darker paths of ontogeny, and that we should be almost +unable to find our way in it without the aid of the former. We shall +have the same experience in the study of the organs in detail, and I +shall be compelled to give simultaneously their ontogenetic and +phylogenetic origin. The more we go into the details of organic +development, and the more closely we follow the rise of the various +parts, the more we see the inseparable connection of embryology and +stem-history. The ontogeny of the organs can only be understood in the +light of their phylogeny, just as we found of the embryology of the +whole body. Each embryonic form is determined by a corresponding +stem-form. This is true of details as well as of the whole. + +We will consider first the animal and then the vegetal systems of +organs of the body. The first group consists of the psychic and the +motor apparatus. To the former belong the skin, the nervous system, and +the sense-organs. The motor apparatus is composed of the passive and +the active organs of movement (the skeleton and the muscles). The +second or vegetal group consists of the nutritive and the reproductive +apparatus. To the nutritive apparatus belong the alimentary canal with +all its appendages, the vascular system, and the renal (kidney) system. +The reproductive apparatus comprises the different organs of sex +(embryonic glands, sexual ducts, and copulative organs). + +As we know from previous chapters (XI–XIII), the animal systems of +organs (the organs of sensation and presentation) develop for the most +part out of the _outer_ primary germ-layer, or the cutaneous (skin) +layer. On the other hand, the vegetal systems of organs arise for the +most part from the _inner_ primary germ-layer, the visceral layer. It +is true that this antithesis of the animal and vegetal spheres of the +body in man and all the higher animals is by no means rigid; several +parts of the animal apparatus (for instance, the greater part of the +muscles) are formed from cells that come originally from the entoderm; +and a great part of the vegetative apparatus (for instance, the +mouth-cavity and the gonoducts) are composed of cells that come from +the ectoderm. + +In the more advanced animal body there is so much interlacing and +displacement of the various parts that it is often very difficult to +indicate the sources of them. But, broadly speaking, we may take it as +a positive and important fact that in man and the higher animals the +chief part of the animal organs comes from the ectoderm, and the +greater part of the vegetative organs from the entoderm. It was for +this reason that Carl Ernst von Baer called the one the animal and the +other the vegetative layer (see p. 16). + +The solid foundation of this important thesis is the _gastrula,_ the +most instructive embryonic form in the animal world, which we still +find in the same shape in the most diverse classes of animals. This +form points demonstrably to a +common stem-form of all the Metazoa, the _Gastræa;_ in this +long-extinct stem-form the whole body consisted throughout life of the +two primary germinal layers, as is now the case temporarily in the +gastrula; in the Gastræa the simple cutaneous (skin) layer _actually_ +represented all the animal organs and functions, and the simple +visceral (gut) layer all the vegetal organs and functions. This is the +case with the modern Gastræads (Fig. 233); and it is also the case +potentially with the gastrula. + +We shall easily see that the gastræa theory is thus able to throw a +good deal of light, both morphologically and physiologically, on some +of the chief features of embryonic development, if we take up first the +consideration of the chief element in the animal sphere, the psychic +apparatus or sensorium and its evolution. This apparatus consists of +two very different parts, which seem at first to have very little +connection with each other—the outer skin, with all its hairs, nails, +sweat-glands, etc., and the nervous system. The latter comprises the +central nervous system (brain and spinal cord), the peripheral, +cerebral, and spinal nerves, and the sense-organs. In the fully-formed +vertebrate body these two chief elements of the sensorium lie far +apart, the skin being external to, and the central nervous system in +the very centre of, the body. The one is only connected with the other +by a section of the peripheral nervous system and the sense-organs. +Nevertheless, as we know from human embryology, the medullary tube is +formed from the cutaneous layer. The organs that discharge the most +advanced functions of the animal body—the organs of the soul, or of +psychic life—develop from the external skin. This is a perfectly +natural and necessary process. If we reflect on the historical +evolution of the psychic and sensory functions, we are forced to +conclude that the cells which accomplish them must originally have been +located on the outer surface of the body. Only elementary organs in +this superficial position could directly receive the influences of the +environment. Afterwards, under the influence of natural selection, the +cellular group in the skin which was specifically “sensitive” withdrew +into the inner and more protected part of the body, and formed there +the foundation of a central nervous organ. As a result of increased +differentiation, the skin and the central nervous system became further +and further separated, and in the end the two were only permanently +connected by the afferent peripheral sensory nerves. + + +Fig.244. The human skin in vertical section. Fig. 284—The human skin in +vertical section (from _Ecker_), highly magnified, _a_ horny layer of +the epidermis, _b_ mucous layer of the epidermis, _c_ papillæ of the +corium, _d_ blood-vessels of same, _ef_ ducts of the sweat-glands +(_g_), _h_ fat-glands in the corium, _i_ nerve, passing into a tactile +corpuscle above. + + +The observations of the comparative anatomist are in complete accord +with this view. He tells us that large numbers of the lower animals +have no nervous system, though they exercise the functions of sensation +and will like the higher animals. In the unicellular Protozoa, which do +not form germinal layers, there is, of course, neither nervous system +nor skin. But in the second division of the animal kingdom also, the +Metazoa, there is at first no nervous system. Its functions are +represented by the simple cell-layer of the ectoderm, which the lower +Metazoa have inherited from the Gastræa (Fig. 30 _e_). We find this in +the lowest Zoophytes—the Gastræads, Physemaria, and Sponges (Figs. +233–238). The lowest Cnidaria (the hydroid polyps) also are little +superior to the Gastræads in structure. Their vegetative functions are +accomplished by the simple visceral layer, and their animal functions +by the simple cutaneous layer. In these +cases the simple cell-layer of the ectoderm is at once skin, locomotive +apparatus, and nervous system. + + +Fig.285. Epidermic cells of a human embryo of two months. Fig. +285—Epidermic cells of a human embryo of two months. (From _Kölliker._) + + +When we come to the higher Metazoa, in which the sensory functions and +their organs are more advanced, we find a division of labour among the +ectodermic cells. Groups of sensitive nerve cells separate from the +ordinary epidermic cells; they retire into the more protected tissue of +the mesodermic under-skin, and form special neural ganglia there. Even +in the Platodes, especially the _Turbellaria,_ we find an independent +nervous system, which has separated from the outer skin. This is the +“upper pharyngeal ganglion,” or _acroganglion,_ situated above the +gullet (Fig. 241 _g_).From this rudimentary structure has been +developed the elaborate central nervous system of the higher animals. +In some of the higher worms, such as the earth-worm, the first rudiment +of the central nervous system (Fig. 74 _n_) is a local thickening of +the skin-sense layer (_hs_), which afterwards separates altogether from +the horny plate. In the earliest Platodes (_Cryptocœla_) and Vermalia +(_Gastrotricha_) the acroganglion remains in the epidermis. But the +medullary tube of the Vertebrates originates in the same way. Our +embryology has taught us that this first structure of the central +nervous system also develops originally from the outer germinal layer. + +Let us now examine more closely the evolution of the human skin, with +its various appendages, the hairs and glands. This external covering +has, physiologically, a double and important part to play. It is, in +the first place, the common integument that covers the whole surface of +the body, and forms a protective envelope for the other organs. As such +it also effects a certain exchange of matter between the body and the +surrounding atmosphere (exhalation, perspiration). In the second place, +it is the earliest and original sense organ, the common organ of +feeling that experiences the sensation of the temperature of the +environment and the pressure or resistance of bodies that come into +contact. + +The human skin (like that of all the higher animals) is composed of two +layers, the outer and the inner or underlying skin. The outer skin or +_epidermis,_ consists of simple ectodermic cells, and contains no +blood-vessels (Fig. 284 _a, b_). It develops from the outer germinal +layer, or skin-sense layer. The underlying skin (_corium_ or +_hypodermis_) consists chiefly of connective tissue, contains numerous +blood-vessels and nerves, and has a totally different origin. It comes +from the outermost parietal stratum of the middle germinal layer, or +the skin-fibre layer. The corium is much thicker than the epidermis. In +its deeper strata (the _subcutis_) there are clusters of fat-cells +(Fig. 284 _h_). Its uppermost stratum (the cutis proper, or the +papillary stratum) forms, over almost the whole surface of the body, a +number of conical microscopic papillæ (something like warts), which +push into the overlying epidermis (_c_). These tactile or sensory +particles contain the finest sensory organs of the skin, the touch +corpuscles. Others contain merely end-loops of the blood-vessels that +nourish the skin (_c, d_). The various parts of the corium arise by +division of labour from the originally homogeneous cells of the +cutis-plate, the outermost lamina of the mesodermic skin-fibre layer +(Fig. 145 _hpr,_ and Figs. 161, 162 _cp_). + +In the same way, all the parts and appendages of the epidermis develop +by differentiation from the homogeneous cells of this horny plate (Fig. +285). At an early stage the simple cellular layer of this horny plate +divides into two. The inner and softer stratum (Fig. 284 _b_) is known +as the mucous stratum, the outer and harder (_a_) as the horny +(corneous) stratum. This horny layer is being constantly used up and +rubbed away at the surface; new layers of cells grow up in their place +out of the underlying mucous stratum. At first the epidermis is a +simple covering of the surface of the body. Afterwards various +appendages develop from it, some internally, others externally. The +internal appendages are the cutaneous glands—sweat, fat, etc. +The external appendages are the hairs and nails. + +The cutaneous glands are originally merely solid cone-shaped growths of +the epidermis, which sink into the underlying corium (Fig. 286 _1_). +Afterwards a canal (_2, 3_) is formed inside them, either by the +softening and dissolution of the central cells or by the secretion of +fluid internally. Some of the glands, such as the sudoriferous, do not +ramify (Fig. 284 _efg_). These glands, which secrete the perspiration, +are very long, and have a spiral coil at the end, but they never +ramify; so also the wax-glands of the ears. Most of the other cutaneous +glands give out buds and ramify; thus, for instance, the lachrymal +glands of the upper eye-lid that secrete tears (Fig. 286), and the +sebaceous glands which secrete the fat in the skin and generally open +into the hair-follicles. Sudoriferous and sebaceous glands are found +only in mammals. But we find lachrymal glands in all the three classes +of Amniotes—reptiles, birds, and mammals. They are wanting in the lower +aquatic vertebrates. + + +Fig.286. Rudimentary lachrymal glands from a human embryo of four +months. Fig. 286—Rudimentary lachrymal glands from a human embryo of +four months. (From _Kölliker._) _1_ earliest structure, in the shape of +a simple solid cone, _2_ and _3_ more advanced structures, ramifying +and hollowing out. _a_ solid buds, _e_ cellular coat of the hollow +buds, _f_ structure of the fibrous envelope, which afterwards forms the +corium about the glands. + + +The mammary glands (Figs. 287, 288) are very remarkable; they are found +in all mammals, and in these alone. They secrete the milk for the +feeding of the new-born mammal. In spite of their unusual size these +structures are nothing more than large sebaceous glands in the skin. +The milk is formed by the liquefaction of the fatty milk-cells inside +the branching mammary-gland tubes (Fig. 287 _c_), in the same way as +the skin-grease or hair-fat, by the solution of fatty cells inside the +sebaceous glands. The outlets of the mammary glands enlarge and form +sac-like mammary ducts (_b_); these narrow again (_a_), and open in the +teats or nipples of the breast by sixteen to twenty-four fine +apertures. The first structure of this large and elaborate gland is a +very simple cone in the epidermis, which penetrates into the corium and +ramifies. In the new-born infant it consists of twelve to eighteen +radiating lobes (Fig. 288). These gradually ramify, their ducts become +hollow and larger, and rich masses of fat accumulate between the lobes. +Thus is formed the prominent female breast (_mamma_), on the top of +which rises the teat or nipple (_mammilla_). The latter is only +developed later on, when the mammary gland is fully-formed; and this +ontogenetic phenomenon is extremely interesting, because the earlier +mammals (the stem-forms of the whole class) have no teats. In them the +milk comes out through a flat portion of the ventral skin that is +pierced like a sieve, as we still find in the lowest living mammals, +the oviparous Monotremes of Australia. The young animal licks the milk +from the mother instead of sucking it. In many of the lower mammals we +find a number of milk-glands at different parts of the ventral surface. +In the human female there is usually only one pair of glands, at the +breast; and it is the same with the apes, bats, elephants, and several +other mammals. Sometimes, however, we find two successive pairs of +glands (or even more) in the human female. Some women have four or five +pairs of breasts, like pigs and hedgehogs (Fig. 103). This polymastism +points back to an older stem-form. We often find these accessory +breasts in the male also (Fig. 103 _D_). Sometimes, moreover, the +normal mammary glands are fully developed and can suckle in the male; +but as a rule they are merely rudimentary organs without functions in +the male. We have already (Chapter XI) dealt with this remarkable and +interesting instance of atavism. + +While the cutaneous glands are inner growths of the epidermis, the +appendages +which we call hairs and nails are external local growths in it. The +nails (_Ungues_) which form important protective structures on the back +of the most sensitive parts of our limbs, the tips of the fingers and +toes, are horny growths of the epidermis, which we share with the apes. +The lower mammals usually have claws instead of them; the ungulates, +hoofs. The stem-form of the mammals certainly had claws; we find them +in a rudimentary form even in the salamander. The horny claws are +highly developed in most of the reptiles (Fig. 264), and the mammals +have inherited them from the earliest representatives of this class, +the stem-reptiles (_Tocosauria_). Like the hoofs (_ungulæ_) of the +Ungulates, the nails of apes and men have been evolved from the claws +of the older mammals. In the human embryo the first rudiment of the +nails is found (between the horny and the mucous stratum of the +epidermis) in the fourth month. But their edges do not penetrate +through until the end of the sixth month. + + +Fig.287. The female breast (mamma) in vertical section. Fig. 287—The +female breast (_mamma_) in vertical section. _c_ racemose glandular +lobes, _b_ enlarged milk-ducts, a narrower outlets, which open into the +nipple. (From _H. Meyer._) + + +The most interesting and important appendages of the epidermis are the +hairs; on account of their peculiar composition and origin we must +regard them as highly characteristic of the whole mammalian class. It +is true that we also find hairs in many of the lower animals, such as +insects and worms. But these hairs, like the hairs of plants, are +thread-like appendages of the surface, and differ entirely from the +hairs of the mammals in the details of their structure and development. + +The embryology of the hairs is known in all its details, but there are +two different views as to their phylogeny. On the older view the hairs +of the mammals are equivalent or homologous to the feathers of the bird +or the horny scales of the reptile. As we deduce all three classes of +Amniotes from a common stem-group, we must assume that these Permian +stem-reptiles had a complete scaly coat, inherited from their +Carboniferous ancestors, the mailed amphibia (_Stegocephala_); the bony +scales of their corium were covered with horny scales. In passing from +aquatic to terrestrial life the horny scales were further developed, +and the bony scales degenerated in most of the reptiles. As regards the +bird’s feathers, it is certain that they are modifications of the horny +scales of their reptilian ancestors. But it is otherwise with the hairs +of the mammals. In their case the hypothesis has lately been advanced +on the strength of very extensive research, especially by Friedrich +Maurer, that they have been evolved from the cutaneous sense-organs of +amphibian ancestors by modification of functions; the epidermic +structure is very similar in both in its embryonic rudiments. This +modern view, which had the support of the greatest expert on the +vertebrates, Carl Gegenbaur, can be harmonised with the older theory to +an extent, in the sense that both formations, scales and hairs, were +very closely connected originally. Probably the conical budding of the +skin-sense layer grew up _under the protection of the horny scale,_ and +became an organ of touch subsequently by the cornification of the +hairs; many hairs are still sensory organs (tactile hairs on the muzzle +and cheeks of many mammals: pubic hairs). + +This middle position of the genetic connection of scales and hairs was +advanced in my _Systematic Phylogeny of the Vertebrates_ (p. 433). It +is confirmed by the similar arrangement of the two cutaneous +formations. As Maurer pointed out, the hairs, as well as the cutaneous +sense-organs and the scales, are at first arranged in regular +longitudinal series, and they afterwards break into alternate groups. +In the embryo of a bear two +inches long, which I owe to the kindness of Herr von Schmertzing (of +Arva Varallia, Hungary), the back is covered with sixteen to twenty +alternating longitudinal rows of scaly protuberances (Fig. 289). They +are at the same time arranged in regular transverse rows, which +converge at an acute angle from both sides towards the middle of the +back. The tip of the scale-like wart is turned inwards. Between these +larger hard scales (or groups of hairs) we find numbers of rudimentary +smaller hairs. + +The human embryo is, as a rule, entirely clothed with a thick coat of +fine wool during the last three or four weeks of gestation. This +embryonic woollen coat (_Lanugo_) generally disappears in part during +the last weeks of fœtal life but in any case, as a rule, it is lost +immediately after birth, and is replaced by the thinner coat of the +permanent hair. These permanent hairs grow out of hair-follicles, which +are formed from the root-sheaths of the disappearing wool-fibres. The +embryonic wool-coat usually, in the case of the human embryo, covers +the whole body, with the exception of the palms of the hands and soles +of the feet. These parts are always bare, as in the case of apes and of +most other mammals. Sometimes the wool-coat of the embryo has a +striking effect, by its colour, on the later permanent hair-coat. Hence +it happens occasionally, for instance, among our Indo-Germanic races, +that children of blond parents seem—to the dismay of the latter—to be +covered at birth with a dark brown or even a black woolly coat. Not +until this has disappeared do we see the permanent blond hair which the +child has inherited. Sometimes the darker coat remains for weeks, and +even months, after birth. This remarkable woolly coat of the human +embryo is a legacy from the apes, our ancient long-haired ancestors. + + +Fig.288. Mammary gland of a new-born infant. Fig. 288—Mammary gland of +a new-born infant, _a_ original central gland, _b_ small and _c_ large +buds of same. (From _Langer._) + + +It is not less noteworthy that many of the higher apes approach man in +the thinness of the hair on various parts of the body. With most of the +apes, especially the higher Catarrhines (or narrow-nosed apes), the +face is mostly, or entirely, bare, or at least it has hair no longer or +thicker than that of man. In their case, too, the back of the head is +usually provided with a thicker growth of hair; this is lacking, +however, in the case of the bald-headed chimpanzee (_Anthropithecus +calvus_). The males of many species of apes have a considerable beard +on the cheeks and chin; this sign of the masculine sex has been +acquired by sexual selection. Many species of apes have a very thin +covering of hair on the breast and the upper side of the limbs—much +thinner than on the back or the under side of the limbs. On the other +hand, we are often astonished to find tufts of hair on the shoulders, +back, and extremities of members of our Indo-Germanic and of the +Semitic races. Exceptional hair on the face, as on the whole body, is +hereditary in certain families of hairy men. The quantity and the +quality of the hair on head and chin are also conspicuously transmitted +in families. These extraordinary variations in the total and partial +hairy coat of the body, which are so noticeable, not only in comparing +different races of men, but also in comparing different families of the +same race, can only be explained on the assumption that in man the +hairy coat is, on the whole, a rudimentary organ, a useless inheritance +from the more thickly-coated apes. In this man resembles the elephant, +rhinoceros, hippopotamus, whale, and other mammals of various orders, +which have also, almost entirely or for the most part, lost their hairy +coats by adaptation. + +The particular process of adaptation by which man lost the growth of +hair on most parts of his body, and retained or augmented it at some +points, was most probably sexual selection. As Darwin luminously showed +in his _Descent of Man,_ sexual selection has been very active +in this respect. As the male anthropoid apes chose the females with the +least hair, and the females favoured the males with the finest growths +on chin and head, the general coating of the body gradually +degenerated, and the hair of the beard and head was more strongly +developed. The growth of hair at other parts of the body (arm-pit, +pubic region) was also probably due to sexual selection. Moreover, +changes of climate, or habits, and other adaptations unknown to us, may +have assisted the disappearance of the hairy coat. + + +Fig.289. Embryo of a bear (Ursus arctos). Fig. 289—Embryo of a bear +(_Ursus arctos_). _A_ seen from ventral side, _B_ from the left. + + +The fact that our coat of hair is inherited directly from the +anthropoid apes is proved in an interesting way, according to Darwin, +by the direction of the rudimentary hairs on our arms, which cannot be +explained in any other way. Both on the upper and the lower part of the +arm they point towards the elbow. Here they meet at an obtuse angle. +This curious arrangement is found only in the anthropoid apes—gorilla, +chimpanzee, orang, and several species of gibbons—besides man (Figs. +203, 207). In other species of gibbon the hairs are pointed towards the +hand both in the upper and lower arm, as in the rest of the mammals. We +can easily explain this remarkable peculiarity of the anthropoids and +man on the theory that our common ancestors were accustomed (as the +anthropoid apes are to-day) to place their hands over their heads, or +across a branch above their heads, during rain. In this position, the +fact that the hairs point downwards helps the rain to run off. Thus the +direction of the hair on the lower part of our arm reminds us to-day of +that useful custom of our anthropoid ancestors. + +The nervous system in man and all the other Vertebrates is, when fully +formed, an extremely complex apparatus, that we may compare, in +anatomic structure and physiological function, with an extensive +telegraphic system. The chief station of +the system is the central marrow or central nervous system, the +innumerable ganglionic cells or _neurona_ (Fig. 9)of which are +connected by branching processes with each other and with numbers of +very fine conducting wires. The latter are the peripheral and +ubiquitous nerve-fibres; with their terminal apparatus, the +sense-organs, etc., they constitute the conducting marrow or peripheral +nervous system. Some of them—the sensory nerve-fibres—conduct the +impressions from the skin and other sense-organs to the central marrow; +others—the motor nerve-fibres—convey the commands of the will to the +muscles. + +The central nervous system or central marrow (_medulla centralis_) is +the real organ of psychic action in the narrower sense. However we +conceive the intimate connection of this organ and its functions, it is +certain that its characteristic actions, which we call sensation, will, +and thought, are inseparably dependent on the normal development of the +material organ in man and all the higher animals. We must, therefore, +pay particular attention to the evolution of the latter. As it can give +us most important information regarding the nature of the “soul,” it +should be full of interest. If the central marrow develops in just the +same way in the human embryo as in the embryo of the other mammals, the +evolution of the human psychic organ from the central organ of the +other mammals, and through them from the lower vertebrates, must be +beyond question. No one can doubt the momentous bearing of these +embryonic phenomena. + + +CaFig.290. Human embryo, three months old, from the dorsal side: brain +and spinal cord exposed. Fig. 291. Central marrow of a human embryo, +four months old, from the back.nyon Fig. 290—Human embryo, three months +old, from the dorsal side: brain and spinal cord exposed. (From +_Kölliker._) _h_ cerebral hemispheres (fore brain), _m_ corpora +quadrigemina (middle brain), _c_ cerebellum (hind brain): under the +latter is the triangular medulla oblongata (after brain). Fig. +291—Central marrow of a human embryo, four months old, from the back. +(From _Kölliker._) _h_ large hemispheres, _v_ quadrigemina, _c_ +cerebellum, _mo_ medulla oblongata: underneath it the spinal cord. + + +In order to understand them fully we must first say a word or two of +the general form and the anatomic composition of the mature human +central marrow. Like the central nervous system of all the other +Craniotes, it consists of two parts, the head-marrow or brain (_medulla +capitis_ or _encephalon_) and the spinal-marrow (_medulla spinalis_ or +_notomyelon_). The one is enclosed in the bony skull, the other in the +bony vertebral column. Twelve pairs of cerebral nerves proceed from the +brain, and thirty-one pairs of spinal nerves from the spinal cord, to +the rest of the body (Fig. 171). On general anatomic investigation the +spinal marrow is found to be a cylindrical cord, with a spindle-shaped +bulb both in the region of the neck above (at the last cervical +vertebra) and the region of the loins (at the first lumbar vertebra) +below (Fig. 291). At the cervical bulb the strong nerves of the upper +limbs, and at the lumbar bulb those of the lower limbs, proceed from +the spinal cord. Above, the latter passes into the brain through the +medulla oblongata (Fig. 291 _mo_). The spinal cord seems to be a thick +mass of nervous matter, but it has a narrow canal at its axis, which +passes into the further cerebral ventricles above, and is filled, like +these, with a clear fluid. + +The brain is a large nerve-mass, occupying the greater part of the +skull, of most elaborate structure. On general examination it divides +into two parts, the cerebrum and cerebellum. The cerebrum lies in front +and above, and has the familiar characteristic convolutions and furrows +on its surface (Figs. 292, 293). On the upper side it is divided by a +deep longitudinal fissure into two halves, the +cerebral hemispheres; these are connected by the _corpus callosum._ The +large cerebrum is separated from the small cerebellum by a deep +transverse furrow. The latter lies behind and below, and has also +numbers of furrows, but much finer and more regular, with convolutions +between, at its surface. The cerebellum also is divided by a +longitudinal fissure into two halves, the “small hemispheres”; these +are connected by a worm-shaped piece, the _vermis cerebelli,_ above, +and by the broad _pons Varolii_ below (Fig. 292 _VI_). + + +Fig.292. The human brain, seen from below. Fig. 292—The human brain, +seen from below. (From _H. Meyer._) Above (in front) is the cerebrum +with its extensive branching furrows; below (behind) the cerebellum +with its narrow parallel furrows. The Roman numbers indicate the roots +of the twelve pairs of cerebral nerves in a series towards the rear. + + +But comparative anatomy and ontogeny teach us that in man and all the +other Craniotes the brain is at first composed, not of these two, but +of three, and afterwards five, consecutive parts. These are found in +just the same form—as five consecutive vesicles—in the embryo of all +the Craniotes, from the Cyclostoma and fishes to man. But, however much +they agree in their rudimentary condition, they differ considerably +afterwards. In man and the higher mammals the first of these +ventricles, the cerebrum, grows so much that in its mature condition it +is by far the largest and heaviest part of the brain. To it belong not +only the large hemispheres, but also the corpus callosum that unites +them, the olfactory lobes, from which the olfactory nerves start, and +most of the structures that are found at the roof and bottom of the +large lateral ventricles inside the two hemispheres, such as the +_corpora striata._ On the other hand, the _optic thalami,_ which lie +between the latter, belong to the second division, which develops from +the “intermediate brain ”; to the same section belong the single third +cerebral ventricle and the structures that are known as the corpora +geniculata, the infundibulum, and the pineal gland. Behind these parts +we find, between the cerebrum and cerebellum, a small ganglion composed +of two prominences, which is called the _corpus quadrigeminum_ on +account of a superficial transverse fissure cutting across (Figs. 290 +_m_ and 291 _v_). Although this quadrigeminum is very insignificant in +man and the higher mammals, it forms a special third section, greatly +developed in the lower vertebrates, the “middle brain.” The fourth +section is the “hind-brain” or little brain (cerebellum) in the +narrower sense, with the single median part, the vermis, and the pair +of lateral parts, the “small hemispheres” (Fig. 291 _c_). Finally, we +have the fifth and last section, the medulla oblongata (Fig. 291 _mo_), +which contains the single fourth cerebral cavity and the contiguous +parts (pyramids, olivary bodies, corpora restiformia). The medulla +oblongata passes straight into the medulla spinalis (spinal cord). The +narrow central canal of the spinal cord continues above into the +quadrangular fourth cerebral cavity of the medulla oblongata, the floor +of which is the quadrangular depression. From here a narrow duct, +called “the aqueduct of Sylvius,” passes through the corpus +quadrigeminum to the third cerebral ventricle, which lies between the +two optic thalami; and this in turn is connected with the pairs of +lateral ventricles which lie to the right and left in the large +hemispheres. Thus all the cavities of the central marrow are directly +interconnected. All these parts of the brain have an infinitely complex +structure in detail, but we cannot go into this. Although it is much +more elaborate in man and the higher Vertebrates than in the lower +classes, it develops in them all from the same rudimentary structure, +the five simple cerebral vesicles of the embryonic brain. + +But before we consider the development of the complicated structure of +the brain from this simple series of vesicles, let +us glance for a moment at the lower animals, which have no brain. Even +in the skull-less vertebrate, the Amphioxus, we find no independent +brain, as we have seen. The whole central marrow is merely a simple +cylindrical cord which runs the length of the body, and ends equally +simply at both extremities—a plain medullary tube. All that we can +discover is a small vesicular bulb at the foremost part of the tube, a +degenerate rudiment of a primitive brain. We meet the same simple +medullary tube in the first structure of the ascidia larva, in the same +characteristic position, above the chorda. On closer examination we +find here also a small vesicular swelling at the fore end of the tube, +the first trace of a differentiation of it into brain and spinal cord. +It is probable that this differentiation was more advanced in the +extinct Provertebrates, and the brain-bulb more pronounced (Figs. +98–102). The brain is phylogenetically older than the spinal cord, as +the trunk was not developed until after the head. If we consider the +undeniable affinity of the Ascidiæ to the Vermalia, and remember that +we can trace all the Chordonia to lower Vermalia, it seems probable +that the simple central marrow of the former is equivalent to the +simple nervous ganglion, which lies above the gullet in the lower +worms, and has long been known as the “upper pharyngeal ganglion” +(_ganglion pharyngeum superius_); it would be better to call it the +primitive or vertical brain (acroganglion). + +Probably this upper pharyngeal ganglion of the lower worms is the +structure from which the complex central marrow of the higher animals +has been evolved. The medullary tube of the Chordonia has been formed +by the lengthening of the vertical brain on the dorsal side. In all the +other animals the central nervous system has been developed in a +totally different way from the upper pharyngeal ganglion; in the +Articulates, especially, a pharyngeal ring, with ventral marrow, has +been added. The Molluscs also have a pharyngeal ring, but it is not +found in the Vertebrates. In these the central marrow has been +prolonged down the dorsal side; in the Articulates down the ventral +side. This fact proves of itself that there is no direct relationship +between the Vertebrates and the Articulates. The unfortunate attempts +to derive the dorsal marrow of the former from the ventral marrow of +the latter have totally failed (cf. p. 219). + + +Fig.293. The human brain, seen from the left. Fig. 293—The human brain, +seen from the left. (From _H. Meyer._) The furrows of the cerebrum are +indicated by thick, and those of the cerebellum by finer lines. Under +the latter we can see the medulla oblongata. _f1–f2_ frontal +convolutions, _C_ central convolutions, _S_ fissure of Sylvius, _T_ +temporal furrow, _Pa_ parietal lobes, _An_ angular gyrus, _Po_ +parieto-occipital fissure. + + +When we examine the embryology of the human nervous system, we must +start from the important fact, which we have already seen, that the +first structure of it in man and all the higher Vertebrates is the +simple medullary tube, and that this separates from the outer germinal +layer in the middle line of the sole-shaped embryonic shield. As the +reader will remember, the straight medullary furrow first appears in +the middle of the sandal-shaped embryonic shield. At each side of it +the parallel borders curve over in the form of dorsal or medullary +swellings. These bend together with their free borders, and thus form +the closed medullary tube (Figs. 133–137). At first this tube lies +directly underneath the horny plate; but it afterwards travels inwards, +the upper edges of the provertebral plates growing together between the +horny plate and the tube, joining above the latter, and forming a +completely closed canal. As Gegenbaur very properly observes, “this +gradual imbedding in the +inner part of the body is a process acquired with the progressive +differentiation and the higher potentiality that this secures; by this +process the organ of greater value to the organism is buried within the +frame.” (Cf. Figs. 143–146). + +In the Cyclostoma—a stage above the Acrania—the fore end of the +cylindrical medullary tube begins early to expand into a pear-shaped +vesicle; this is the first outline of an independent brain. In this way +the central marrow of the Vertebrates divides clearly into its two +chief sections, brain and spinal cord. The simple vesicular form of the +brain, which persists for some time in the Cyclostoma, is found also at +first in all the higher Vertebrates (Fig. 153 _hb_). But in these it +soon passes away, the one vesicle being divided into several successive +parts by transverse constrictions. There are first two of these +constrictions, dividing the brain into three consecutive vesicles (fore +brain, middle brain, and hind brain, Fig. 154 _v, m, h_). Then the +first and third are sub-divided by fresh constrictions, and thus we get +five successive sections (Fig. 155). + + +Fig.294. Central marrow of the human embryo from the seventh week, 4/5 +inch long. Fig. 294. The brain from above. Fig. 295. The brain with the +uppermost part of the cord, from the left. Fig. 296. Back view of the +whole embryo: brain and spinal cord exposed. Fig. 294–296—Central +marrow of the human embryo from the seventh week, 4/5 inch long. (From +_Kölliker._) Fig. 294. The brain from above, _v_ fore brain, _z_ +intermediate brain, _m_ middle brain, _h_ hind brain, _n_ after brain. +Fig. 2955. The brain with the uppermost part of the cord, from the +left. Fig. 296. Back view of the whole embryo: brain and spinal cord +exposed. + + +In all the Craniotes, from the Cyclostoma up to man, the same parts +develop from these five original cerebral vesicles, though in very +different ways. The first vesicle, the fore brain (Fig. 155 _v_), forms +by far the largest part of the cerebrum—namely, the large hemispheres, +the olfactory lobes, the corpora striata, the callosum, and the fornix. +From the second vesicle, the intermediate brain (_z_), originate +especially the optic thalami, the other parts that surround the third +cerebral ventricle, and the infundibulum and pineal gland. The third +vesicle, the middle brain (_m_), produces the corpora quadrigemina and +the aqueduct of Sylvius. From the fourth vesicle, the hind brain (_h_), +develops the greater part of the cerebellum—namely, the vermis and the +two small hemispheres. Finally, the fifth vesicle, the after brain +(_n_), forms the medulla oblongata, with the quadrangular pit (the +floor of the fourth ventricle), the pyramids, olivary bodies, etc. + +We must certainly regard it as a comparative-anatomical and ontogenetic +fact of the greatest significance that in all the Craniotes, from the +lowest Cyclostomes and fishes up to the apes and man, the brain +develops in just the same way in the embryo. The first rudiment of it +is always a simple vesicular enlargement of the fore end of the +medullary tube. In every case, first three, then five, vesicles develop +from this bulb, and the permanent brain with all its complex anatomic +structures, of so great a variety in the various classes of +Vertebrates, is formed from the five primitive vesicles. When we +compare the mature brain of a fish, an amphibian, a reptile, a bird, +and a mammal, it seems incredible that we can trace the various parts +of these organs, that differ so much internally and externally, to +common types. Yet all these different Craniote brains have started with +the same rudimentary structure. To convince ourselves of this we have +only to compare the corresponding stages of development of the embryos +of these different animals. + +This comparison is extremely instructive. If we extend it through the +whole series of the Craniotes, we soon discover this interesting fact: +In the Cyclostomes (the Myxinoida and Petromyzonta), which we have +recognised as the lowest and earliest Craniotes, the whole brain +remains throughout life at a very low stage, which is very brief and +passing in the embryos of the higher Craniotes; they retain the five +original sections of the brain unchanged. In the fishes we find an +essential and considerable modification of the five vesicles; it is +clearly the brain of the Selachii in the first place, and subsequently +the brain of the Ganoids, from which the brain of the rest of the +fishes on the one hand and of the Dipneusts and Amphibia, and through +these of the higher Vertebrates, on the other hand, must be derived. In +the fishes and Amphibia (Fig. 300) there is a preponderant development +of the middle brain, and also the after brain, the first, second, and +fourth sections remaining very primitive. It is just the reverse in the +higher Vertebrates, in which the first and third sections, the cerebrum +and cerebellum, are exceptionally developed; while the middle brain and +after brain remain small. The corpora quadrigemina are mostly covered +by the cerebrum, and the oblongata by the cerebellum. But we find a +number of stages of development within the higher Vertebrates +themselves. From the Amphibia upwards the brain (and with it the +psychic life) develops in two different directions; one of these is +followed by the reptiles and birds, and the other by the mammals. The +development of the first section, the fore brain, is particularly +characteristic of the mammals. It is only in them that the cerebrum +becomes so large as to cover all the other parts of the brain (Figs. +293, 301–304). + + +Fig.297. Head of a chick embryo (hatched fifty-eight hours), from the +back. Fig. 297—Head of a chick embryo (hatched fifty-eight hours), from +the back. (From _Mihalkovics._) _vw_ anterior wall of the fore brain. +_vh_ its ventricle. _au_ optic vesicles, _mh_ middle brain, _kh_ hind +brain, _nh_ after brain, _hz_ heart (seen from below), _vw_ vitelline +veins, _us_ primitive segment, _rm_ spinal cord. + + + +Fig.298. Brain of three craniote embryos in vertical section. Fig. 299. +Brain of a shark (Scyllium), back view. Fig. 298—Brain of three +craniote embryos in vertical section. _A_ of a shark (_Heptarchus_), +_B_ of a serpent (_Coluber_), _C_ of a goat (_Capra_). _a_ fore brain, +_b_ intermediate brain, _c_ middle brain, _d_ hind brain, _e_ after +brain, _s_ primitive cleft. (From _Gegenbaur._) +Fig. 299—Brain of a shark (_Scyllium_), back view. _g_ fore-brain, _h_ +olfactory lobes, which send the large olfactory nerves to the nasal +capsule (_o_), _d_ intermediate brain, _b_ middle brain; behind this +the insignificant structure of the hind brain, _a_ after brain. (From +_Gegenbaur._) + + +There are also notable variations in the relative position of the +cerebral vesicles. In the lower Craniotes they lie originally almost in +the same plane. When we examine the brain laterally, we can cut through +all five vesicles with a straight line. But in the Amniotes there is a +considerable curve in the brain along with the bending of the head and +neck; the whole of the upper dorsal surface of the brain develops much +more than the under ventral surface. This causes a curve, so that the +parts come to lie as follows: The fore brain is right in front and +below, the intermediate brain a little higher, and the middle brain +highest of all; the hind brain lies a little lower, and the after brain +lower still. We find this only in the Amniotes—the reptiles, birds, and +mammals. + +Thus, while the brain of the mammals agrees a good deal in general +growth with that of the birds and reptiles, there are some striking +differences between the two. In the Sauropsids (birds and reptiles) the +middle brain and the middle part of the hind brain are well developed. +In the mammals these parts do not grow, and the fore-brain develops so +much that it overlies the other vesicles. As it continues to grow +towards the rear, it at last covers the whole of the rest of the brain, +and also encloses the middle parts from the sides (Figs. 301–303). This +process is of great importance, because the fore brain is the organ of +the higher psychic life, and in it those functions of the nerve-cells +are discharged which we sum up in +the word “soul.” The highest achievements of the animal body—the +wonderful manifestations of consciousness and the complex molecular +processes of thought—have their seat in the fore brain. We can remove +the large hemispheres, piece by piece, from the mammal without killing +it, and we then see how the higher functions of consciousness, thought, +will, and sensation, are gradually destroyed, and in the end completely +extinguished. If the animal is fed artificially, it may be kept alive +for a long time, as the destruction of the psychic organs by no means +involves the extinction of the faculties of digestion, respiration, +circulation, urination—in a word, the vegetative functions. It is only +conscious sensation, voluntary movement, thought, and the combination +of various higher psychic functions that are affected. + + +Fig.300. Brain and spinal cord of the frog. Fig. 300—Brain and spinal +cord of the frog. _A_ from the dorsal, _B_ from the ventral side. _a_ +olfactory lobes before the (_b_) fore brain, _i_ infundibulum at the +base of the intermediate brain, _c_ middle brain, _d_ hind brain, _s_ +quadrangular pit in the after brain, _m_ spinal cord (very short in the +frog), _m_′ roots of the spinal nerves, _t_ terminal fibres of the +spinal cord. (From _Gegenbaur._) + + +The fore brain, the organ of these functions, only attains this high +level of development in the more advanced Placentals, and thus we have +the simple explanation of the intellectual superiority of the higher +mammals. The soul of most of the lower Placentals is not much above +that of the reptiles, but among the higher Placentals we find an +uninterrupted gradation of mental power up to the apes and man. In +harmony with this we find an astonishing variation in the degree of +development of their fore brain, not only qualitatively, but also +quantitatively. The mass and weight of the brain are much greater in +modern mammals, and the differentiation of its various parts more +important, than in their extinct Tertiary ancestors. This can be shown +paleontologically in any particular order. The brains of the living +ungulates are (relatively to the size of the body) four to six times +(in the highest groups even eight times) as large as those of their +earlier Tertiary ancestors, the well-preserved skulls of which enable +us to determine the size and weight of the brain. + + +Fig.301. Brain of an ox-embryo, two inches in length. +Fig. 301—Brain of an ox-embryo, two inches in length. (From +_Mihalkovics._) Left view; the lateral wall of the left hemisphere has +been removed, _st_ corpora striata, _ml_ Monro-foramen, _ag_ arterial +plexus, _ah_ Ammon’s horn, _mh_ middle brain, _kh_ cerebellum, _dv_ +roof of the fourth ventricle, _bb_ pons Varolii, _na_ medulla +oblongata. + + +Fig. 302. Brain of a human embryo, twelve weeks old. Fig. 302—Brain of +a human embryo, twelve weeks old. (From _Mihalkovics._) Seen from +behind and above. _ms_ mantle-furrow, _mh_ corpora quadrigemina (middle +brain), _vs_ anterior medullary ala, _kh_ cerebellum, _vv_ fourth +ventricle, _na_ medulla oblongata. + + +In the lower mammals the surface of the cerebral hemispheres is quite +smooth and level, as in the rabbit (Fig. 304). Moreover, the fore brain +remains so small that it does not cover the middle brain. At a stage +higher the middle +brain is covered, but the hind brain remains free. Finally, in the apes +and man, the latter also is covered by the fore brain. We can trace a +similar gradual development in the fissures and convolutions that are +found on the surface of the cerebrum of the higher mammals (Figs. 292, +293). If we compare different groups of mammals in regard to these +fissures and convolutions, we find that their development proceeds step +by step with the advance of mental life. + + +Fig.303. Brain of a human embryo, twenty-four weeks old, halved in the +median plane: right hemisphere seen from inside. Fig. 303—Brain of a +human embryo, twenty-four weeks old, halved in the median plane: right +hemisphere seen from inside. (From _Mihalkovics._) _rn_ olfactory +nerve, _tr_ funnel of the intermediate brain, _vc_ anterior commissure, +_ml_ Monro-foramen, _gw_ fornix, _ds_ transparent sheath, _bl_ corpus +callosum, _br_ fissure at its border, _hs_ occipital fissure, _zh_ +cuneus, _sf_ occipital transverse fissure, _zb_ pineal gland, _mh_ +corpora quadrigemina, _kh_ cerebellum. + + +Of late years great attention has been paid to this special branch of +cerebral anatomy, and very striking individual differences have been +detected within the limits of the human race. In all human beings of +special gifts and high intelligence the convolutions and fissures are +much more developed than in the average man; and they are more +developed in the latter than in idiots and others of low mental +capacity. There is a similar gradation among the mammals in the +internal structure of the fore brain. In particular the corpus +callosum, that unites the two cerebral hemispheres, is only developed +in the Placentals. Other structures—for instance, in the lateral +ventricles—that seem at first to be peculiar to man, are also found in +the higher apes, and these alone. It was long thought that man had +certain distinctive organs in his cerebrum which were not found in any +other animal. But careful examination has discovered that this is not +the case, but that the characteristic features of the human brain are +found in a rudimentary form in the lower apes, and are more or less +fully developed in the higher apes. Huxley has convincingly shown, in +his _Man’s Place in Nature_ (1863), that the differences in the +formation of the brain within the ape-group constitute a deeper gulf +between the lower and higher apes than between the higher apes and man. + + +Fig.304. Brain of the rabbit. Fig. 304—Brain of the rabbit. _A_ from +the dorsal, _B_ from the ventral side, _lo_ olfactory lobes, _I_ fore +brain, _h_ hypophysis at the base of the intermediate brain, _III_ +middle brain, _IV_ hind brain, _V_ after brain, _2_ optic nerve, _3_ +oculo-motor nerve, _5–8_ cerebral nerves. In _A_ the roof of the right +hemisphere (_I_) is removed, so that we can see the corpora striata in +the lateral ventricle. (From _Gegenbaur._) + + +The comparative anatomy and physiology of the brain of the higher and +lower mammals are very instructive, and give important information in +connection with the chief questions of psychology. + +The central marrow (brain and spinal cord) develops from the medullary +tube in man just as in all the other mammals, and the same applies to +the conducting marrow or “peripheral nervous system.” It consists of +the _sensory_ nerves, which conduct centripetally the impressions from +the skin and the sense-organs to the central marrow, and of the _motor_ +nerves, which convey centrifugally the movements of the will from the +central marrow to the muscles. All these +peripheral nerves grow out of the medullary tube (Fig. 171), and are, +like it, products of the skin-sense layer. + +The complete agreement in the structure and development of the psychic +organs which we find between man and the highest mammals, and which can +only be explained by their common origin, is of profound importance in +the monistic psychology. This is only seen in its full light when we +compare these morphological facts with the corresponding physiological +phenomena, and remember that every psychic action requires the complete +and normal condition of the correlative brain structure for its full +and normal exercise. The very complex molecular movements inside the +neural cells, which we describe comprehensively as “the life of the +soul,” can no more exist in the vertebrate, and therefore in man, +without their organs than the circulation without the heart and blood. +And as the central marrow develops in man from the same medullary tube +as that of the other vertebrates, and as man shares the characteristic +structure of his cerebrum (the organ of thought) with the anthropoid +apes, his psychic life also must have the same origin as theirs. + +If we appreciate the full weight of these morphological and +physiological facts, and put a proper phylogenetic interpretation on +the observations of embryology, we see that the older idea of the +personal immortality of the human soul is scientifically untenable. +Death puts an end, in man as in any other vertebrate, to the +physiological function of the cerebral neurona, the countless +microscopic ganglionic cells, the collective activity of which is known +as “the soul.” I have shown this fully in the eleventh chapter of my +_Riddle of the Universe._ + + + + +Chapter XXV. +EVOLUTION OF THE SENSE-ORGANS + + +The sense-organs are indubitably among the most important and +interesting parts of the human body; they are the organs by means of +which we obtain our knowledge of objects in the surrounding world. +_Nihil est in intellectu quod non prius fuerit in sensu._ They are the +first sources of the life of the soul. There is no other part of the +body in which we discover such elaborate anatomical structures, +co-operating with a definite purpose; and there is no other organ in +which the wonderful and purposive structure seems so clearly to compel +us to admit a Creator and a preconceived plan. Hence we find special +efforts made by dualists to draw our attention here to the “wisdom of +the Creator” and the design visible in his works. As a matter of fact, +you will discover, on mature reflection, that on this theory the +Creator is at bottom only playing the part of a clever mechanic or +watch-maker; all these familiar teleological ideas of Creator and +creation are based, in the long run, on a similar childlike +anthropomorphism. + +However, we must grant that at the first glance the teleological theory +seems to give the simplest and most satisfactory explanation of these +purposive structures. If we merely examine the structure and functions +of the most advanced sense-organs, it seems impossible to explain them +without postulating a creative act. Yet evolution shows us quite +clearly that this popular idea is totally wrong. With its assistance we +discover that the purposive and remarkable sense-organs were developed, +like all other organs, without any preconceived design—developed by the +same mechanical process of natural selection, the same constant +correlation of adaptation and heredity, by which the other purposive +structures in the animal frame were slowly and gradually brought forth +in the struggle for life. + +Like most other Vertebrates, man has six sensory organs, which serve +for eight +different classes of sensations. The skin serves for sensations of +pressure and temperature. This is the oldest, lowest, and vaguest of +the sense-organs; it is distributed over the surface of the body. The +other sensory activities are localised. The sexual sense is bound up +with the skin of the external sexual organs, the sense of taste with +the mucous lining of the mouth (tongue and palate), and the sense of +smell with the mucous lining of the nasal cavity. For the two most +advanced and most highly differentiated sensory functions there are +special and very elaborate mechanical structures—the eye for the sense +of sight, and the ear for the sense of hearing and space (equilibrium). + +Comparative anatomy and physiology teach us that there are no +differentiated sense-organs in the lower animals; all their sensations +are received by the surface of the skin. The undifferentiated +skin-layer or ectoderm of the Gastræa is the simple stratum of cells +from which the differentiated sense-organs of all the Metazoa +(including the Vertebrates) have been evolved. Starting from the +assumption that necessarily only the superficial parts of the body, +which are in direct touch with the outer world, could be concerned in +the origin of sensations, we can see at once that the sense-organs also +must have arisen there. This is really the case. The chief part of all +the sense-organs originates from the skin-sense layer, partly directly +from the horny plate, partly from the brain, the foremost part, of the +medullary tube, after it has separated from the horny plate. If we +compare the embryonic development of the various sense-organs, we see +that they all make their appearance in the simplest conceivable form; +the wonderful contrivances that make the higher sense-organs among the +most remarkable and elaborate structures in the body develop only +gradually. In the phylogenetic explanation of them comparative anatomy +and ontogeny achieve their greatest triumphs. But at first all the +sense-organs are merely parts of the skin in which sensory nerves +expand. These nerves themselves were originally of a homogeneous +character. The different functions or specific energies of the +differentiated sense-nerves were only gradually developed by division +of labour. At the same time, their simple terminal expansions in the +skin were converted into extremely complex organs. + +The great instructiveness of these historical facts in connection with +the life of the soul is not difficult to see. The whole philosophy of +the future will be transformed as soon as psychology takes cognisance +of these genetic phenomena and makes them the basis of its +speculations. When we examine impartially the manuals of psychology +that have been published by the most distinguished speculative +philosophers and are still widely distributed, we are astonished at the +naivete with which the authors raise their airy metaphysical +speculations, regardless of the momentous embryological facts that +completely refute them. Yet the science of evolution, in conjunction +with the great advance of the comparative anatomy and physiology of the +sense-organs, provides the one sound empirical basis of a natural +psychology. + + +Fig.305. Head of a shark (Scyllium), from the ventral side. Fig. +305—Head of a shark (_Scyllium_), from the ventral side. _m_ mouth, _o_ +olfactory pits, _r_ nasal groove, _n_ nasal fold in natural position, +_n′_ nasal fold drawn up. (The dots are openings of the mucous canals.) +(From _Gegenbaur._) + + +In respect of the terminal expansions of the sensory nerves, we can +distribute the human sense-organs in three groups, which correspond to +three stages of development. The first group comprises those organs the +nerves of which spread out quite simply in the free surface of the skin +itself (organs of the sense of pressure, warmth, and sex). In the +second group the nerves spread out in the mucous coat of cavities which +are at first depressions in or invaginations of the skin (organs of the +sense of smell and taste). The third group is formed of the very +elaborate organs, the nerves of which spread out in an internal +vesicle, separated from the skin (organs of the sense of sight, +hearing, and space). + +There is little to be said of the development of the lower +sense-organs. We +have already considered (p. 268) the organ of touch and temperature in +the skin. I need only add that in the corium of man and all the higher +Vertebrates countless microscopic sense-organs develop, but the precise +relation of these to the sensations of pressure or resistance, of +warmth and cold, has not yet been explained. Organs of this kind, in or +on which sensory cutaneous nerves terminate, are the “tactile +corpuscles” (or the Pacinian corpuscles) and end-bulbs. We find similar +corpuscles in the organs of the sexual sense, the male penis and the +female clitoris; they are processes of the skin, the development of +which we will consider later (together with the rest of the sexual +parts, Chapter XXIX). The evolution of the organ of taste, the tongue +and palate, will also be treated later, together with that of the +alimentary canal to which these parts belong (Chapter XXVII). I will +only point out for the present that the mucous coat of the tongue and +palate, in which the gustatory nerve ends, originates from a part of +the outer skin. As we have seen, the whole of the mouth-cavity is +formed, not as a part of the gut-tube proper, but as a pit-like fold in +the outer skin (p. 139). Its mucous lining is therefore formed, not +from the visceral, but from the cutaneous layer, and the taste-cells at +the surface of the tongue and palate are not products of the gut-fibre +layer, but of the skin-sense layer. + + +Figs. 306 and 307. Head of a chick embryo, three days old. Fig. 308. +Head of a chick embryo, four days old, from below. Figs. 309 and 310. +Heads of chick embryos: 309 from the end of the fourth, 310 from the +beginning of the fifth week. Fig. 306 and 307—Head of a chick embryo, +three days old: 2.306 front view, 2.307 from the right. _n_ rudimentary +nose (olfactory pits), _l_ rudimentary eyes (optic pits), _g_ +rudimentary ear (auscultory pit), _v_ fore brain, _gl_ eye-cleft, _o_ +process of upper jaw, _u_ process of lower jaw of the first gill-arch. + + +Fig. 308—Head of a chick embryo, four days old, from below. _n_ nasal +pit, _o_ upper-jaw process of the first gill-arch, _u_ lower-jaw +process of same, _k″_ second gill-arch, _sp_ choroid fissure of eye, +_s_ gullet. + +Fig. 309 and 310—Heads of chick embryos: 309 from the end of the +fourth, 310 from the beginning of the fifth week. Letters as in Fig. +308, except: in inner, an outer, nasal process, _nf_ nasal furrow, _st_ +frontal process, _m_ mouth. (From _Kölliker._). + +This applies also to the mucous lining of the olfactory organ, the +nose. However, the development of this organ is much more interesting. +Although the nose seems superficially to be simple and single, it +really consists, in man and all +other Gnathostomes, of two completely separated halves, the right and +left cavities. They are divided by a vertical partition, so that the +right nostril leads into the right cavity alone and the left nostril +into the left cavity. They open internally (and separately) by the +posterior nasal apertures into the pharynx, so that we can get direct +into the gullet through the nasal passages without touching the mouth. +This is the way the air usually passes in respiration; the mouth being +closed, it goes through the nose into the gullet, and through the +larynx and bronchial tubes into the lungs. The nasal cavities are +separated from the mouth by the horizontal bony palate, to which is +attached behind (as a dependent process) the soft palate with the +uvula. In the upper and hinder parts of the nasal cavities the +olfactory nerve, the first pair of cerebral nerves, expands in the +mucous coat which clothes them. The terminal branches of it spread +partly over the septum (partition), partly on the side walls of the +internal cavities, to which are attached the turbinated bones. These +bones are much more developed in many of the higher mammals than in +man, but there are three of them in all mammals. The sensation of smell +arises by the passage of a current of air containing odorous matter +over the mucous lining of the cavities, and stimulating the olfactory +cells of the nerve-endings. + +Man has all the features which distinguish the olfactory organ of the +mammals from that of the lower Vertebrates. In all essential points the +human nose entirely resembles that of the Catarrhine apes, some of +which have quite a human external nose (compare the face of the +long-nosed apes). However, the first structure of the olfactory organ +in the human embryo gives no indication of the future ample proportions +of our catarrhine nose. It has the form in which we find it permanently +in the fishes—a couple of simple depressions in the skin at the outer +surface of the head. We find these blind olfactory pits in all the +fishes; sometimes they lie near the eyes, sometimes more forward at the +point of the muzzle, sometimes lower down, near the mouth (Fig. 249). + + +Fig.311. Frontal section of the mouth and throat of a human embryo, +neck half-inch long. Fig. 311—Frontal section of the mouth and throat +of a human embryo, neck half-inch long. “Invented” by _Wilhelm His._ +The vertical section (in the frontal plane, from left to right) is so +constructed that we see the nasal pits in the upper third of the figure +and the eyes at the sides: in the middle third the primitive gullet +with the gill-clefts (gill-arches in section); in the lower third the +pectoral cavity with the bronchial tubes and the rudimentary lungs. + + +This first rudimentary structure of the double nose is the same in all +the Gnathostomes; it has no connection with the primitive mouth. But +even in a section of the fishes a connection of this kind begins to +make its appearance, a furrow in the surface of the skin running from +each side of the nasal pit to the nearest corner of the mouth. This +furrow, the nasal groove or furrow (Fig. 305 _r_), is very important. +In many of the sharks, such as the _Scyllium,_ a special process of the +frontal skin, the nasal fold or internal nasal process, is formed +internally over the groove (_n, n″_). In contrast to this the outer +edge of the furrow rises in an “external nasal process.” As the two +processes meet and coalesce over the nasal groove in the Dipneusts and +Amphibia, it is converted into a canal, the nasal canal. Henceforth we +can penetrate from the external pits through the nasal canals direct +into the mouth, which has been formed quite independently. In the +Dipneusts and the lower Amphibia the internal aperture of the nasal +canals lies in front (behind the lips); in the higher Amphibia it is +right behind. Finally, in the three higher classes of Vertebrates the +primary mouth-cavity is divided by the formation of the horizontal +palate-roof into two distinct cavities—the upper (secondary) nasal +cavity and the lower (secondary) mouth-cavity. The nasal cavity in turn +is divided by the construction of the vertical septum into two +halves—right and left. + + +Fig.312. Diagrammatic section of the mouth-nose cavity. Fig. +312—Diagrammatic section of the mouth-nose cavity. While the +palate-plates (_p_) divide the original mouth-cavity into the lower +secondary mouth (_m_) and the upper nasal cavity, the latter in turn is +divided by the vertical partition (_e_) into two halves (_n, n_). (From +_Gegenbaur._) + + +Comparative anatomy shows us to-day, in the series of the double-nosed +Vertebrates, from the fishes up to man, all the different stages in the +development of the nose, which the advanced olfactory organ of the +higher mammals has passed through at various periods in the course of +its phylogeny. It first appears in the embryo of man and the higher +Vertebrates, in which the double fish-nose persists throughout life. At +an early stage, before there is any trace of the characteristic human +face, a pair of small pits are formed in the head over the original +mouth-cavity; these were first discovered by Baer, and rightly called +the “olfactory pits” (Figs. 306 _n_, 307 _n_). These primitive nasal +pits are quite separate from the rudimentary mouth, which also +originates as a pit-like depression in the skin, in front of the blind +fore end of the gut. Both the pair of nasal pits and the single +mouth-pit (Fig. 310 _m_) are clothed with the horny plate. The original +separation of the former from the latter is, however, presently +abolished, a process forming above the mouth-pit—the “frontal process” +(Fig. 309 _st_). Its outer edge rises to the right and left in the +shape of two lateral processes; these are the inner nasal processes or +folds (_in_). Opposite to these a parallel ridge is formed on either +side between the eye and the nasal pit; these are the outer nasal +processes (_an_). Thus between the inner and outer nasal processes a +groove-like depression is formed on either side, which leads from the +nasal pit towards the mouth-pit (_m_); this groove is, as the reader +will guess, the same nasal furrow or groove that we have already seen +in the shark (Fig. 305 _r_). As the parallel edges of the inner and +outer nasal processes bend towards each other and join above the nasal +groove, this is converted into a tube, the primitive nasal canal. Hence +the nose of man and all the other Amniotes consists at this embryonic +stage of a couple of narrow tubes, the nasal canals, which lead from +the outer surface of the forehead into the rudimentary mouth. This +transitory condition resembles that in which we find the nose +permanently in the Dipneusts and Amphibia. + +A cone-shaped structure, which grows from below towards the lower ends +of the two nasal processes and joins with them, plays an important part +in the conversion of the open nasal groove into the closed canal. This +is the upper-jaw process (Figs. 306–310 _o_). Below the mouth-pit are +the gill-arches, which are separated by the gill-clefts. The first of +these gill-arches, and the most important for our purpose, which we may +call the maxillary (jaw) arch, forms the skeleton of the jaws. Above at +the basis a small process grows out of this first gill-arch; this is +the upper-jaw process. The first gill-arch itself develops a cartilage +at one of its inner sides, the “Meckel cartilage” (named after its +discoverer), on the outer surface of which the lower jaw is formed +(Figs. 306–310 _u_). The upper-jaw process forms the chief part of the +skeleton of that jaw, the palate bone, and the pterygoid bone. On its +outer side is afterwards formed the upper-jaw bone, in the narrower +sense, while the middle part of the skeleton of the upper jaw, the +intermaxillary, develops from the foremost part of the frontal process. + +The two upper-jaw processes are of great importance in the further +development of the face. From them is formed, growing into the +primitive mouth-cavity, the important horizontal partition (the palate) +that divides the former into two distinct cavities. The upper cavity, +into which the nasal canals open, now develops into the nasal cavity, +the air-passage and the organ of smell. The lower cavity forms the +permanent secondary mouth (Fig. 312 _m_), the food-passage and the +organ of taste. Both the upper and lower cavities open behind into the +gullet (pharynx). The hard +palate that separates them is formed by the joining of two lateral +halves, the horizontal plates of the two upper-jaw processes, or the +palate-plates (_p_). When these do not, sometimes, completely join in +the middle, a longitudinal cleft remains, through which we can +penetrate from the mouth straight into the nasal cavity. This is the +malformation known as “wolf’s throat.” “Hare-lip” is the lesser form of +the same defect. At the same time as the horizontal partition of the +hard palate a vertical partition is formed by which the single nasal +cavity is divided into two sections—a right and left half (Fig. 312 _n, +n_). + + +Figs. 313 and 314. Upper part of the body of a human embryo, two-thirds +of an inch long, of the sixth week; Fig. 313 from the left, Fig. 314 +from the front. Figs. 313 and 314—Upper part of the body of a human +embryo, two-thirds of an inch long, of the sixth week; Fig. 313 from +the left, Fig. 314 from the front. The origin of the nose and the upper +lip from two lateral and originally separate halves can be clearly +seen. Nose and upper lip are large in proportion to the rest of the +face, and especially to the lower lip. (From _Kollmann._) + + +The double nose has now acquired the characteristic form that man +shares with the other mammals. Its further development is easy to +follow; it consists of the formation of the inner and outer processes +of the walls of the two cavities. The external nose is not formed until +long after all these essential parts of the internal organ of smell. +The first traces of it in the human embryo are found about the middle +of the second month (Figs. 313–316). As can be seen in any human embryo +during the first month, there is at first no trace of the external +nose. It only develops afterwards from the foremost nasal part of the +primitive skull, growing forwards from behind. The characteristic human +nose is formed very late. Much stress is at times laid on this organ as +an exclusive privilege of man. But there are apes that have similar +noses, such as the long-nosed ape. + +The evolution of the eye is not less interesting and instructive than +that of the nose. Although this noblest of the sensory organs is one of +the most elaborate and purposive on account of its optic perfection and +remarkable structure, it nevertheless develops, without preconceived +design, from a simple process of the outer germinal layer. The +fully-formed human eye is a round capsule, the eye-ball (Fig. 317). +This lies in the bony cavity of the skull, surrounded by protective fat +and motor muscles. The greater part of it is taken up with a +semi-fluid, transparent gelatinous substance, the _corpus vitreum._ The +crystalline lens is fitted into the anterior surface of the ball (Fig. +317 _l_). It is a lenticular, bi-convex, transparent body, the most +important of the refractive media in the eye. Of this group we have, +besides the corpus vitreum and the lens, the watery fluid (_humor +aqueus_) that is found in front of the lens (at the letter _m_ in Fig. +317). These three transparent refractive media, by which the rays of +light that +enter the eye are broken up and re-focussed, are enclosed in a solid +round capsule, composed of several different coats, something like the +concentric layers of an onion. The outermost and thickest of these +envelopes is the white sclerotic coat of the eye. It consists of tough +white connective tissue. In front of the lens a circular, +strongly-curved, transparent plate is fitted into the sclerotic, like +the glass of a watch—the _cornea_ (_b_). At its outer surface the +cornea is covered with a very thin layer of the epidermis; this is +known as the _conjunctiva._ It goes from the cornea over the inner +surface of the eye-lids, the upper and lower folds which we draw over +the eye in closing it. At the inner corner of the eye we have a +rudimentary organ in the shape of the relic of a third (inner) eye-lid, +which is greatly developed, as “nictitating (winking) membrane,” in the +lower Vertebrates (p. 5). Underneath the upper eye-lid are the +lachrymal glands, the product of which, the lachrymal fluid, keeps the +outer surface of the eye smooth and clean. + + +Fig.315. Face of a human embryo, seven weeks old. Fig. 315—Face of a +human embryo, seven weeks old. (From _Kollmann._) Joining of the nasal +processes (_e_ outer, _i_ inner) with the upper-jaw process (_o_), _n_ +nasal wall, _a_ ear-opening. + + +Immediately under the sclerotic we find a very delicate, dark-red +membrane, very rich in blood-vessels—the _choroid coat_—and inside this +the retina (_o_), the expansion of the optic nerve (_i_). The latter is +the second cerebral nerve. It proceeds from the optic thalami (the +second cerebral vesicle) to the eye; penetrates its outer envelopes, +and then spreads out like a net between the choroid and the corpus +vitreum. Between the retina and the choroid there is a very delicate +membrane, which is usually (but wrongly) associated with the latter. +This is the black pigment-membrane (_n_). It consists of a single +stratum of graceful, hexagonal, regularly-joined cells, full of +granules of black colouring matter. This pigment membrane clothes, not +only the inner surface of the choroid proper, but also the hind surface +of its anterior muscular continuation, which covers the edge of the +lens in front as a circular membrane, and arrests the rays of light at +the sides. This is the well-known _iris_ of the eye (_h_), coloured +differently in different individuals (blue, grey, brown, etc.); it +forms the anterior border of the choroid. The circular opening that is +left in the middle is the _pupil,_ through which the rays of light +penetrate into the eye. At the point where the iris leaves the anterior +border of the choroid proper the latter is very thick, and forms a +delicate crown of folds (_g_), which surrounds the edge of the lens +with about seventy large and many smaller rays (_corona ciliaris._) + + +Fig.316. Face of a human embryo, eight weeks old. Fig. 316—Face of a +human embryo, eight weeks old. (From _Ecker._) + + +At a very early stage a couple of pear-shaped vesicles develop from the +foremost part of the first cerebral vesicle in the embryo of man and +the other Craniotes (Figs. 155 _a_, 297 _au_). These growths are the +primary optic vesicles. They are at first directed outwards and +forwards, but presently grow downward, so that, after the complete +separation of the five cerebral vesicles, they lie at the base of the +intermediate brain. The inner cavities of these pear-shaped vesicles, +which soon attain a considerable size, are openly connected with the +ventricle of the intermediate brain by their hollow stems. They are +covered externally by the epidermis. + +At the point where this comes into direct contact with the most curved +part of the primary optic vesicle there is a thickening (_l_) and also +a depression (_o_) of the horny plate (Fig. 318, _I_). This pit, which +we may call the lens-pit, is converted into a closed sac, the thick- +walled lens-vesicle (_2, l_), the thick edges of the pit joining +together above it. In the same way in which the medullary tube +separates from the outer germinal layer, we now see this lens-sac sever +itself entirely from the horny plate (_h_), its source of origin. The +hollow of the sac is afterwards filled with the cells of its thick +walls, and thus we get the solid crystalline lens. This is, therefore, +a purely epidermic structure. Together with the lens the small +underlying piece of corium-plate also separates from the skin. + +As the lens separates from the corneous plate and grows inwards, it +necessarily hollows out the contiguous primary optic vesicle (Fig. 318, +_1–3_). This is done in just the same way as the invagination of the +blastula, which gives rise to the gastrula in the amphioxus (Fig. 38 +_C–F_). In both cases the hollowing of the closed vesicle on one side +goes so far that at last the inner, folded part touches the outer, not +folded part, and the cavity disappears. As in the gastrula the first +part is converted into the entoderm and the latter into the ectoderm, +so in the invagination of the primary optic vesicle the retina (_r_) is +formed from the first (inner) part, and the black pigment membrane +(_u_) from the latter (outer, non-invaginated) part. The hollow stem of +the primary optic vesicle is converted into the optic nerve. The lens +(_l_), which has so important a part in this process, lies at first +directly on the invaginated part, or the retina (_r_). But they soon +separate, a new structure, the corpus vitreum (_gl_), growing between +them. While the lenticular sac is being detached and is causing the +invagination of the primary optic vesicle, another invagination is +taking place from below; this proceeds from the superficial part of the +skin-fibre layer—the corium of the head. Behind and under the lens a +last-shaped process rises from the cutis-plate (Fig. 319 _g_), hollows +out the cup-shaped optic vesicle from below, and presses between the +lens (_l_) and the retina (_i_). In this way the optic vesicle acquires +the form of a hood. + + +Fig.317. The human eye in section. Fig. 317—The human eye in section. +_a_ sclerotic coat, _b_ cornea, _c_ conjunctiva, _d_ circular veins of +the iris, _e_ choroid coat, _f_ ciliary muscle, _g_ corona ciliaris, +_h_ iris, _i_ optic nerve, _k_ anterior border of the retina, _l_ +crystalline lens, _m_ inner covering of the cornea (aqueous membrane), +_n_ pigment membrane, _o_ retina, _p_ Petit’s canal, _q_ yellow spot of +the retina. (From _Helmholtz._) + + +Finally, a complete fibrous envelope, the fibrous capsule of the +eye-ball, is formed about the secondary optic vesicle and its stem (the +secondary optic nerve). It originates from the part of the head-plates +which immediately encloses the eye. This fibrous envelope takes the +form of a closed round vesicle, surrounding the whole of the ball and +pushing between the lens and the horny plate at its outer side. The +round wall of the capsule soon divides into two different membranes by +surface-cleavage. The inner membrane becomes the choroid or vascular +coat, and in front the ciliary corona and iris. The outer membrane is +converted into the white protective or sclerotic coat—in front, the +transparent cornea. The eye is now formed in all its essential parts. +The further development—the complicated differentiation and composition +of the various parts—is a matter of detail. + +The chief point in this remarkable evolution of the eye is the +circumstance that the optic nerve, the retina, and the pigment membrane +originate really from a part of the brain—an outgrowth of the +intermediate brain—while the lens, the chief refractive body, develops +from the outer skin. From the skin—the horny +plate—also arises the delicate conjunctiva, which afterwards covers the +outer surface of the eyeball. The lachrymal glands are ramified growths +from the conjunctiva (Fig. 286). All these important parts of the eye +are products of the outer germinal layer. The remaining parts—the +corpus vitreum (with the vascular capsule of the lens), the choroid +(with the iris), and the sclerotic (with the cornea)—are formed from +the middle germinal layer. + + +Fig.318. Eye of the chick embryo in longitudinal section (1. from an +embryo sixty-five hours old; 2. from a somewhat older embryo; 3. from +an embryo four days old). Fig. 318—Eye of the chick embryo in +longitudinal section (_1._ from an embryo sixty-five hours old; _2._ +from a somewhat older embryo; _3._ from an embryo four days old). _h_ +horny plate, _o_ lens-pit, _l_ lens (in _1._ still part of the +epidermis, in _2._ and _3._ separated from it), _x_ thickening of the +horny plate at the point where the lens has severed itself, _gl_ corpus +vitreum, _r_ retina, _u_ pigment membrane. (From _Remak._) + + +The outer protection of the eye, the eye-lids, are merely folds of the +skin, which are formed in the third month of human embryonic life. In +the fourth month the upper eye-lid reaches the lower, and the eye +remains covered with them until birth. As a rule, they open wide +shortly before birth (sometimes only after birth). Our craniote +ancestors had a third eye-lid, the nictitating membrane, which was +drawn over the eye from its inner angle. It is still found in many of +the Selachii and Amniotes. In the apes and man it has degenerated, and +there is now only a small relic of it at the inner corner of the eye, +the semi-lunar fold, a useless rudimentary organ (cf. p. 32). The apes +and man have also lost the Harderian gland that opened under the +nictitating membrane; we find this in the rest of the mammals, and the +birds, reptiles, and amphibia. + +The peculiar embryonic development of the vertebrate eye does not +enable us to draw any definite conclusions as to its obscure phylogeny; +it is clearly cenogenetic to a great extent, or obscured by the +reduction and curtailment of its original features. It is probable that +many of the earlier stages of its phylogeny have disappeared without +leaving a trace. It can only be said positively that the peculiar +ontogeny of the complicated optic apparatus in man follows just the +same laws as in all the other Vertebrates. Their eye is a part of the +fore brain, which has grown forward towards the skin, not an original +cutaneous sense-organ, as in the Invertebrates. + + +Fig.319. Horizontal transverse section of the eye of a human embryo, +four weeks old. Fig. 319—Horizontal transverse section of the eye of a +human embryo, four weeks old. (From _Kölliker._) _t_ lens (the dark +wall of which is as thick as the diameter of the central cavity), _g_ +corpus vitreum (connected by a stem, _g,_ with the corium), _v_ +vascular loop (pressing behind the lens inside the corpus vitreum by +means of this stem _g_), _i_ retina (inner thicker, invaginated layer +of the primary optic vesicle), _a_ pigment membrane (outer, thin, +non-invaginated layer of same), _h_ space between retina and pigment +membrane (remainder of the cavity of the primary optic vesicle). + + +The vertebrate ear resembles the eye and nose in many important +respects, but is different in others, in its development. The +auscultory organ in the fully-developed man is like that of the other +mammals, and especially the apes, in the main features. As in them, it +consists of two chief parts—an apparatus for conducting sound (external +and middle ear) and an apparatus for the sensation of sound (internal +ear). The external ear opens in the shell at the side of the head (Fig. +320 _a_). From this point the external passage (_b_), about an inch in +length, leads into the head. The inner end of it is closed by the +tympanum, a vertical, but not quite upright, thin membrane of an oval +shape (_c_). This tympanum separates the external passage from the +tympanic cavity (_d_). This is a small cavity, filled with air, in the +temporal bone; it is connected with the mouth by a special tube. This +tube is rather longer, but much narrower, than the outer passage, leads +inwards obliquely from the anterior wall of the tympanic cavity, and +opens in the throat below, behind the nasal +openings. It is called the Eustachian tube (_e_); it serves to equalise +the pressure of the air within the tympanic cavity and the outer +atmosphere that enters by the external passage. Both the Eustachian +tube and the tympanic cavity are lined with a thin mucous coat, which +is a direct continuation of the mucous lining of the throat. Inside the +tympanic cavity there are three small bones which are known (from their +shape) as the hammer, anvil, and stirrup (Fig. 320, _f, g, h_). The +hammer (_f_) is the outermost, next to the tympanum. The anvil (_g_) +fits between the other two, above and inside the hammer. The stirrup +(_h_) lies inside the anvil, and touches with its base the outer wall +of the internal ear, or auscultory vesicle. All these parts of the +external and middle ear belong to the apparatus for conducting sound. +Their chief task is to convey the waves of sound through the thick wall +of the head to the inner-lying auscultory vesicle. They are not found +at all in the fishes. In these the waves of sound are conveyed directly +by the wall of the head to the auscultory vesicle. + + +Fig.320. The human ear (left ear, seen from the front). Fig. 320—The +human ear (left ear, seen from the front), _a_ shell of ear, _b_ +external passage, _c_ tympanum, _d_ tympanic cavity, _e_ Eustachian +tube, _f, g, h_ the three bones of the ear (_f_ hammer, _g_ anvil, _h_ +stirrup), _i_ utricle, _k_ the three semi-circular canals, _l_ the +sacculus, _m_ cochlea, _n_ auscultory nerve. + + +Fig. 321. The bony labyrinth of the human ear (left side). Fig. +321—The bony labyrinth of the human ear (left side). _a_ vestibulum, +_b_ cochlea, _c_ upper canal, _d_ posterior canal, _e_ outer canal, _f_ +oval fenestra, _g_ round fenestra. (From _Meyer._) + + +The internal apparatus for the sensation of sound, which receives the +waves of sound from the conducting apparatus, consists in man and all +other mammals of a closed auscultory vesicle filled with fluid and an +auditory nerve, the ends of which expand over the wall of this vesicle. +The vibrations of the sound-waves are conveyed by these media to the +nerve-endings. In the labyrinthic water that fills the auscultory +vesicle there are small stones at the points of entry of the acoustic +nerves, which are composed of groups of microscopic calcareous crystals +(otoliths). The auscultory organ of most of the Invertebrates has +substantially the same composition. It usually consists of a closed +vesicle, filled with fluid, and containing otoliths, with the acoustic +nerve expanding on its wall. But, while the auditory vesicle is usually +of a simple round or oval shape in the Invertebrates, it has in the +Vertebrates a special and curious structure, the labyrinth. This +thin-membraned labyrinth is enclosed in a bony capsule of the same +shape, the osseous labyrinth (Fig. 321), and this lies in the middle of +the petrous bone of the skull. The labyrinth is divided into two +vesicles in all the Gnathostomes. The larger one is called the +_utriculus,_ and has three arched appendages, called the “semi-circular +canals” (_c, d, e_). The smaller vesicle is called the sacculus, and is +connected with a peculiar appendage, with (in man and the higher +mammals) a spiral form something like a snail’s shell, and therefore +called the _cochlea_ (= snail, _b_). On the thin wall of this delicate +labyrinth the acoustic nerve, which comes from the after-brain, spreads +out in most elaborate fashion. It divides into two main branches—a +cochlear nerve (for the cochlea) and a vestibular nerve (for the rest +of the labyrinth). The former seems to have more to do with the +quality, the latter with the quantity, of the acoustic sensations. +Through the cochlear nerves we learn the height and timbre, through the +vestibular nerves the intensity, of tones. + +The first structure of this highly elaborate organ is very simple in +the embryo of man and all the other Craniotes; it is a +pit-like depression in the skin. At the back part of the head at both +sides, near the after brain, a small thickening of the horny plate is +formed at the upper end of the second gill-cleft (Fig. 322 _A fl_). +This sinks into a sort of pit, and severs from the epidermis, just as +the lens of the eye does. In this way is formed at each side, directly +under the horny plate of the back part of the head, a small vesicle +filled with fluid, the primitive auscultory vesicle, or the primary +labyrinth. As it separates from its source, the horny plate, and +presses inwards and backwards into the skull, it changes from round to +pear-shaped (Figs. 322 _B lv_, 323 _o_). The outer part of it is +lengthened into a thin stem, which at first still opens outwards by a +narrow canal. This is the labyrinthic appendage (Fig. 322 _lr_). In the +lower Vertebrates it develops into a special cavity filled with +calcareous crystals, which remains open permanently in some of the +primitive fishes, and opens outwards in the upper part of the skull. +But in the mammals the labyrinthic appendage degenerates. In these it +has only a phylogenetic interest as a rudimentary organ, with no actual +physiological significance. The useless relic of it passes through the +wall of the petrous bone in the shape of a narrow canal, and is called +the vestibular aqueduct. + + +Fig.322. Development of the auscultory labyrinth of the chick, in five +successive stages (A to E). Fig. 322—Development of the auscultory +labyrinth of the chick, in five successive stages (_A–E_). (Vertical +transverse sections of the skull.) _fl_ auscultory pits, _lv_ +auscultory vesicles, _lr_ labyrinthic appendage, _c_ rudimentary +cochlea, _csp_ posterior canal, _cse_ external canal, _jv_ jugular +vein. (From _Reissner._) + + +It is only the inner and lower bulbous part of the separated auscultory +vesicle that develops into the highly complex and differentiated +structure that is afterwards known as the secondary labyrinth. This +vesicle divides at an early stage into an upper and larger and a lower +and smaller section. From the one we get the _utriculus_ with the +semi-circular canals; from the other the _sacculus_ and the cochlea +(Fig. 320 _c_). The canals are formed in the shape of simple pouch-like +involutions of the utricle (_cse_ and _csp_). The edges join together +in the middle part of each fold, and separate from the utricle, the two +ends remaining in open connection with its cavity. All the Gnathostomes +have these three canals like man, whereas among the Cyclostomes the +lampreys have only two and the hag-fishes only one. The very complex +structure of the cochlea, one of the most elaborate and wonderful +outcomes of adaptation in the mammal body, develops originally in very +simple fashion as a flask-like projection from the sacculus. As Hasse +and Retzius have pointed out, we find the successive ontogenetic stages +of its growth represented permanently in the series of the higher +Vertebrates. The cochlea is wanting even in the Monotremes, and is +restricted to the rest of the mammals and man. + +The auditory nerve, or eighth cerebral nerve, expands with one branch +in the cochlea, and with the other in the remaining parts of the +labyrinth. This nerve is, as Gegenbaur has shown, the sensory dorsal +branch of a cerebro-spinal nerve, the motor ventral branch of which +acts for the muscles of the face (_nervus facialis_). It has therefore +originated phylogenetically from an ordinary cutaneous nerve, and so is +of quite different origin from the optic and olfactory nerves, which +both represent direct outgrowths of the brain. In this respect the +auscultory organ is essentially different from the organs of sight and +smell. The acoustic nerve is formed from ectodermic cells of the hind +brain, and develops from the nervous structure that appears at its +dorsal limit. On the other hand, all the membranous, cartilaginous, and +osseous coverings of the labyrinth are formed from the mesodermic +head-plates. + +The apparatus for conducting sound which we find in the external and +middle ear of mammals develops quite separately from the apparatus for +the sensation of sound. It is both phylogenetically and ontogenetically +an independent secondary formation, a later accession +to the primary internal ear. Nevertheless, its development is not less +interesting, and is explained with the same ease by comparative +anatomy. In all the fishes and in the lowest Vertebrates there is no +special apparatus for conducting sound, no external or middle ear; they +have only a labyrinth, an internal ear, which lies within the skull. +They are without the tympanum and tympanic cavity, and all its +appendages. From many observations made in the last few decades it +seems that many of the fishes (if not all) cannot distinguish tones; +their labyrinth seems to be chiefly (if not exclusively) an organ for +the sense of space (or equilibrium). If it is destroyed, the fishes +lose their balance and fall. In the opinion of recent physiologists +this applies also to many of the Invertebrates (including the nearer +ancestors of the Vertebrates). The round vesicles which are considered +to be their auscultory vesicles, and which contain an otolith, are +supposed to be merely organs of the sense of space (“static vesicles or +statocysts”). + + +Fig.323. Primitive skull of the human embryo, four weeks old, vertical +section, left half seen internally. Fig. 323—Primitive skull of the +human embryo, four weeks old, vertical section, left half seen +internally. _v, z, m, h, n_ the five pits of the cranial cavity, in +which the five cerebral vesicles lie (fore, intermediate, middle, hind, +and after brains), _o_ pear-shaped primary auscultory vesicle +(appearing through), _a_ eye (appearing through), _no_ optic nerve, _p_ +canal of the hypophysis, _t_ central prominence of the skull. (From +_Kölliker._) + + +The middle ear makes its first appearance in the amphibian class, where +we find a tympanum, tympanic cavity, and Eustachian tube; these +animals, and all terrestrial Vertebrates, certainly have the faculty of +hearing. All these essential parts of the middle ear originate from the +first gill-cleft and its surrounding part; in the Selachii this remains +throughout life an open squirting-hole, and lies between the first and +second gill-arch. In the embryo of the higher Vertebrates it closes up +in the centre, and thus forms the tympanic membrane. The outlying +remainder of the first gill-cleft is the rudiment of the external +meatus. From its inner part we get the tympanic cavity, and, further +inward still, the Eustachian tube. Connected with this is the +development of the three bones of the mammal ear from the first two +gill-arches; the hammer and anvil are formed from the first, the +stirrup from the upper end of the second, gill-arch. + + +Fig.324. The rudimentary muscles of the ear in the human skull. Fig. +324—The rudimentary muscles of the ear in the human skull. _a_ raising +muscle (_M. attollens_), _b_ drawing muscle (_M. attrahens_), _c_ +withdrawing muscle (_M. retrahens_), _d_ large muscle of the helix (_M. +helicis major_), _e_ small muscle of the helix (_M. helicis minor_), +_f_ muscle of the angle of the ear (_M. tragicus_), _g_ anti-angular +muscle (_M. antitragicus_). (From _H. Meyer._) + + +Finally, the shell (pinna or concha) and external meatus (passage to +the tympanum) of the outer ear are developed in a very simple fashion +from the skin that borders the external aperture of the first +gill-cleft. The shell rises in the shape of a circular fold of the +skin, in which cartilage and muscles are afterwards formed (Figs. 313, +315). This organ is only found in the mammalian class. It is very +rudimentary in the lowest section, the Monotremes. In the others it is +found at very different stages of development, and sometimes of +degeneration. It is degenerate in most of the aquatic mammals. The +majority of them have lost it altogether—for instance, the walruses and +whales and most of the seals. On the other hand, the pinna is well +developed in the great majority of the Marsupials and Placentals; it +receives and collects the waves of sound, and is equipped with a very +elaborate muscular apparatus, by means of which the pinna +can be turned freely in any direction and its shape be altered. It is +well known how readily domestic animals—horses, cows, dogs, hares, +etc.—point their ears and move them in different directions. Most of +the apes do the same, and our earlier ape ancestors were also able to +do it. But our later simian ancestors, which we have in common with the +anthropoid apes, abandoned the use of these muscles, and they gradually +became rudimentary and useless. However, we possess them still (Fig. +324). In fact, some men can still move their ears a little backward and +forward by means of the drawing and withdrawing muscles (_b_ and _c_); +with practice this faculty can be much improved. But no man can now +lift up his ears by the raising muscle (_a_), or change the shape of +them by the small inner muscles (_d, e, f, g_). These muscles were very +useful to our ancestors, but are of no consequence to us. This applies +to most of the anthropoid apes as well. + +We also share with the higher anthropoid apes (gorilla, chimpanzee, and +orang) the characteristic form of the human outer ear, especially the +folded border, the helix and the lobe. The lower apes have pointed +ears, without folded border or lobe, like the other mammals. But Darwin +has shown that at the upper part of the folded border there is in many +men a small pointed process, which most of us do not possess. In some +individuals this process is well developed. It can only be explained as +the relic of the original point of the ear, which has been turned +inwards in consequence of the curving of the edge. If we compare the +pinna of man and the various apes in this respect, we find that they +present a connected series of degenerate structures. In the common +catarrhine ancestors of the anthropoids and man the degeneration set in +with the folding together of the pinna. This brought about the helix of +the ear, in which we find the significant angle which represents the +relic of the salient point of the ear in our earlier simian ancestors. +Here again, therefore, comparative anatomy enables us to trace with +certainty the human ear to the similar, but more developed, organ of +the lower mammals. At the same time, comparative physiology shows that +it was a more or less useful implement in the latter, but it is quite +useless in the anthropoids and man. The conducting of the sound has +scarcely been affected by the loss of the pinna. We have also in this +the explanation of the extraordinary variety in the shape and size of +the shell of the ear in different men; in this it resembles other +rudimentary organs. + + + + +Chapter XXVI. +EVOLUTION OF THE ORGANS OF MOVEMENT + + +The peculiar structure of the locomotive apparatus is one of the +features that are most distinctive of the vertebrate stem. The chief +part of this apparatus is formed, as in all the higher animals, by the +active organs of movement, the muscles; in consequence of their +contractility they have the power to draw up and shorten themselves. +This effects the movement of the various parts of the body, and thus +the whole body is conveyed from place to place. But the arrangement of +these muscles and their relation to the solid skeleton are different in +the Vertebrates from the Invertebrates. + +In most of the lower animals, especially the Platodes and Vermalia, we +find that the muscles form a simple, thin layer of flesh immediately +underneath the skin. This muscular layer is very closely connected with +the skin itself; it is the same in the Mollusc stem. Even in the large +division of the Articulates, the classes of crabs, spiders, myriapods, +and + +insects, we find a similar feature, with the difference that in this +case the skin forms a solid armour—a rigid cutaneous skeleton made of +chitine (and often also of carbonate of lime). This external chitine +coat undergoes a very elaborate articulation both on the trunk and the +limbs of the Articulates, and in consequence the muscular system also, +the contractile fibres of which are attached inside the chitine tubes, +is highly articulated. The Vertebrates form a direct contrast to this. +In these alone a solid internal skeleton is developed, of cartilage or +bone, to which the muscles are attached. This bony skeleton is a +complex lever apparatus, or _passive_ apparatus of movement. Its rigid +parts, the arms of the levers, or the bones, are brought together by +the actively mobile muscles, as if by drawing-ropes. This admirable +locomotorium, especially its solid central axis, the vertebral column, +is a special feature of the Vertebrates, and has given the name to the +group. + + +Fig.325. The human skeleton from the right. Fig. 326. The human +skeleton. Front. Fig. 325—The human skeleton. From the right. Fig. +326—The human skeleton. Front. + + +Fig.327. The human vertebral column (standing upright, from the right +side). Fig. 327—The human vertebral column (standing upright, from the +right side). (From _H. Meyer._) + + +Fig.328. A piece of the axial rod (chorda dorsalis), from a sheep +embryo. Fig. 328—A piece of the axial rod (_chorda dorsalis_), from a +sheep embryo. _a_ cuticular sheath, _b_ cells. (From _Kölliker._) + + +In order to get a clear idea of the chief features of the development +of the human skeleton, we must first examine its composition in the +adult frame (Fig. 325, the human skeleton seen from the right; Fig. +326, front view of the whole skeleton). As in other mammals, we +distinguish first between the axial or dorsal skeleton and the skeleton +of the limbs. The axial skeleton consists of the vertebral column (the +skeleton of the trunk) and the skull (skeleton of the head); the latter +is a peculiarly modified part of the former. As appendages of the +vertebral column we have the ribs, and of the skull we have the hyoid +bone, the lower jaw, and the other products of the gill-arches. + +The skeleton of the limbs or extremities is composed of two groups of +parts—the skeleton of the extremities proper and the zone-skeleton, +which connects these with the vertebral column. The zone-skeleton of +the arms (or fore legs) is the shoulder-zone; the zone-skeleton of the +legs (or hind legs) is the pelvic zone. + +The vertebral column (Fig. 327) in man is composed of thirty-three to +thirty-five ring-shaped bones in a continuous series (above each other, +in man’s upright position). These _vertebræ_ are separated from each +other by elastic ligaments, and at the same time connected by joints, +so that the whole column forms a firm and solid, but flexible and +elastic, axial skeleton, moving freely in all directions. The vertebræ +differ in shape and connection at the various parts of the trunk, and +we distinguish the following groups in the series, beginning at the +top: Seven cervical vertebræ, twelve dorsal vertebræ, five lumbar +vertebræ, five sacral vertebræ, and four to six caudal vertebræ. The +uppermost, or those next to the skull, are the cervical vertebræ (Fig. +327); they have a hole in each of the lateral processes. There are +seven of these vertebræ in man and almost all the other mammals, even +if the neck is as long as that of the camel or giraffe, or as short as +that of the mole or hedgehog. This constant number, which has few +exceptions (due to adaptation), is a strong proof of the common descent +of the mammals; it can only be explained by faithful heredity from a +common stem-form, a primitive mammal with seven cervical vertebræ. If +each species had been created separately, it would have been better to +have given the long-necked mammals more, and the short-necked animals +less, cervical vertebræ. Next to these come the dorsal (or pectoral) +vertebræ, which number twelve to thirteen (usually twelve) in man and +most of the other mammals. Each dorsal vertebra (Fig. 165) has at the +side, connected by joints, a couple of ribs, long bony arches that lie +in and protect the wall of the chest. The twelve pairs of ribs, +together with the connecting intercostal muscles and the sternum, which +joins the ends of the right and left ribs in front, form the chest +(_thorax_). In this strong and elastic frame are the lungs, and between +them the heart. Next to the dorsal vertebræ comes a short but stronger +section of the column, formed of five large vertebræ. These are the +lumbar vertebræ (Fig. 166); they have no ribs and no holes in the +transverse processes. To these succeeds the sacral bone, which is +fitted between the two halves of the pelvic zone. The sacrum is formed +of five vertebræ, completely blended together. Finally, we have at the +end a small rudimentary caudal column, the _coccyx._ This consists of a +varying number (usually four, more rarely three, or five or six) of +small degenerated vertebræ, and is a useless rudimentary organ with no +actual physiological significance. Morphologically, however, it is of +great interest as an irrefragable proof of the descent of man and the +anthropoids from long-tailed apes. On no other theory can we explain +the existence of this rudimentary tail. In the earlier stages of +development the tail of the human embryo protrudes considerably. It +afterwards atrophies; but the relic of the atrophied caudal vertebræ +and of the rudimentary muscles that once moved it remains permanently. +Sometimes, in fact, the external tail is preserved. The older +anatomists say that the tail is usually one vertebra longer in the +human female than in the male (or four against five); Steinbach says it +is the reverse. + + +Fig.329. Three dorsal vertebræ, from a human embryo, eight weeks old, +in lateral longitudinal section. Fig. 329—Three dorsal vertebræ, from a +human embryo, eight weeks old, in lateral longitudinal section. _v_ +cartilaginous vertebral body, _li_ inter-vertebral disks, _ch_ chorda. +(From _Kölliker._) + + +Fig.330. A dorsal vertebra of the same embryo, in lateral transverse +section. Fig. 330—A dorsal vertebra of the same embryo, in lateral +transverse section. _cv_ cartilaginous vertebral body, _ch_ chorda, +_pr_ transverse process, _a_ vertebral arch (upper arch), _c_ upper end +of the rib (lower arch). (From _Kölliker._) + + +In the human vertebral column there are usually thirty-three vertebræ. +It is interesting to find, however, that the number often changes, one +or two vertebræ dropping out or an additional one appearing. Often, +also, a mobile rib is formed at the last cervical or the first lumbar +vertebra, so that there are then thirteen dorsal vertebræ, besides six +cervical and four lumbar. In this way the contiguous vertebræ of the +various sections of the column may take each other’s places. + +In order to understand the embryology of the human vertebral column we +must first carefully consider the shape and connection of the vertebræ. +Each vertebra has, in general, the shape of a seal-ring (Figs. +164–166). The thicker portion, which is turned towards the ventral +side, is called the body of the vertebra, and forms a short osseous +disk; the thinner part forms a semi-circular arch, the _vertebral +arch,_ and is turned towards the back. The arches of the successive +vertebræ are connected by thin intercrural ligaments in such a way that +the cavity they collectively enclose represents a long canal. In this +vertebral canal we find the trunk part of the central nervous system, +the spinal cord. Its head part, the brain, is enclosed by the skull, +and the skull itself is merely the uppermost part of the vertebral +column, distinctively modified. The base or ventral side of the +vesicular cranial capsule corresponds originally to a number of +developed vertebral bodies; its vault or dorsal side to their combined +upper vertebral arches. + +While the solid, massive bodies of the vertebræ represent the real +central axis of the skeleton, the dorsal arches serve to protect the +central marrow they enclose. But similar arches develop on the ventral +side for the protection of the viscera in the breast and belly. These +lower or +ventral vertebral arches, proceeding from the ventral side of the +vertebral bodies, form, in many of the lower Vertebrates, a canal in +which the large blood-vessels are enclosed on the lower surface of the +vertebral column (aorta and caudal vein). In the higher Vertebrates the +majority of these vertebral arches are lost or become rudimentary. But +at the thoracic section of the column they develop into independent +strong osseous arches, the ribs (_costæ_). In reality the ribs are +merely large and independent lower vertebral arches, which have lost +their original connection with the vertebral bodies. + + +Fig.331. Intervertebral disk of a new-born infant, transverse section. +Fig. 331—Intervertebral disk of a new-born infant, transverse section. +_a_ rest of the chorda. (From _Kölliker._) + + +If we turn from this anatomic survey of the composition of the column +to the question of its development, I may refer the reader to earlier +pages with regard to the first and most important points (pp. 145–148). +It will be remembered that in the human embryo and that of the other +vertebrates we find at first, instead of the segmented column, only a +simple unarticulated cartilaginous rod. This solid but flexible and +elastic rod is the axial rod (or the _chorda dorsalis_). In the lowest +Vertebrate, the Amphioxus, it retains this simple form throughout life, +and permanently represents the whole internal skeleton (Fig. 210 _i_). +In the Tunicates, also, the nearest Invertebrate relatives of the +Vertebrates, we meet the same chorda—transitorily in the passing larva +tail of the Ascidia, permanently in the Copelata (Fig. 225 _c_). +Undoubtedly both the Tunicates and Acrania have inherited the chorda +from a common unsegmented stem-form; and these ancient, long-extinct +ancestors of all the chordonia are our hypothetical Prochordonia. + +Long before there is any trace of the skull, limbs, etc., in the embryo +of man or any of the higher Vertebrates—at the early stage in which the +whole body is merely a sole-shaped embryonic shield—there appears in +the middle line of the shield, directly under the medullary furrow, the +simple chorda. (Cf. Figs. 131–135 _ch_). It follows the long axis of +the body in the shape of a cylindrical axial rod of elastic but firm +composition, equally pointed at both ends. In every case the chorda +originates from the dorsal wall of the primitive gut; the cells that +compose it (Fig. 328 _b_) belong to the entoderm (Figs. 216–221). At an +early stage the chorda develops a transparent structureless sheath, +which is secreted from its cells (Fig. 328 _a_). This _chordalemma_ is +often called the “inner chorda-sheath,” and must not be confused with +the real external sheath, the mesoblastic perichorda. + + +Fig. 332. Human skull. Fig. 332—Human skull. + + +But this unsegmented primary axial skeleton is soon replaced by the +segmented secondary axial skeleton, which we know as the vertebral +column. The provertebral plates (Fig. 124 _s_) differentiate from the +innermost, median part of the visceral layer of the cœlom-pouches at +each side of the chorda. As they grow round the chorda and enclose it +they form the skeleton plate or skeletogenetic layer—that is to say, +the skeleton-forming stratum of cells, which provides the mobile +foundation of the permanent vertebral column and skull (scleroblast). +In the head-half of the embryo the skeletal plate remains a continuous, +simple, undivided layer of tissue, and presently enlarges into a +thin-walled capsule enclosing the brain, the primordial skull. In the +trunk-half the provertebral +plate divides into a number of homogeneous, cubical, successive pieces; +these are the several primitive vertebræ. They are not numerous at +first, but soon increase as the embryo grows longer (Figs. 153–155). + + +Fig. 333. Skull of a new-born child. Fig. 333—Skull of a new-born +child. (From _Kollmann._) Above, in the three bones of the roof of the +skull, we see the lines that radiate from the central points of +ossification; in front, the frontal bone; behind, the occipital bone; +between the two the large parietal bone, _p. s_ the scurf bone, _w_ +mastoid fontanelle, _f_ petrous bone, _t_ tympanic bone, _l_ lateral +part, _b_ bulla, _j_ cheek-bone, _a_ large wing of cuneiform bone, _k_ +fontanelle of cuneiform bone. + + +In all the Craniotes the soft, indifferent cells of the mesoderm, which +originally compose the skeletal plate, are afterwards converted for the +most part into cartilaginous cells, and these secrete a firm and +elastic intercellular substance between them, and form cartilaginous +tissue. Like most of the other parts of the skeleton, the membranous +rudiments of the vertebræ soon pass into a cartilaginous state, and in +the higher Vertebrates this is afterwards replaced by the hard osseous +tissue with its characteristic stellate cells (Fig. 6). The primary +axial skeleton remains a simple chorda throughout life in the Acrania, +the Cyclostomes, and the lowest fishes. In most of the other +Vertebrates the chorda is more or less replaced by the cartilaginous +tissue of the secondary perichorda that grows round it. In the lower +Craniotes (especially the fishes) a more or less considerable part of +the chorda is preserved in the bodies of the vertebræ. In the mammals +it disappears for the most part. By the end of the second month in the +human embryo the chorda is merely a slender thread, running through the +axis of the thick, cartilaginous vertebral column (Figs. 182 _ch,_ 329 +_ch_). In the cartilaginous vertebral bodies themselves, which +afterwards ossify, the slender remnant of the chorda presently +disappears (Fig. 330 _ch_). But in the elastic inter-vertebral disks, +which develop from the skeletal plate between each pair of vertebral +bodies (Fig. 329 _li_), a relic of the chorda remains permanently. In +the new-born child there is a large pear-shaped cavity in each +intervertebral disk, filled with a gelatinous mass of cells (Fig. 331 +_a_). + + +Fig.334. Head-skeleton of a primitive fish. Fig. 334—Head-skeleton of a +primitive fish. _n_ nasal pit, _eth_ cribriform bone region, _orb_ +orbit of eye, _la_ wall of auscultory labyrinth, _occ_ occipital region +of primitive skull, _cv_ vertebral column, _a_ fore, _bc_ hind-lip +cartilage, _o_ primitive upper jaw (_palato-quadratum_), _u_ primitive +lower jaw, _II_ hyaloid bone, _III–VIII_ first to sixth branchial +arches. (From _Gegenbaur._) + + +Though less sharply defined, this gelatinous nucleus of the elastic +cartilaginous disks persists throughout life in the mammals, but in the +birds and most reptiles the last trace of the chorda disappears. In the +subsequent ossification of the cartilaginous vertebra the first deposit +of bony matter (“first osseous nucleus”) takes place in the vertebral +body immediately round the remainder of the chorda, and soon displaces +it altogether. Then there is a special osseous nucleus formed in each +half of the vertebral arch. The ossification does not reach the point +at which the three nuclei are joined until after birth. In the first +year the two osseous halves of the arches unite; but it is much +later—in the second to the eighth year— + +that they connect with the osseous vertebral bodies. + + +Fig.345. Roofs of the skulls of nine Primates (Cattarrhines), seen from +above and reduced to a common size. Fig. 335—Roofs of the skulls of +nine Primates (_Cattarrhines_), seen from above and reduced to a common +size. _1_ European, _2_ Brazilian, _3_ Pithecanthropus, _4_ Gorilla, +_5_ Chimpanzee, _6_ Orang, _7_ Gibbon, _8_ Tailed ape, _9_ Baboon. + + +The bony skull (_cranium_), the head-part of the secondary axial +skeleton, develops in just the same way as the vertebral column. The +skull forms a bony envelope for the brain, just as the vertebral canal +does for the spinal cord; and as the brain is only a peculiarly +differentiated part of the head, while the spinal cord represents the +longer trunk-section of the originally homogeneous medullary tube, we +shall expect to find that the osseous coat of the one is a special +modification of the osseous envelope of the other. When we examine the +adult human skull in itself (Fig. 332), it is difficult to conceive how +it can be merely the modified fore part of the vertebral column. It is +an elaborate and extensive bony structure, composed of no less than +twenty bones of different shapes and sizes. Seven of them form the +spacious shell that surrounds the brain, in which we distinguish the +solid ventral base below and the curved dorsal vault above. The other +thirteen bones form the facial skull, which is especially the bony +envelope of the higher sense-organs, and at the same time encloses the +entrance of the alimentary canal. The lower jaw is articulated at the +base of the skull (usually regarded as the XXI cranial bone). Behind +the lower jaw we find the hyoid bone at the root of the tongue, also +formed from the gill-arches, and a part of the lower arches that have +developed as “head-ribs” from the ventral side of the base of the +cranium. + + +Fig.336. Skeleton of the breast-fin of Ceratodus (biserial feathered +skeleton). Fig. 337. Skeleton of the breast-fin of an early Selachius +(Acanthias). Fig. 338. Skeleton of the breast-fin of a young Selachius. +Fig. 336—Skeleton of the breast-fin of Ceratodus (biserial feathered +skeleton). _A, B,_ cartilaginous series of the fin-stem. _rr_ +cartilaginous fin-radii. (From _Gunther._) +Fig. 337—Skeleton of the breast-fin of an early Selachius +(_Acanthias_). The radii of the median fin-border (_B_) have +disappeared for the most part; a few only (_R_) are left. _R, R,_ radii +of the lateral fin-border, _mt_ metapterygium, _ms_ mesopterygium, _p_ +propterygium. (From _Gegenbaur._) Fig. 338—Skeleton of the breast-fin +of a young Selachius. The radii of the median fin-border have wholly +disappeared. The shaded part on the right is the section that persists +in the five-fingered hand of the higher Vertebrates. (_b_ the three +basal pieces of the fin: _mt_ metapterygium, rudiment of the humerus, +_ms_ mesopterygium, _p_ propterygium.) (From _Gegenbaur._) + + +Although the fully-developed skull of the higher Vertebrates, with its +peculiar shape, its enormous size, and its complex composition, seems +to have nothing in common with the ordinary vertebræ, nevertheless even +the older comparative anatomists came to recognise at the end of the +eighteenth century that it is really nothing else originally than a +series of modified vertebræ. When Goethe in 1790 “picked up the skull +of a slain victim from the sand of the Jewish cemetery at Venice, he +noticed at once +that the bones of the face also could be traced to vertebræ (like the +three hind-most cranial vertebræ).” And when Oken (without knowing +anything of Goethe’s discovery) found at Ilenstein, “a fine bleached +skull of a hind, the thought flashed across him like lightning: ‘It is +a vertebral column.’” + +This famous vertebral theory of the skull has interested the most +distinguished zoologists for more than a century: the chief +representatives of comparative anatomy have devoted their highest +powers to the solution of the problem, and the interest has spread far +beyond their circle. But it was not until 1872 that it was happily +solved, after seven years’ labour, by the comparative anatomist who +surpassed all other experts of this science in the second half of the +nineteenth century by the richness of his empirical knowledge and the +acuteness and depth of his philosophic speculations. Carl Gegenbaur has +shown, in his classic _Studies of the Comparative Anatomy of the +Vertebrates_ (third section), that we find the most solid foundation +for the vertebral theory of the skull in the head-skeleton of the +Selachii. Earlier anatomists had wrongly started from the mammal skull, +and had compared the several bones that compose it with the several +parts of the vertebra (Fig. 333) they thought they could prove in this +way that the fully-formed mammal skull was made of from three to six +vertebræ. + + + +Fig.339. Skeleton of the fore leg of an amphibian. Fig. 340. Skeleton +of gorilla’s hand. Fig. 341. Skeleton of human hand, back. Fig. +339—Skeleton of the fore leg of an amphibian. _h_ upper-arm (humerus), +_ru_ lower arm (_r_ radius, _u_ ulna), _rcicu′,_ wrist-bones of first +series (_r_ radiale, _i_ intermedium, _c_ centrale, _u′_ ulnare). _1, +2, 3, 4, 5_ wrist-bones of the second series. (From _Gegenbaur._) +Fig. 340—Skeleton of gorilla’s hand. (From _Huxley._) +Fig. 341—Skeleton of human hand, back. (From _Meyer._) + + +The older theory was refuted by simple and obvious facts, which were +first pointed out by Huxley. Nevertheless, the fundamental idea of +it—the belief that the skull is formed from the head-part of the +perichordal axial skeleton, just as the brain is from the simple +medullary tube, by differentiation and modification—remained. The work +now was to discover the proper way of supplying this philosophic theory +with an empirical foundation, and it was reserved for Gegenbaur to +achieve this. He first opened out the phylogenetic path which here, as +in all morphological questions, leads most confidently to the goal. He +showed that the primitive fishes (Figs. 249–251), the ancestors of all +the Gnathostomes, still preserve permanently in the form of their skull +the structure out of which the transformed skull of the higher +Vertebrates, including man, has been evolved. He further showed that +the branchial arches of the Selachii prove that their skull originally +consisted of a large number of (at least nine or ten) provertebræ, and +that the cerebral nerves that proceed from the base of the brain +entirely confirm this. These cerebral nerves are (with the exception of +the first and second pair, the olfactory and optic nerves) merely +modifications of spinal nerves, and are essentially similar to them in +their peripheral expansion. The comparative anatomy of these cerebral +nerves, their origin and their expansion, furnishes one of the +strongest arguments for the new vertebral theory of the skull. + + +Fig.342. Skeleton of the hand or fore foot of six mammals. I man, II +dog, III pig, IV ox, V tapir, VI horse. Fig. 342—Skeleton of the hand +or fore foot of six mammals. _I_ man, _II_ dog, _III_ pig, _IV_ ox, _V_ +tapir, _VI_ horse. _r_ radius, _u_ ulna, _a_ scaphoideum, _b_ lunare, +_a_ triquetrum, _d_ trapezium, _e_ trapezoid, _f_ capitatum, _g_ +hamatum, _p_ pisiforme. _1_ thumb, _2_ index finger, _3_ middle finger, +_4_ ring finger, _5_ little finger. (From _Gegenbaur._) + + +We have not space here to go into the details of Gegenbaur’s theory of +the skull. I must be content to refer the reader to the great work I +have mentioned, in which it is thoroughly established from the +empirico-philosophical point of view. He has also given a comprehensive +and up-to-date treatment of the subject in his _Comparative Anatomy of +the Vertebrates_ (1898). Gegenbaur indicates as original “cranial +ribs,” or “lower arches of the cranial vertebræ,” at each side of the +head of the Selachii (Fig. 334), the following pairs of arches: _I_ and +_II,_ two lip-cartilages, the anterior (_a_) of which is composed of an +upper piece only, the posterior (_bc_) from an upper and lower piece; +_III,_ the maxillary arches, also consisting of two pieces on each +side—the primitive upper jaw (_os palato-quadratum, o_) and the +primitive lower jaw (_u_); _IV,_ the hyaloid bone (_II_); finally, +_V–X,_ six branchial arches in the narrower sense (_III–VIII_). From +the anatomic features of these nine to ten cranial ribs or “lower +vertebral arches” and the cranial nerves that spread over them, it is +clear that the apparently simple cartilaginous primitive skull of the +Selachii was originally formed from so many (at least nine) somites or +provertebræ. The blending of these primitive segments into a single +capsule is, however, so ancient that, in virtue of the law of curtailed +heredity, the original division seems to have disappeared; in the +embryonic development it is very difficult to detect it in isolated +traces, and in some respects quite impossible. It is claimed that +several (three to six) traces of provertebræ have been discovered in +the anterior (pre-chordal) part of the Selachii-skull; this would bring +up the number of cranial somites to twelve or sixteen, or even more. + +In the primitive skull of man (Fig. 323) and the higher Vertebrates, +which has been evolved from that of the Selachii, five consecutive +sections are discoverable at a certain early period of development, and +one might be induced to trace these to five primitive vertebræ; but +these sections are due entirely to adaptation to + +the five primitive cerebral vesicles, and correspond, like these, to a +large number of metamera. That we have in the primitive skull of the +mammals a greatly modified and transformed organ, and not at all a +primitive formation, is clear from the circumstance that its original +soft membranous form only assumes the cartilaginous character for the +most part at the base and the sides, and remains membranous at the +roof. At this part the bones of the subsequent osseous skull develop as +external coverings over the membranous structure, without an +intermediate cartilaginous stage, as there is at the base of the skull. +Thus a large part of the cranial bones develop originally as covering +bones from the corium, and only secondarily come into close touch with +the primitive skull (Fig. 333). We have previously seen how this very +rudimentary beginning of the skull in man is formed ontogenetically +from the “head-plates,” and thus the fore end of the chorda is enclosed +in the base of the skull. (Cf. Fig. 145 and pp. 138, 144, and 149.) + + +Fig.343-345. Arm and hand of three anthropoids. Fig. 343. Chimpanzee +(Anthropithecus niger). Fig. 344. Veddah of Ceylon (Homo veddalis). +Fig. 345. European (Homo mediterraneus). Figs. 343–345—Arm and hand of +three anthropoids. Fig. 343—Chimpanzee (_Anthropithecus niger_). Fig. +344—Veddah of Ceylon (_Homo veddalis_). Fig. 345—European (_Homo +mediterraneus_). (From _Paul_ and _Fritz Sarasin._) + + +The phylogeny of the skull has made great progress during the last +three decades through the joint attainments of comparative anatomy, +ontogeny, and paleontology. By the judicious and comprehensive +application of the phylogenetic method (in the sense of Gegenbaur) we +have found the key to the great and important problems that arise from +the thorough comparative study of the skull. Another school of +research, the school of what is called “exact craniology” (in the sense +of Virchow), has, meantime, made fruitless efforts to obtain this +result. We may gratefully acknowledge all that this descriptive school +has done in the way of accurately describing the various forms and +measurements of the human skull, as compared with those of other +mammals. But the vast empirical material that it has accumulated in its +extensive literature is mere dead and sterile erudition until it is +vivified and illumined by phylogenetic speculation. + +Virchow confined himself to the most careful analysis of large numbers +of human skulls and those of anthropoid mammals. He saw only the +differences between them, and sought to express these in figures. + + +Fig.346. Transverse section of a fish’s tail (from the tunny). Fig. +346—Transverse section of a fish’s tail (from the tunny). (From +_Johannes Müller._) _a_ upper (dorsal) lateral muscles, _a′, b′_ lower +(ventral) lateral muscles, _d_ vertebral bodies, _b_ sections of +incomplete conical mantle, _B_ attachment lines of the inter-muscular +ligaments (from the side). + + +Without adducing a single solid reason, or offering any alternative +explanation, he rejected evolution as an unproved hypothesis. He played +a most unfortunate part in the controversy as to the significance of +the fossil human skulls of Spy and Neanderthal, and the comparison of +them with the skull of the Pithecanthropus (Fig. 283). All the +interesting features of these skulls that clearly indicated the +transition from the anthropoid to the man were declared by Virchow to +be chance pathological variations. He said that the roof of the skull +of Pithecanthropus (Fig. 335, _3_) must have belonged to an ape, +because so pronounced an _orbital stricture_ (the horizontal +constriction between the outer edge of the eye-orbit and the temples) +is not found in any human being. Immediately afterwards Nehring showed +in the skull of a Brazilian Indian (Fig. 335, _2_), found in the +Sambaquis of Santos, that this stricture can be even deeper in man than +in many of the apes. It is very instructive in this connection to +compare the roofs of the skulls (seen from above) of different +primates. I have, therefore, arranged nine such skulls in Fig. 335, and +reduced them to a common size. + +We turn now to the branchial arches, which were regarded even by the +earlier natural philosophers as “head-ribs.” (Cf. Figs. 167–170). Of +the four original gill-arches of the mammals the first lies between the +primitive mouth and the first gill-cleft. From the base of this arch is +formed the upper-jaw process, which joins with the inner and outer +nasal processes on each side, in the manner we have previously +explained, and forms the chief parts of the skeleton of the upper jaw +(palate bone, pterygoid bone, etc.) (Cf. p. 284.) The remainder of the +first branchial arch, which is now called, by +way of contrast, the “upper-jaw process,” forms from its base two of +the ear-ossicles (hammer and anvil), and as to the rest is converted +into a long strip of cartilage that is known, after its discoverer, as +“Meckel’s cartilage,” or the _promandibula._ At the outer surface of +the latter is formed from the cellular matter of the corium, as +covering or accessory bone, the permanent bony lower jaw. From the +first part or base of the second branchial arch we get, in the mammals, +the third ossicle of the ear, the stirrup; and from the succeeding +parts we get (in this order) the muscle of the stirrup, the styloid +process of the temporal bone, the styloid-hyoid ligament, and the +little horn of the hyoid bone. The third branchial arch is only +cartilaginous at the foremost part, and here the body of the hyoid bone +and its larger horn are formed at each side by the junction of its two +halves. The fourth branchial arch is only found transitorily in the +mammal embryo as a rudimentary organ, and does not develop special +parts; and there is no trace in the embryo of the higher Vertebrates of +the posterior branchial arches (fifth and sixth pair), which are +permanent in the Selachii. They have been lost long ago. Moreover, the +four gill-clefts of the human embryo are only interesting as +rudimentary organs, and they soon close up and disappear. The first +alone (between the first and second branchial arches) has any permanent +significance; from it are developed the tympanic cavity and the +Eustachian tube. (Cf. Figs. 169, 320.) + +It was Carl Gegenbaur again who solved the difficult problem of tracing +the skeleton of the limbs of the Vertebrates to a common type. Few +parts of the vertebrate body have undergone such infinitely varied +modifications in regard to size, shape, and adaptation of structure as +the limbs or extremities; yet we are in a position to reduce them all +to the same hereditary standard. We may generally distinguish three +groups among the Vertebrates in relation to the formation of their +limbs. The lowest and earliest Vertebrates, the Acrania and +Cyclostomes, had, like their invertebrate ancestors, no pairs of limbs, +as we see in the Amphioxus and the Cyclostomes to-day (Figs. 210, 247). +The second group is formed of the two classes of the true fishes and +the Dipneusts; here there are always two pairs of limbs at first, in +the shape of many-toed fins—one pair of breast-fins or fore legs, and +one pair of belly-fins or hind legs (Figs. 248–259). The third group +comprises the four higher classes of Vertebrates—the amphibia, +reptiles, birds, and mammals; in these quadrupeds there are at first +the same two pairs of limbs, but in the shape of five-toed feet. +Frequently we find less than five toes, and sometimes the feet are +wholly atrophied (as in the serpents). But the original stem-form of +the group had five toes or fingers before and behind (Figs. 263–265). + +The true primitive form of the pairs of limbs, such as they were found +in the primitive fishes of the Silurian period, is preserved for us in +the Australian dipneust, the remarkable _Ceratodus_ (Fig. 257). Both +the breast-fin and the belly-fin are flat oval paddles, in which we +find a biserial cartilaginous skeleton (Fig. 336). This consists, +firstly, of a much segmented fin-rod or “stem” (_A, B_), which runs +through the fin from base to tip; and secondly of a double row of thin +articulated fin-radii (_r, r_), which are attached to both sides of the +fin-rod, like the feathers of a feathered leaf. This primitive fin, +which Gegenbaur first recognised, is attached to the vertebral column +by a simple zone in the shape of a cartilaginous arch. It has probably +originated from the branchial arches.[31] + + [31] While Gegenbaur derives the fins from two pairs of posterior + separated branchial arches, Balfour holds that they have been + developed from segments of a pair of originally continuous lateral + fins or folds of the skin.) + + +We find the same biserial primitive fin more or less preserved in the +fossilised remains of the earliest Selachii (Fig. 248), Ganoids (Fig. +253), and Dipneusts (Fig. 256). It is also found in modified form in +some of the actual sharks and pikes. But in the majority of the +Selachii it has already degenerated to the extent that the radii on one +side of the fin-rod have been partly or entirely lost, and are retained +only on the other (Fig. 337). We thus get the uniserial fin, which has +been transmitted from the Selachii to the rest of the fishes (Fig. +338). + +Gegenbaur has shown how the five-toed leg of the Amphibia, that has +been inherited by the three classes of Amniotes, was evolved from the +uniserial fish-fin.[32] + + + [32] The limb of the four higher classes of Vertebrates is now + explained in the sense that the original fin-rod passes along its + outer (ulnar or fibular) side, and ends in the fifth toe. It was + formerly believed to go along the inner (radial or tibial) side, and + end in the first toe, as Fig. 339 shows.) In the dipneust ancestors of + the Amphibia the radii gradually atrophy, and are lost, for the most + part, on the other side of the fin-rod as well (the lighter cartilages + in Fig. 338). Only the four lowest radii (shaded in the illustration) + are preserved; and these are the four inner toes of the foot (first to + fourth). The little or fifth toe is developed from the lower end of + the fin-rod. From the middle and upper part of the fin-rod was + developed the long stem of the limb—the important radius and ulna + (Fig. 339 _r_ and _u_) and humerus (_h_) of the higher Vertebrates. + + + +Fig.347. Human skeleton. Fig. 348. Skeleton of the giant gorilla. Fig. +347—Human skeleton. (Cf. Figure 326.) Fig. 348—Skeleton of the giant +gorilla. (Cf. Figure 209.) + + +In this way the five-toed foot of the Amphibia, which we first meet in +the Carboniferous Stegocephala (Fig. 260), and which was inherited from +them by the reptiles on one side and the mammals on the other, was +formed by gradual degeneration and differentiation from the many-toed +fish-fin (Fig. 341). The reduction of the radii to four was accompanied +by a further differentiation of the fin-rod, its transverse +segmentation into upper and lower halves, and the formation of the zone +of the limb, which is composed originally of three limbs before and +behind in the higher Vertebrates. The simple arch of the original +shoulder-zone divides on each side into an upper (dorsal) piece, the +shoulder-blade (_scapula_), and a lower (ventral) piece; the anterior +part of the latter forms the primitive clavicle (_procoracoideum_), and +the posterior part the _coracoideum._ In the same way the simple arch +of the pelvic zone breaks up into an upper (dorsal) piece, the +iliac-bone (_os ilium_), and a lower (ventral) piece; the anterior part +of the latter forms the pubic bone (_os pubis_), and the posterior the +ischial bone (_os ischii_). + +There is also a complete agreement between the fore and hind limb in +the stem or shaft. The first section of the stem is supported by a +single strong bone—the humerus in the fore, the femur in the hind limb. +The second section contains two bones: in front the radius (_r_) and +ulna (_u_), behind the tibia and fibula. (Cf. the skeletons in Figs. +260, 265, 270, 278–282, and 348.) The succeeding numerous small bones +of the wrist (_carpus_) and ankle (_tarsus_) are also similarly +arranged in the fore and hind extremities, and so are the five bones of +the middle-hand (_metacarpus_) and middle-foot (_metatarsus_). Finally, +it is the same with the toes themselves, which have a similar +characteristic composition from a series of bony pieces before and +behind. We find a complete parallel in all the parts of the fore leg +and the hind leg. + +When we thus learn from comparative anatomy that the skeleton of the +human limbs is composed of just the same bones, put together in the +same way, as the skeleton in the four higher classes of Vertebrates, we +may at once infer a common descent of them from a single stem-form. +This stem-form was the earliest amphibian that had five toes on each +foot. It is particularly the outer parts of the limbs that have been +modified by adaptation to different conditions. We need only recall the +immense variations they offer within the mammal class. We have the +slender legs of the deer and the strong springing legs of the kangaroo, +the climbing feet of the sloth and the digging feet of the mole, the +fins of the whale and the wings of the bat. It will readily be granted +that these organs of locomotion differ as much in regard to size, +shape, and special function as can be conceived. Nevertheless, the bony +skeleton is substantially the same in every case. In the different +limbs we always find the same characteristic bones in essentially the +same rigidly hereditary connection; this is as splendid a proof of the +theory of evolution as comparative anatomy can discover in any organ of +the body. It is true that the skeleton of the limbs of the various +mammals undergoes many distortions and degenerations besides the +special adaptations (Fig. 342). Thus we find the first finger or the +thumb atrophied in the fore-foot (or hand) of the dog (_II_). It has +entirely disappeared in the pig (_III_) and tapir (_V_). In the +ruminants (such as the ox, _IV_) the second and fifth toes are also +atrophied, and only the third and fourth are well developed (_VI, 3_). +Nevertheless, all these different fore-feet, as well as the hand of the +ape (Fig. 340) and of man (Fig. 341), were originally developed from a +common pentadactyle stem-form. This is proved by the rudiments of the +degenerated toes, and by the similarity of the arrangement of the +wrist-bones in all the pentanomes (Fig. 342 _a–p_). + +If we candidly compare the bony skeleton of the human arm and hand with +that of the nearest anthropoid apes, we find an almost perfect +identity. This is especially true of the chimpanzee. In regard to the +proportions of the various +parts, the lowest living races of men (the Veddahs of Ceylon, Fig. 344) +are midway between the chimpanzee (Fig. 343) and the European (Fig. +345). More considerable are the differences in structure and the +proportions of the various parts between the different genera of +anthropoid apes (Figs. 278–282); and still greater is the morphological +distance between these and the lowest apes (the _Cynopitheca_). Here, +again, impartial and thorough anatomic comparison confirms the accuracy +of Huxley’s pithecometra principle p. 171. + +The complete unity of structure which is thus revealed by the +comparative anatomy of the limbs is fully confirmed by their +embryology. However different the extremities of the four-footed +Craniotes may be in their adult state, they all develop from the same +rudimentary structure. In every case the first trace of the limb in the +embryo is a very simple protuberance that grows out of the side of the +hyposoma. These simple structures develop directly into fins in the +fishes and Dipneusts by differentiation of their cells. In the higher +classes of Vertebrates each of the four takes the shape in its further +growth of a leaf with a stalk, the inner half becoming narrower and +thicker and the outer half broader and thinner. The inner half (the +stalk of the leaf) then divides into two sections—the upper and lower +parts of the limb. Afterwards four shallow indentations are formed at +the free edge of the leaf, and gradually deepen; these are the +intervals between the five toes (Fig. 174). The toes soon make their +appearance. But at first all five toes, both of fore and hind feet, are +connected by a thin membrane like a swimming-web; they remind us of the +original shaping of the foot as a paddling fin. The further development +of the limbs from this rudimentary structure takes place in the same +way in all the Vertebrates according to the laws of heredity. + +The embryonic development of the muscles, or _active_ organs of +locomotion, is not less interesting than that of the skeleton, or +_passive_ organs. But the comparative anatomy and ontogeny of the +muscular system are much more difficult and inaccessible, and +consequently have hitherto been less studied. We can therefore only +draw some general phylogenetic conclusions therefrom. + +It is incontestable that the musculature of the Vertebrates has been +evolved from that of lower Invertebrates; and among these we have to +consider especially the unarticulated Vermalia. They have a simple +cutaneous muscular layer, developing from the mesoderm. This was +afterwards replaced by a pair of internal lateral muscles, that +developed from the middle wall of the cœlom-pouches; we still find the +first rudiments of the muscles arising from the muscle-plate of these +in the embryos of all the Vertebrates (cf. Figs. 124, 158–160, 222–224 +_mp_). In the unarticulated stem-forms of the Chordonia, which we have +called the Prochordonia, the two cœlom-pouches, and therefore also the +muscle-plates of their walls, were not yet segmented. A great advance +was made in the articulation of them, as we have followed it step by +step in the Amphioxus (Figs. 124, 158). This segmentation of the +muscles was the momentous historical process with which vertebration, +and the development of the vertebrate stem, began. The articulation of +the skeleton came after this segmentation of the muscular system, and +the two entered into very close correlation. + +The episomites or dorsal cœlom-pouches of the Acrania, Cyclostomes, and +Selachii (Fig. 161 _h_) first develop from their inner or median wall +(from the cell-layer that lies directly on the skeletal plate [_sk_] +and the medullary tube [_nr_]) a strong muscle-plate (_mp_). By dorsal +growth (_w_) it also reaches the external wall of the cœlom-pouches, +and proceeds from the dorsal to the ventral wall. From these segmental +muscle-plates, which are chiefly concerned in the segmentation of the +Vertebrates, proceed the lateral muscles of the stem, as we find in the +simplest form in the Amphioxus (Fig. 210). By the formation of a +horizontal frontal septum they divide on each side into an upper and +lower series of myotomes, dorsal and ventral lateral muscles. This is +seen with typical regularity in the transverse section of the tail of a +fish (Fig. 346). From these earlier lateral muscles of the trunk +develop the greater part of the subsequent muscles of the trunk, and +also the much later “muscular buds” of the limbs.[33] + + [33] The ontogeny of the muscles is mostly cenogenetic. The greater + part of the muscles of the head (or the visceral muscles) belong + originally to the hyposoma of the vertebrate organism, and develop + from the wall of the hyposomites or ventral cœlom-pouches. This also + applies originally to the primary muscles of the limbs, as these too + belong phylogenetically to the hyposoma. (Cf. Chapter XIV.) + + + + +Chapter XXVII. +THE EVOLUTION OF THE ALIMENTARY SYSTEM + + +The chief of the vegetal organs of the human frame, to the evolution of +which we now turn our attention, is the alimentary canal. The gut is +the oldest of all the organs of the metazoic body, and it leads us back +to the earliest age of the formation of organs—to the first section of +the Laurentian period. As we have already seen, the result of the first +division of labour among the homogeneous cells of the earliest +multicellular animal body was the formation of an alimentary cavity. +The first duty and first need of every organism is self-preservation. +This is met by the functions of the nutrition and the covering of the +body. When, therefore, in the primitive globular _Blastæa_ the +homogeneous cells began to effect a division of labour, they had first +to meet this twofold need. One half were converted into alimentary +cells and enclosed a digestive cavity, the gut. The other half became +covering cells, and formed an envelope round the alimentary tube and +the whole body. Thus arose the primary germinal layers—the inner, +alimentary, or vegetal layer, and the outer, covering, or animal layer. +(Cf. pp. 214–17.) + +When we try to construct an animal frame of the simplest conceivable +type, that has some such primitive alimentary canal and the two primary +layers constituting its wall, we inevitably come to the very remarkable +embryonic form of the gastrula, which we have found with extraordinary +persistence throughout the whole range of animals, with the exception +of the unicellulars—in the Sponges, Cnidaria, Platodes, Vermalia, +Molluscs, Articulates, Echinoderms, Tunicates, and Vertebrates. In all +these stems the gastrula recurs in the same very simple form. It is +certainly a remarkable fact that the gastrula is found in various +animals as a larva-stage in their individual development, and that this +gastrula, though much disguised by cenogenetic modifications, has +everywhere essentially the same palingenetic structure (Figs. 30–35). +The elaborate alimentary canal of the higher animals develops +ontogenetically from the same simple primitive gut of the _gastrula._ + +This gastræa theory is now accepted by nearly all zoologists. It was +first supported and partly modified by Professor Ray-Lankester; he +proposed three years afterwards (in his essay on the development of the +Molluscs, 1875) to give the name of _archenteron_ to the primitive gut +and _blastoporus_ to the primitive mouth. + +Before we follow the development of the human alimentary canal in +detail, it is necessary to say a word about the general features of its +composition in the fully-developed man. The mature alimentary canal in +man is constructed in all its main features like that of all the higher +mammals, and particularly resembles that of the Catarrhines, the +narrow-nosed apes of the Old World. The entrance into it, the mouth, is +armed with thirty-two teeth, fixed in rows in the upper and lower jaws. +As we have seen, our dentition is exactly the same as that of the +Catarrhines, and differs from that of all other animals p. 257. Above +the mouth-cavity is the double nasal cavity; they are separated by the +palate-wall. But we saw that this separation is not there from the +first, and that originally there is a common mouth-nasal cavity in the +embryo; and this is only divided afterwards by the hard palate into +two—the nasal cavity above and that of the mouth below (Fig. 311). + +At the back the cavity of the mouth is half closed by the vertical +curtain that we call the soft palate, in the middle of which is the +uvula. A glance into a mirror with the mouth wide open will show its +shape. The uvula is interesting because, besides man, it is only found +in the ape. At each side of the soft palate are the tonsils. Through +the curved opening that we find +underneath the soft palate we penetrate into the gullet or pharynx +behind the mouth-cavity. Into this opens on either side a narrow canal +(the Eustachian tube), through which there is direct communication with +the tympanic cavity of the ear (Fig. 320 _e_). The pharynx is continued +in a long, narrow tube, the œsophagus ( _sr_). By this the food passes +into the stomach when masticated and swallowed. Into the gullet also +opens, right above, the trachea ( _lr_), that leads to the lungs. The +entrance to it is covered by the epiglottis, over which the food +slides. The cartilaginous epiglottis is found only in the mammals, and +has developed from the fourth branchial arch of the fishes and +amphibia. The lungs are found, in man and all the mammals, to the right +and left in the pectoral cavity, with the heart between them. At the +upper end of the trachea there is, under the epiglottis, a specially +differentiated part, strengthened by a cartilaginous skeleton, the +larynx. This important organ of human speech also develops from a part +of the alimentary canal. In front of the larynx is the thyroid gland, +which sometimes enlarges and forms goitre. + +The œsophagus descends into the pectoral cavity along the vertebral +column, behind the lungs and the heart, pierces the diaphragm, and +enters the visceral cavity. The diaphragm is a membrano-muscular +partition that completely separates the thoracic from the abdominal +cavity in all the mammals (and these alone). This separation is not +found in the beginning; there is at first a common breast-belly cavity, +the cœloma or pleuro-peritoneal cavity. The diaphragm is formed later +on as a muscular horizontal partition between the thoracic and +abdominal cavities. It then completely separates the two cavities, and +is only pierced by several organs that pass from the one to the other. +One of the chief of these organs is the œsophagus. After this has +passed through the diaphragm, it expands into the gastric sac in which +digestion chiefly takes place. The stomach of the adult man (Fig. 349) +is a long, somewhat oblique sac, expanding on the left into a blind +sac, the fundus of the stomach ( _b′_), but narrowing on the right, and +passing at the pylorus ( _e_) into the small intestine. At this point +there is a valve, the pyloric valve ( _d_), between the two sections of +the canal; it opens only when the pulpy food passes from the stomach +into the intestine. In man and the higher Vertebrates the stomach +itself is the chief organ of digestion, and is especially occupied with +the solution of the food; this is not the case in many of the lower +Vertebrates, which have no stomach, and discharge its function by a +part of the gut farther on. The muscular wall of the stomach is +comparatively thick; it has externally strong muscles that accomplish +the digestive movements, and internally a large quantity of small +glands, the peptic glands, which secrete the gastric juice. + + +Fig.349. Human stomach and duodenum, longitudinal section. Fig. +349—Human stomach and duodenum, longitudinal section. _a_ cardiac (end +of œsophagus), _b_ fundus (blind sac of the left side), _c_ +pylorus-fold, _d_ pylorus-valves, _e_ pylorus-cavity, _fgh_ duodenum, +_i_ entrance of the gall-duct and the pancreatic duct. (From _Meyer._) + + +Next to the stomach comes the longest section of the alimentary canal, +the middle gut or small intestine. Its chief function is to absorb the +peptonised fluid mass of food, or the chyle, and it is subdivided into +several sections, of which the first (next to the stomach) is called +the duodenum (Fig. 349 _fgh_). It is a short, horseshoe-shaped loop of +the gut. The largest glands of the alimentary canal open into it—the +liver, the chief digestive gland, that secretes the gall, and the +pancreas, which secretes the pancreatic juice. The two glands pour +their secretions, the bile and pancreatic juice, close together into +the duodenum ( _i_). The opening of the gall-duct is of particular +phylogenetic importance, as it is the same in all the Vertebrates, and +indicates the principal point of the hepatic or trunk-gut (Gegenbaur). +The liver, phylogenetically older than the stomach, is a large gland, +rich in blood, in the adult man, immediately under the diaphragm on the +left +side, and separated by it from the lungs. The pancreas lies a little +further back and more to the left. The remaining part of the small +intestine is so long that it has to coil itself in many folds in order +to find room in the narrow space of the abdominal cavity. It is divided +into the jejunum above and the ileum below. In the last section of it +is the part of the small intestine at which in the embryo the yelk-sac +opens into the gut. This long and thin intestine then passes into the +large intestine, from which it is cut off by a special valve. +Immediately behind this “Bauhin-valve” the first part of the large +intestine forms a wide, pouch-like structure, the cæcum. The atrophied +end of the cæcum is the famous rudimentary organ, the vermiform +appendix. The large intestine ( _colon_) consists of three parts—an +ascending part on the right, a transverse middle part, and a descending +part on the left. The latter finally passes through an S-shaped bend +into the last section of the alimentary canal, the rectum, which opens +behind by the anus. Both the large and small intestines are equipped +with numbers of small glands, which secrete mucous and other fluids. + + +Fig.350. Median section of the head of a hare-embryo, one-fourth of an +inch in length. Fig. 350—Median section of the head of a hare-embryo, +one-fourth of an inch in length. (From _Mihalcovics._) The deep +mouth-cleft ( _hp_) is separated by the membrane of the throat ( _rh_) +from the blind cavity of the head-gut ( _kd_). _hz_ heart, _ch_ chorda, +_hp_ the point at which the hypophysis develops from the mouth-cleft, +_vh_ ventricle of the cerebrum, _v3_ , third ventricle (intermediate +brain), _v4_ fourth ventricle (hind brain), _ck_ spinal canal. + + +For the greater part of its length the alimentary canal is attached to +the inner dorsal surface of the abdominal cavity, or to the lower +surface of the vertebral column. The fixing is accomplished by means of +the thin membranous plate that we call the mesentery. + +Although the fully-formed alimentary canal is thus a very elaborate +organ, and although in detail it has a quantity of complex structural +features into which we cannot enter here, nevertheless the whole +complicated structure has been historically evolved from the very +simple form of the primitive gut that we find in our +gastræad-ancestors, and that every gastrula brings before us to-day. We +have already pointed out (Chapter IX) how the epigastrula of the +mammals (Fig. 67) can be reduced to the original type of the +bell-gastrula, which is now preserved by the amphioxus alone (Fig. 35). +Like the latter, the human gastrula and that of all other mammals must +be regarded as the ontogenetic reproduction of the phylogenetic form +that we call the Gastræa, in which the whole body is nothing but a +double-walled gastric sac. + +We already know from embryology the manner in which the gut develops in +the embryo of man and the other mammals. From the gastrula is first +formed the spherical embryonic vesicle filled with fluid ( +_gastrocystis,_ Fig. 106). In the dorsal wall of this the sole-shaped +embryonic shield is developed, and on the under-side of this a shallow +groove appears in the middle line, the first trace of the later, +secondary alimentary tube. The gut-groove becomes deeper and deeper, +and its edges bend towards each other, and finally form a tube. + +As we have seen, this simple cylindrical gut-tube is at first +completely closed before and behind in man and in the Vertebrates +generally (Fig. 148); the permanent openings of the alimentary canal, +the mouth and anus, are only formed later on, and from the outer skin. +A mouth-pit appears in the skin in front (Fig. 350 _hp_), and this +grows towards the blind fore-end of the cavity of the head-gut ( _kd_), +and at length breaks into it. In the same way a shallow anus-pit is +formed in the skin behind, which grows deeper and deeper, advances +towards the blind hinder end of the pelvic gut, and at last connects +with it. There is at first, both before and behind, a thin partition +between the external cutaneous pit and the blind end of the gut—the +throat-membrane in front and the anus-membrane behind; these disappear +when the connection takes place. + +Directly in front of the anus-opening the allantois develops from the +hind gut; this is the important embryonic structure +that forms into the placenta in the Placentals (including man). In this +more advanced form the human alimentary canal (and that of all the +other mammals) is a slightly bent, cylindrical tube, with an opening at +each end, and two appendages growing from its lower wall: the anterior +one is the umbilical vesicle or yelk-sac, and the posterior the +allantois or urinary sac (Fig. 195). + +The thin wall of this simple alimentary tube and its ventral appendages +is found, on microscopic examination, to consist of two strata of +cells. The inner stratum, lining the entire cavity, consists of larger +and darker cells, and is the gut-gland layer. The outer stratum +consists of smaller and lighter cells, and is the gut-fibre layer. The +only exception is in the cavities of the mouth and anus, because these +originate from the skin. The inner coat of the mouth-cavity is not +provided by the gut-gland layer, but by the skin-sense layer; and its +muscular substratum is provided, not by the gut-fibre, but the +skin-fibre, layer. It is the same with the wall of the small +anus-cavity. + +If it is asked how these constituent layers of the primitive gut-wall +are related to the various tissues and organs that we find afterwards +in the fully-developed system, the answer is very simple. It can be put +in a single sentence. The epithelium of the gut—that is to say, the +internal soft stratum of cells that lines the cavity of the alimentary +canal and all its appendages, and is immediately occupied with the +processes of nutrition—is formed solely from the gut-gland layer; all +other tissues and organs that belong to the alimentary canal and its +appendages originate from the gut-fibre layer. From the latter is also +developed the whole of the outer envelope of the gut and its +appendages; the fibrous connective tissue and the smooth muscles that +compose its muscular layer, the cartilages that support it (such as the +cartilages of the larynx and the trachea), the blood-vessels and +lymph-vessels that absorb the nutritive fluid from the intestines—in a +word, all that there is in the alimentary system besides the epithelium +of the gut. From the same layer we also get the whole of the mesentery, +with all the organs embedded in it—the heart, the large blood-vessels +of the body, etc. + + +Fig.351. Scales or cutaneous teeth of a shark (Centrophorus calceus). +Fig. 351—Scales or cutaneous teeth of a shark ( _Centrophorus +calceus_). A three-pointed tooth rises obliquely on each of the +quadrangular bony plates that lie in the corium. (From _Gegenbaur._) + + +Let us now leave this original structure of the mammal gut for a +moment, in order to compare it with the alimentary canal of the lower +Vertebrates, and of those Invertebrates that we have recognised as +man’s ancestors. We find, first of all, in the lowest Metazoa, the +Gastræads, that the gut remains permanently in the very simple form in +which we find it transitorily in the palingenetic gastrula of the other +animals; it is thus in the Gastremaria ( _Pemmatodiscus_), the +Physemaria ( _Prophysema_), the simplest Sponges ( _Olynthus_), the +freshwater Polyps ( _Hydra_), and the ascula-embryos of many other +Cœlenteria (Figs. 233–238). Even in the simplest forms of the Platodes, +the Rhabdocœla (Fig. 240), the gut is still a simple straight tube, +lined with the entoderm; but with the important difference that in this +case its single opening, the primitive mouth ( _m_), has formed a +muscular gullet ( _sd_) by invagination of the skin. + +We have the same simple form in the gut of the lowest Vermalia +(Gastrotricha, Fig. 242, Nematodes, Sagitta, etc.). But in these a +second important opening of the gut has been formed at the opposite end +to the mouth, the anus (Fig. 242 _a_). + + +We see a great advance in the structure of the vermalian gut in the +remarkable _Balanoglossus_ (Fig. 245), the sole survivor of the +Enteropneust class. Here we have the first appearance of the division +of the alimentary tube into two sections that characterises the +Chordonia. The fore half, the head-gut ( _cephalogaster_), becomes the +organ of respiration (branchial gut, Fig. 245 _k_); the hind half, the +trunk-gut ( _truncogaster_), alone acts as digestive organ (hepatic +gut, _d_). The differentiation of these two parts of the gut in the +Enteropneust is just the same as in all the Tunicates and Vertebrates. + + +Fig.352. Gut of a human embryo, one-sixth of an inch long. Fig. 352—Gut +of a human embryo, one-sixth of an inch long. (From _His._) + + +It is particularly interesting and instructive in this connection to +compare the Enteropneusts with the Ascidia and the Amphioxus (Figs. +220, 210)—the remarkable animals that form the connecting link between +the Invertebrates and the Vertebrates. In both forms the gut is of +substantially the same construction; the anterior section forms the +respiratory branchial gut, the posterior the digestive hepatic gut. In +both it develops palingenetically from the primitive gut of the +gastrula, and in both the hinder end of the medullary tube covers the +primitive mouth to such an extent that the remarkable medullary +intestinal duct is formed, the passing communication between the neural +and intestinal tubes ( _canalis neurentericus,_ Figs. 83, 85 _ne_). In +the vicinity of the closed primitive mouth, possibly in its place, the +later anus is developed. In the same way the mouth is a fresh formation +in the Amphioxus and the Ascidia. It is the same with the human mouth +and that of the Craniotes generally. The secondary formation of the +mouth in the Chordonia is probably connected with the development of +the gill-clefts which are formed in the gut-wall immediately behind the +mouth. In this way the anterior section of the gut is converted into a +respiratory organ. I have already pointed out that this modification is +distinctive of the +Vertebrates and Tunicates. The phylogenetic appearance of the +gill-clefts indicates the commencement of a new epoch in the +stem-history of the Vertebrates. + +In the further ontogenetic development of the alimentary canal in the +human embryo the appearance of the gill-clefts is the most important +process. At a very early stage the gullet-wall joins with the external +body-wall in the head of the human embryo, and this is followed by the +formation of four clefts, which lead directly into the gullet from +without, on the right and left sides of the neck, behind the mouth. +These are the gill or gullet clefts, and the partitions that separate +them are the gill or gullet-arches (Fig. 171). These are most +interesting embryonic structures. They show us that all the higher +Vertebrates reproduce in their earlier stages, in harmony with the +biogenetic law, the process that had so important a part in the rise of +the whole Chordonia-stem. This process was the differentiation of the +gut into two sections—an anterior respiratory section, the branchial +gut, that was restricted to breathing, and a posterior digestive +section, the hepatic gut. As we find this highly characteristic +differentiation of the gut into two different sections in all the +Vertebrates and all the Tunicates, we may conclude that it was also +found in their common ancestors, the Prochordonia—especially as even +the Enteropneusts have it. (Cf. pp. 119, 151, 227, Figs. 210, 220, +245.) It is entirely wanting in all the other Invertebrates. + + +Fig.353. Gut of a dog-embryo (shown in Fig. 202, from Bischoff), seen +from the ventral side. Fig. 354. The same gut seen from the right. Fig. +353—Gut of a dog-embryo (shown in Fig. 202, from _Bischoff_), seen from +the ventral side. _a_ gill-arches (four pairs), _b_ rudiments of +pharynx and larynx, _c_ lungs, _d_ stomach, _f_ liver, _g_ walls of the +open yelk-sac (into which the middle gut opens with a wide aperture), +_h_ rectum. + +Fig. 354—The same gut seen from the right. _a_ lungs, _b_ stomach, _c_ +liver, _d_ yelk-sac, _e_ rectum.) + + +There is at first only one pair of gill-clefts in the Amphioxus, as in +the Ascidia and Enteropneusts; and the Copelata (Fig. 225) have only +one pair throughout life. But the number presently increases in the +former. In the Craniotes, however, it decreases still further. The +Cyclostomes have six to eight pairs (Fig. 247); some of the Selachii +six or seven pairs, most of the fishes only four or five pairs. In the +embryo of man, and the higher Vertebrates generally, where they make an +appearance at an early stage, only three or four pairs are developed. +In the fishes they remain throughout life, and form an exit for the +water taken in at the mouth (Figs. 249–251). But they are partly lost +in the amphibia, and entirely in the higher Vertebrates. In these +nothing is left but a relic of the first gill-cleft. This is formed +into a part of the organ of hearing; from it are developed the external +meatus, the tympanic cavity, and the Eustachian tube. We have already +considered these remarkable structures, and need only point here to the +interesting fact that our middle and external ear is a modified +inheritance from the fishes. The branchial arches also, which separate +the clefts, develop into very different parts. In the fishes they +remain gill-arches, supporting the respiratory gill-leaves. It is the +same with the lowest amphibia, but in the higher amphibia they undergo +various modifications; and in the three higher classes of Vertebrates +(including man) the hyoid bone and the ossicles of the ear develop from +them. (Cf. p. 291.) + +From the first gill-arch, from the inner surface of which the muscular +tongue proceeds, we get the first structure of the maxillary +skeleton—the upper and lower jaws, which surround the mouth and support +the teeth. These important parts are wholly wanting in the two lowest +classes of Vertebrates, the Acrania and Cyclostoma. They appear first +in the earliest Selachii (Figs. 248–251), and have been transmitted +from this stem-group of the Gnathostomes to the higher +Vertebrates. Hence the original formation of the skeleton of the mouth +can be traced to these primitive fishes, from which we have inherited +it. The teeth are developed from the skin that clothes the jaws. As the +whole mouth cavity originates from the outer integument (Fig. 350), the +teeth also must come from it. As a fact, this is found to be the case +on microscopic examination of the development and finer structure of +the teeth. The scales of the fishes, especially of the shark type (Fig. +351), are in the same position as their teeth in this respect (Fig. +252). The osseous matter of the tooth (dentine) develops from the +corium; its enamel covering is a secretion of the epidermis that covers +the corium. It is the same with the cutaneous teeth or placoid scales +of the Selachii. At first the whole of the mouth was armed with these +cutaneous teeth in the Selachii and in the earliest amphibia. +Afterwards the formation of them was restricted to the edges of the +jaws. + + +Fig.355. Median section of the head of a Petromyzon-larva. Fig. +355—Median section of the head of a Petromyzon-larva. (From +_Gegenbaur._) _h_ hypobranchial groove (above it in the gullet we see +the internal openings of the seven gill-clefts), _v_ velum, _o_ mouth, +_c_ heart, _a_ auditory vesicle, _n_ neural tube, _ch_ chorda. + + +Hence our human teeth are, in relation to their original source, +modified fish-scales. For the same reason we must regard the salivary +glands, which open into the mouth, as epidermic glands, as they are +formed, not from the glandular layer of the gut like the rest of the +alimentary glands, but from the epidermis, from the horny plate of the +outer germinal layer. Naturally, in harmony with this evolution of the +mouth, the salivary glands belong genetically to one series with the +sudoriferous, sebaceous, and mammary glands. + +Thus the human alimentary canal is as simple as the primitive gut of +the gastrula in its original structure. Later it resembles the gut of +the earliest Vermalia (Gastrotricha). It then divides into two +sections, a fore or branchial gut and a hind or hepatic gut, like the +alimentary canal of the Balanoglossus, the Ascidia, and the Amphioxus. +The formation of the jaws and the branchial arches changes it into a +real fish-gut ( _Selachii_). But the branchial gut, the one +reminiscence of our fish-ancestors, is afterwards atrophied as such. +The parts of it that remain are converted into entirely different +structures. + + +Fig.356. Transverse section of the head of a Petromyzon-larva. Fig. +356—Transverse section of the head of a Petromyzon-larva. (From +_Gegenbaur._) Beneath the pharynx ( _d_) we see the hypobranchial +groove; above it the chorda and neural tube. _A, B, C_ stages of +constriction. + + +But, although the anterior section of our alimentary canal thus +entirely loses its original character of branchial gut, it retains the +physiological character of respiratory gut. We are now astonished to +find that the permanent respiratory organ of the higher Vertebrates, +the air-breathing lung, is developed from this first part of the +alimentary canal. Our lungs, trachea, and larynx are formed from the +ventral wall of the branchial gut. The whole of the respiratory +apparatus, which occupies the greater part of the pectoral cavity in +the adult man, is at first merely a small pair of vesicles or sacs, +which grow out of the floor of the head-gut immediately behind the +gills (Figs. 354 _c,_ 147 _l_). These vesicles are found in all the +Vertebrates except the two lowest classes, the Acrania and Cyclostomes. +In the lower Vertebrates they do not develop +into lungs, but into a large air-filled bladder, which occupies a good +deal of the body-cavity and has a quite different purport. It serves, +not for breathing, but to effect swimming movements up and down, and so +is a sort of hydrostatic apparatus—the floating bladder of the fishes ( +_nectocystis,_ p. 233). However, the human lungs, and those of all +air-breathing Vertebrates, develop from the same simple vesicular +appendage of the head-gut that becomes the floating bladder in the +fishes. + +At first this bladder has no respiratory function, but merely acts as +hydrostatic apparatus for the purpose of increasing or lessening the +specific gravity of the body. The fishes, which have a fully-developed +floating bladder, can press it together, and thus condense the air it +contains. The air also escapes sometimes from the alimentary canal, +through an air-duct that connects the floating bladder with the +pharynx, and is ejected by the mouth. This lessens the size of the +bladder, and so the fish becomes heavier and sinks. When it wishes to +rise again, the bladder is expanded by relaxing the pressure. In many +of the Crossopterygii the wall of the bladder is covered with bony +plates, as in the Triassic _Undina_ (Fig. 254). + +This hydrostatic apparatus begins in the Dipneusts to change into a +respiratory organ; the blood-vessels in the wall of the bladder now no +longer merely secrete air themselves, but also take in fresh air +through the air-duct. This process reaches its full development in the +Amphibia. In these the floating bladder has turned into lungs, and the +air-passage into a trachea. The lungs of the Amphibia have been +transmitted to the three higher classes of Vertebrates. In the lowest +Amphibia the lungs on either side are still very simple transparent +sacs with thin walls, as in the common water-salamander, the Triton. It +still entirely resembles the floating bladder of the fishes. It is true +that the Amphibia have two lungs, right and left. But the floating +bladder is also double in many of the fishes (such as the early +Ganoids), and divides into right and left halves. On the other hand, +the lung is single in Ceratodus (Fig. 257). + + +Fig.357. Thoracic and abdominal viscera of a human embryo of twelve +weeks. Fig. 357—Thoracic and abdominal viscera of a human embryo of +twelve weeks. (From _Kölliker._) The head is omitted. Ventral and +pectoral walls are removed. The greater part of the body-cavity is +taken up with the liver, from the middle part of which the cæcum and +the vermiform appendix protrude. Above the diaphragm, in the middle, is +the conical heart; to the right and left of it are the two small lungs. + + +In the human embryo and that of all the other Amniotes the lungs +develop from the hind part of the ventral wall of the head-gut (Fig. +149). Immediately behind the single structure of the thyroid gland a +median groove, the rudiment of the trachea, is detached from the +gullet. From its hinder end a couple of vesicles develop—the simple +tubular rudiments of the right and left lungs. They afterwards increase +considerably in size, fill the greater part of the thoracic cavity, and +take the heart between them. Even in the frogs we find that the simple +sac has developed into a spongy body of peculiar froth-like tissue. The +originally short connection of the pulmonary sacs with the head-gut +extends into a long, thin tube. This is the wind-pipe (trachea); it +opens into the gullet above, and divides below into two branches which +go to the two lungs. In the wall of the trachea circular cartilages +develop, and these keep it open. At its upper end, underneath its +pharyngeal opening, the larynx is formed—the organ of voice and speech. +The larynx is found at various stages of development in the Amphibia, +and comparative anatomists are in a position to trace the progressive +growth of this important organ from the rudimentary structure of the +lower Amphibia up to the elaborate and delicate vocal apparatus that we +have in the larynx of man and of the birds. + +We must refer here to an interesting rudimentary organ of the +respiratory gut, the thyroid gland, the large gland in front of the +larynx, that lies below the “Adam’s +apple,” and is often especially developed in the male sex. It has a +certain function—not yet fully understood—in the nutrition of the body, +and arises in the embryo by constriction from the lower wall of the +pharynx. In many mining districts the thyroid gland is peculiarly +liable to morbid enlargement, and then forms goitre, a growth that +hangs at the front of the neck. But it is much more interesting +phylogenetically. As Wilhelm Müller, of Jena, has shown, this +rudimentary organ is the last relic of the hypobranchial groove, which +we considered in a previous chapter, and which runs in the middle line +of the gill-crate in the Ascidia and Amphioxus, and conveys food to the +stomach. (Cf. p. 184,Fig. 246). We still find it in its original +character in the larvæ of the Cyclostomes (Figs. 355, 356). + +The second section of the alimentary canal, the trunk or hepatic gut, +undergoes not less important modifications among our vertebrate +ancestors than the first section. In tracing the further development of +this digestive part of the gut, we find that most complex and elaborate +organs originate from a very rudimentary original structure. For +clearness we may divide the digestive gut into three sections: the fore +gut (with œsophagus and stomach), the middle gut (duodenum, with liver, +pancreas, jejunum, and ileum, and the hind gut (colon and rectum). Here +again we find vesicular growths or appendages of the originally simple +gut developing into a variety of organs. Two of these embryonic +structures, the yelk-sac and allantois, are already known to us. The +two large glands that open into the duodenum, the liver and pancreas, +are growths from the middle and most important part of the trunk-gut. + +Immediately behind the vesicular rudiments of the lungs comes the +section of the alimentary canal that forms the stomach (Figs. 353 _d,_ +354 _b_). This sac-shaped organ, which is chiefly responsible for the +solution and digestion of the food, has not in the lower Vertebrates +the great physiological importance and the complex character that it +has in the higher. In the Acrania and Cyclostomes and the earlier +fishes we can scarcely distinguish a real stomach; it is represented +merely by the short piece from the branchial to the hepatic gut. In +some of the other fishes also the stomach is only a very simple +spindle-shaped enlargement at the beginning of the digestive section of +the gut, running straight from front to back in the median plane of the +body, underneath the vertebral column. In the mammals its first +structure is just as rudimentary as it is permanently in the preceding. +But its various parts soon begin to develop. As the left side of the +spindle-shaped sac grows much more quickly than the right, and as it +turns considerably on its axis at the same time, it soon comes to lie +obliquely. The upper end is more to the left, and the lower end more to +the right. The foremost end draws up into the longer and narrower canal +of the œsophagus. Underneath this on the left the blind sac (fundus) of +the stomach bulges out, and thus the later form gradually develops +(Figs. 349, 184 _e_). The original longitudinal axis becomes oblique, +sinking below to the left and rising to the right, and approaches +nearer and nearer to a transverse position. In the outer layer of the +stomach-wall the powerful muscles that accomplish the digestive +movements develop from the gut-fibre layer. In the inner layer a number +of small glandular tubes are formed from the gut-gland layer; these are +the peptic glands that secrete the gastric juice. At the lower end of +the gastric sac is developed the valve that separates it from the +duodenum (the pylorus, Fig. 349 _d_). + +Underneath the stomach there now develops the disproportionately long +stretch of the small intestine. The development of this section is very +simple, and consists essentially in an extremely rapid and considerable +growth lengthways. It is at first very short, quite straight, and +simple. But immediately behind the stomach we find at an early stage a +horseshoe-shaped bend and loop of the gut, in connection with the +severance of the alimentary canal from the yelk-sac and the development +of the first mesentery. The thin delicate membrane that fastens this +loop to the ventral side of the vertebral column, and fills the inner +bend of the horseshoe formation, is the first rudiment of the mesentery +(Fig. 147 _g_). We find at an early stage a considerable growth of the +small intestine; it is thus forced to coil itself in a number of loops. +The various sections that we have to distinguish in it are +differentiated in a very simple way—the duodenum (next to the stomach), +the succeeding long jejunum, and the last section of the small +intestine, the ileum. + + +From the duodenum are developed the two large glands that we have +already mentioned—the liver and pancreas. The liver appears first in +the shape of two small sacs, that are found to the right and left +immediately behind the stomach (Figs. 353 _f,_ 354 _c_). In many of the +lower Vertebrates they remain separate for a long time (in the +Myxinoides throughout life), or are only imperfectly joined. In the +higher Vertebrates they soon blend more or less completely to form a +single large organ. The growth of the liver is very brisk at first. In +the human embryo it grows so much in the second month of development +that in the third it occupies by far the greater part of the +body-cavity (Fig. 357). At first the two halves develop equally; +afterwards the left falls far behind the right. In consequence of the +unsymmetrical development and turning of the stomach and other +abdominal viscera, the whole liver is now pushed to the right side. +Although the liver does not afterwards grow so disproportionately, it +is comparatively larger in the embryo at the end of pregnancy than in +the adult. Its weight relatively to that of the whole body is 1 : 36 in +the adult, and 1 : 18 in the embryo. Hence it is very important +physiologically during embryonic life; it is chiefly concerned in the +formation of blood, not so much in the secretion of bile. + +Immediately behind the liver a second large visceral gland develops +from the duodenum, the pancreas or sweetbread. It is wanting in most of +the lowest classes of Vertebrates, and is first found in the fishes. +This organ is also an outgrowth from the gut. + +The last section of the alimentary canal, the large intestine, is at +first in the embryo a very simple, short, and straight tube, which +opens behind by the anus. It remains thus throughout life in the lower +Vertebrates. But it grows considerably in the mammals, coils into +various folds, and divides into two sections, the first and longer of +which is the colon, and the second the rectum. At the beginning of the +colon there is a valve (valvula _Bauhini_) that separates it from the +small intestine. Immediately behind this there is a sac-like growth, +which enlarges into the cæcum (Fig. 357 _v_). In the plant-eating +mammals this is very large, but it is very small or completely +atrophied in the flesh-eaters. In man, and most of the apes, only the +first portion of the cæcum is wide; the blind end-part of it is very +narrow, and seems later to be merely a useless appendage of the former. +This “vermiform appendage” is very interesting as a rudimentary organ. +The only significance of it in man is that not infrequently a +cherry-stone or some other hard and indigestible matter penetrates into +its narrow cavity, and by setting up inflammation and suppuration +causes the death of otherwise sound men. Teleology has great difficulty +in giving a rational explanation of, and attributing to a beneficent +Providence, this dreaded appendicitis. In our plant-eating ancestors +this rudimentary organ was much larger and had a useful function. + +Finally, we have important appendages of the alimentary tube in the +bladder and urethra, which belong to the alimentary system. These +urinary organs, acting as reservoir and duct for the urine excreted by +the kidneys, originate from the innermost part of the allantoic +pedicle. In the Dipneusts and Amphibia, in which the allantoic sac +first makes its appearance, it remains within the body-cavity, and +functions entirely as bladder. But in all the Amniotes it grows far +outside of the body-cavity of the embryo, and forms the large embryonic +“primitive bladder,” from which the placenta develops in the higher +mammals. This is lost at birth. But the long stalk or pedicle of the +allantois remains, and forms with its upper part the middle +vesico-umbilical ligament, a rudimentary organ that goes in the shape +of a solid string from the vertex of the bladder to the navel. The +lowest part of the allantoic pedicle (or the “urachus”) remains hollow, +and forms the bladder. At first this opens into the last section of the +gut in man as in the lower Vertebrates; thus there is a real cloaca, +which takes off both urine and excrements. But among the mammals this +cloaca is only permanent in the Monotremes, as it is in all the birds, +reptiles, and amphibia. In all the other mammals (marsupials and +placentals) a transverse partition is afterwards formed, and this +separates the urogenital aperture in front from the anus-opening +behind. (Cf. p. 249 and Chapter 29.) + + + + +Chapter XXVIII. +EVOLUTION OF THE VASCULAR SYSTEM + + +The use that we have hitherto made of our biogenetic law will give the +reader an idea how far we may trust its guidance in phylogenetic +investigation. This differs considerably in the various systems of +organs; the reason is that heredity and variability have a very +different range in these systems. While some of them faithfully +preserve the original palingenetic development inherited from earlier +animal ancestors, others show little trace of this rigid heredity; they +are rather disposed to follow new and divergent _cenogenetic_ lines of +development in consequence of adaptation. The organs of the first kind +represent the _conservative_ element in the multicellular state of the +human frame, while the latter represent the _progressive_ element. The +course of historic development is a result of the correlation of the +two tendencies, and they must be carefully distinguished. + +There is perhaps no other system of organs in the human body in which +this is more necessary than in that of which we are now going to +consider the obscure development—the vascular system, or apparatus of +circulation. If we were to draw our conclusions as to the original +features in our earlier animal ancestors solely from the phenomena of +the development of this system in the embryo of man and the other +higher Vertebrates, we should be wholly misled. By a number of +important embryonic adaptations, the chief of which is the formation of +an extensive food-yelk, the original course of the development of the +vascular system has been so much falsified and curtailed in the higher +Vertebrates that little or nothing now remains in their embryology of +some of the principal phylogenetic features. We should be quite unable +to explain these if comparative anatomy and ontogeny did not come to +our assistance. + +The vascular system in man and all the Craniotes is an elaborate +apparatus of cavities filled with juices or cell-containing fluids. +These “vessels” (_vascula_) play an important part in the nutrition of +the body. They partly conduct the nutritive red blood to the various +parts of the body (blood-vessels); partly absorb from the gut the white +chyle formed in digestion (chyle-vessels); and partly collect the +used-up juices and convey them away from the tissues (lymphatic +vessels). With the latter are connected the large cavities of the body, +especially the body-cavity, or cœloma. The lymphatic vessels conduct +both the colourless lymph and the white chyle into the venous part of +the circulation. The lymphatic glands act as producers of new +blood-cells, and with them is associated the spleen. The centre of +movement for the circulation of the fluids is the heart, a strong +muscular sac, which contracts regularly and is equipped with valves +like a pump. This constant and steady circulation of the blood makes +possible the complex metabolism of the higher animals. + +But, however important the vascular system may be to the more advanced +and larger and highly-differentiated animals, it is not at all so +indispensable an element of animal life as is commonly supposed. The +older science of medicine regarded the blood as the real source of +life. Even in the still prevalent confused notions of heredity the +blood plays the chief part. People speak generally of full blood, half +blood, etc., and imagine that the hereditary transmission of certain +characters “lies in the blood.” The incorrectness of these ideas is +clearly seen from the fact that in the act of generation the blood of +the parents is not directly transmitted to the offspring, nor does the +embryo possess blood in its early stages. We have already seen that not +only the differentiation of the four secondary germinal layers, but +also the first structures of the principal organs in the embryo of all +the Vertebrates, take place long before there is any +trace of the vascular system—the heart and the blood. In accordance +with this ontogenetic fact, we must regard the vascular system as one +of the latest organs from the phylogenetic point of view; just as we +have found the alimentary canal to be one of the earliest. In any case, +the vascular system is much later than the alimentary. + + +Fig.358. Red blood-cells of various Vertebrates. Fig. 359. Vascular +tissues or endothelium (vasalium). A capillary from the mesentery. Fig. +358—Red blood-cells of various Vertebrates (equally magnified). _1._ of +man, _2._ camel, _3._ dove, _4._ proteus, _5._ water-salamander +(_Triton_), _6._ frog, _7._ merlin (_Cobitis_), _8._ lamprey +(_Petromyzon_). _a_ surface-view, _b_ edge-view. (From _Wagner._) Fig. +359—Vascular tissues or endothelium (_vasalium_). A capillary from the +mesentery. _a_ vascular cells, _b_ their nuclei. + + +The important nutritive fluid that circulates as blood and lymph in the +elaborate canals of our vascular system is not a clear, simple fluid, +but a very complex chemical juice with millions of cells floating in +it. These blood-cells are just as important in the complicated life of +the higher animal body as the circulation of money is to the commerce +of a civilised community. Just as the citizens meet their needs most +conveniently by means of a financial circulation, so the various +tissue-cells, the microscopic citizens of the multicellular human body, +have their food conveyed to them best by the circulating cells in the +blood. These blood cells (_hæmocytes_) are of two kinds in man and all +the other Craniotes—red cells (_rhodocytes_ or _erythrocytes_) and +colourless or lymph cells (_leucocytes_). The red colour of the blood +is caused by the great accumulation of the former, the others circulate +among them in much smaller quantity. When the colourless cells increase +at the expense of the red we get anæmia (or chlorosis). + +The lymph-cells (_leucocytes_), commonly called the “white corpuscles” +of the blood, are phylogenetically older and more widely distributed in +the animal world than the red. The great majority of the Invertebrates +that have acquired an independent vascular system have only colourless +lymph-cells in the circulating fluid. There is an exception in the +Nemertines (Fig. 358) and some groups of Annelids. When we examine the +colourless blood of a cray-fish or a snail (Fig. 358) under a high +power of the microscope, we find in each drop numbers of mobile +leucocytes, which behave just like independent Amoebæ (Fig. 17). Like +these unicellular Protozoa, the colourless blood-cells creep slowly +about, their unshapely plasma-body constantly changing its form, and +stretching out finger-like processes first in one direction, then +another. Like the Amoebæ, they take particles into their cell-body. On +account +of this feature these amoeboid plastids are called “eating cells” +(_phagocytes_), and on account of their motions “travelling cells” +(_planocytes_). It has been shown by the discoveries of the last few +decades that these leucocytes are of the greatest physiological and +pathological consequence to the organism. They can absorb either solid +or dissolved particles from the wall of the gut, and convey them to the +blood in the chyle; they can absorb and remove unusable matter from the +tissues. When they pass in large quantities through the fine pores of +the capillaries and accumulate at irritated spots, they cause +inflammation. They can consume and destroy bacteria, the dreaded +vehicles of infectious diseases; but they can also transport these +injurious Monera to fresh regions, and so extend the sphere of +infection. It is probable that the sensitive and travelling leucocytes +of our invertebrate ancestors have powerfully co-operated for millions +of years in the phylogenesis of the advancing animal organisation. + + +Fig.360. Transverse section of the trunk of a chick-embryo, forty-five +hours old. Fig. 360—Transverse section of the trunk of a chick-embryo, +forty-five hours old. (From _Balfour._) _A_ ectoderm (horny-plate), +_Mc_ medullary tube, _ch_ chorda, _C_ entoderm (gut-gland layer), _Pv_ +primitive segment (episomite), _Wd_ prorenal duct, _pp_ cœloma +(secondary body-cavity). _So_ skin-fibre layer, _Sp_ gut-fibre layer, +_v_ blood-vessels in latter, _ao_ primitive aortas, containing red +blood-cells. + + +The red blood-cells have a much more restricted sphere of distribution +and activity. But they also are very important in connection with +certain functions of the craniote-organism, especially the exchange of +gases or respiration. The cells of the dark red, carbonised or venous, +blood, which have absorbed carbonic acid from the animal tissues, give +this off in the respiratory organs; they receive instead of it fresh +oxygen, and thus bring about the bright red colour that distinguishes +oxydised or arterial blood. The red colouring matter of the blood +(_hæmoglobin_) is regularly distributed in the pores of their +protoplasm. The red cells of most of the Vertebrates are elliptical +flat disks, and enclose a nucleus of the same shape; they differ a good +deal in size (Fig. 358). The mammals are distinguished from the other +Vertebrates by the circular form of their biconcave red cells and by +the absence of a nucleus (Fig. 1); only a few genera still have the +elliptic form inherited from the reptiles (Fig. 2). In the embryos of +the mammals the red cells have a nucleus and the power of increasing by +cleavage (Fig. 10). + +The origin of the blood-cells and vessels in the embryo, and their +relation to the germinal layers and tissues, are among the most +difficult problems of ontogeny—those obscure questions on which the +most divergent opinions are still advanced by the most competent +scientists. In general, it is certain that the greater part of the +cells that compose the vessels and their contents come from the +mesoderm—in fact, from the gut-fibre layer; it was on this account that +Baer gave the name of “vascular layer” to this visceral layer of the +coeloma. But other important observers say that a part of these cells +come from other germinal layers, especially from the gut-gland layer. +It seems to be true that blood-cells may be formed from the cells of +the entoderm before the development of the mesoderm. If we examine +sections of chickens, the earliest and most familiar subjects of +embryology, we find at an early stage the “primitive-aortas” we have +already described (Fig. 360 _ao_) in the ventral angle between the +episoma (_Pv_) and hyposoma (_Sp_). The +thin wall of these first vessels of the amniote embryo consists of flat +cells (_endothelia_ or _vascular epithelia_); the fluid within already +contains numbers of red blood-cells; both have been developed from the +gut-fibre layer. It is the same with the vessels of the germinative +area (Fig. 361 _v_), which lie on the entodermic membrane of the +yelk-sac (_c_). These features are seen still more clearly in the +transverse section of the duck-embryo in Fig. 152.In this we see +clearly how a number of stellate cells proceed from the “vascular +layer” and spread in all directions in the “primary body-cavity”—_i.e._ +in the spaces between the germinal layers. A part of these travelling +cells come together and line the wall of the larger spaces, and thus +form the first vessels; others enter into the cavity, live in the fluid +that fills it, and multiply by cleavage—the first blood-cells. + +But, besides these mesodermic cells of the “vascular layer” proper, +other travelling cells, of which the origin and purport are still +obscure, take part in the formation of blood in the meroblastic +Vertebrates (especially fishes). The chief of these are those that +Ruckert has most aptly denominated “merocytes.” These “eating +yelk-cells” are found in large numbers in the food-yelk of the +Selachii, especially in the yelk-wall—the border zone of the germinal +disk in which the embryonic vascular net is first developed. The nuclei +of the merocytes become ten times as large as the ordinary +cell-nucleus, and are distinguished by their strong capacity for taking +colour, or their special richness in chromatin. Their protoplasmic body +resembles the stellate cells of osseous tissue (astrocytes), and +behaves just like a rhizopod (such as Gromia); it sends out numbers of +stellate processes all round, which ramify and stretch into the +surrounding food-yelk. These variable and very mobile processes, the +pseudopodia of the merocytes, serve both for locomotion and for getting +food; as in the real rhizopods, they surround the solid particles of +food (granules and plates of yelk), and accumulate round their nucleus +the food they have received and digested. Hence we may regard them both +as eating-cells (_phagocytes_) and travelling-cells (_planocytes_). +Their lively nucleus divides quickly and often repeatedly, so that a +number of new nuclei are formed in a short time; as each fresh nucleus +surrounds itself with a mantle of protoplasm, it provides a new cell +for the construction of the embryo. Their origin is still much +disputed. + + +Fig.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells +underneath the embryonic cavity (B). Fig. 361—Merocytes of a +shark-embryo, rhizopod-like yelk-cells underneath the embryonic cavity +(_B_). (From _Ruckert._) _z_ two embryonic cells, _k_ nuclei of the +merocytes, which wander about in the yelk and eat small yelk-plates +(_d_), _k_ smaller, more superficial, lighter nuclei, _k′_ a deeper +nucleus, in the act of cleavage, _k*_ chromatin-filled border-nucleus, +freed from the surrounding yelk in order to show the numerous +pseudopodia of the protoplasmic cell-body. + + +Half of the twelve stems of the animal world have no blood-vessels. +They make their first appearance in the Vermalia. Their earliest source +is the primary body-cavity, the simple space between the two primary +germinal layers, which is either a relic of the segmentation-cavity, or +is a subsequent formation. Amoeboid planocytes, which migrate from the +entoderm and reach this fluid-filled primary cavity, live and multiply +there, and form the first colourless blood-cells. We find the vascular +system in this very simple form to-day in the Bryozoa, Rotatoria, +Nematoda, and other lower Vermalia. + +The first step in the improvement of this primitive vascular system is +the formation of larger canals or blood-conducting tubes. The spaces +filled with blood, the relics of the primary body-cavity, receive a +special wall. “Blood-vessels” of this kind (in the narrower sense) are +found among the higher worms in various forms, sometimes very simple, +at other times very complex. The form +that was probably the incipient structure of the elaborate vascular +system of the Vertebrates (and of the Articulates) is found in two +primordial principal vessels—a dorsal vessel in the middle line of the +dorsal wall of the gut, and a ventral vessel that runs from front to +rear in the middle line of its ventral wall. From the dorsal vessel is +evolved the aorta (or principal artery), from the ventral vessel the +principal or subintestinal vein. The two vessels are connected in front +and behind by a loop that runs round the gut. The blood contained in +the two tubes is propelled by their peristaltic contractions. + + +Fig.362. Vascular system of an Annelid (Saenuris), foremost section. +Fig. 362—Vascular system of an Annelid (_Sænuris_), foremost section. +_d_ dorsal vessel, _v_ ventral vessel, _c_ transverse connection of two +(enlarged in shape of heart). The arrows indicate the direction of the +flow of blood. (From _Gegenbaur._ + + +The earliest Vermalia in which we first find this independent vascular +system are the Nemertina (Fig. 244). As a rule, they have three +parallel longitudinal vessels connected by loops, a single dorsal +vessel above the gut and a pair of lateral vessels to the right and +left. In some of the Nemertina the blood is already coloured, and the +red colouring matter is real hæmoglobin, connected with elliptical +discoid cells, as in the Vertebrates. The further evolution of this +rudimentary vascular system can be gathered from the class of the +Annelids in which we find it at various stages of development. First, a +number of transverse connections are formed between the dorsal and +ventral vessels, which pass round the gut ring-wise (Fig. 362). Other +vessels grow into the body-wall and ramify in order to convey blood to +it. In addition to the two large vessels of the middle plane there are +often two lateral vessels, one to the right and one to the left; as, +for instance, in the leech. There are four of these parallel +longitudinal vessels in the Enteropneusts (_Balanoglossus,_ Fig. 245). +In these important Vermalia the foremost section of the gut has already +been converted into a gill-crate, and the vascular arches that rise in +the wall of this from the ventral to the dorsal vessel have become +branchial vessels. + + +Fig.363. Head of a fish-embryo, with rudimentary vascular system, from +the left. Fig. 363—Head of a fish-embryo, with rudimentary vascular +system, from the left. _dc_ Cuvier’s duct (juncture of the anterior and +posterior principal veins), _sv_ venous sinus (enlarged end of Cuvier’s +duct), _a_ auricle, _v_ ventricle, _abr_ trunk of branchial artery, _s_ +gill-clefts (arterial arches between), _ad_ aorta, _c_ carotid artery, +_n_ nasal pit. (From _Gegenbaur._ + + +We have a further important advance in the Tunicates, which we have +recognised as the nearest blood-relatives of our early vertebrate +ancestors. Here we find for the first time a real heart—_i.e._ a +central organ of circulation, driving the blood into the vessels by the +regular contractions of its muscular wall, it is of a very rudimentary +character, a spindle-shaped tube, passing at both ends into a principal +vessel (Fig. 221). By its original position behind the gill-crate, on +ventral side of the Tunicates (sometimes more, sometimes less, +forward), the head shows clearly that it has been formed by the local +enlargement of a section of the ventral vessel. We have already noticed +the remarkable alternation of the direction of the blood stream, the +heart driving it first from one end, then from the other p. 190. This +is very instructive, because in most of the worms (even the +Enteropneust) the blood in the dorsal vessel travels from back to +front, but in the Vertebrates in the opposite direction. As the +Ascidia-heart alternates steadily from one direction to the other, it +shows us permanently, in a sense, the phylogenetic transition from the +earlier forward direction of the dorsal current (in the worms) to the +new backward direction (in the Vertebrates). + +As the new direction became permanent in the earlier Prochordonia, +which gave rise to the Vertebrate stem, the two vessels that proceed +from either end of the tubular heart acquired a fixed function. +The foremost section of the ventral vessel henceforth always conveys +blood from the heart, and so acts as an artery; the hind section of the +same vessel brings the blood from the body to the heart, and so becomes +a vein. In view of their relation to the two sections of the gut, we +may call the latter the intestinal vein and the former the branchial +artery. The blood contained in both vessels, and also in the heart, is +venous or carbonised blood—_i.e._ rich in carbonic acid; on the other +hand, the blood that passes from the gills into the dorsal vessel is +provided with fresh oxygen—arterial or oxydised blood. The finest +branches of the arteries and veins pass into each other in the tissues +by means of a network of very fine, ventral, hair-like vessels, or +capillaries (Fig. 359). + + +Fig.364. The five arterial arches of the Craniotes (1 to 5) in their +original disposition. Fig. 365. The five arterial arches of the birds; +the lighter parts of the structure disappear; only the shaded parts +remain. Fig. 366. The five arterial arches of mammals. Fig. 364—The +five arterial arches of the Craniotes (_1–5_) in their original +disposition. _a_ arterial cone or bulb, _a″_ aorta-trunk, _c_ carotid +artery (foremost continuation of the roots of the aorta). (From +_Rathke._) +Fig. 365—The five arterial arches of the birds; the lighter parts of +the structure disappear; only the shaded parts remain. Letters as in +Fig. 364. _s_ subclavian arteries, _p_ pulmonary artery, _p′_ branches +of same, _c′_ outer carotid, _c″_ inner carotid. (From _Rathke._) Fig. +366—The five arterial arches of mammals; letters as in Fig. 365. _v_ +vertebral artery, _b_ Botall’s duct (open in the embryo, closed +afterwards). (From _Rathke._) + + +When we turn from the Tunicates to the closely-related Amphioxus we are +astonished at first to find an apparent retrogression in the formation +of the vascular system. As we have seen, the Amphioxus has no real +heart; its colourless blood is driven along in its vascular system by +the principal vessel itself, which contracts regularly in its whole +length (cf. Fig. 210). A dorsal vessel that lies above the gut (aorta) +receives the arterial blood from the gills and drives it into the body. +Returning from here, the venous blood gathers in a ventral vessel under +the gut (intestinal vein), and goes back to the gills. A number of +branchial vascular arches, which effect respiration and rise in the +wall of the branchial gut from belly to back, absorb oxygen from the +water and give off carbonic acid; they connect the ventral with the +dorsal vessel. As the same section of the ventral vessel, which also +forms the heart in the Craniotes, has developed in the Ascidia into a +simple tubular heart, we may regard the absence of this in the +Amphioxus as a result of degeneration, a return in this case to the +earlier form of the vascular system, as we find it in many of the +worms. We may assume that the Acrania that really belong to our +ancestral series did not share this retrogression, but inherited the +one-chambered heart of the Prochordonia, and transmitted it directly to +the earliest Craniotes (cf. the ideal Primitive Vertebrate, +_Prospondylus,_ Figs. 98–102). + +The further phylogenetic evolution of the vascular system is revealed +to us by the comparative anatomy of the Craniotes. At the lowest stage +of this group, in the Cyclostomes, we find for the first time the +differentiation of the vasorium into two sections: a system of +blood-vessels proper, which convey the _red_ blood about the body, and +a system of lymphatic vessels, +which absorb the colourless lymph from the tissues and convey it to the +blood. The lymphatics that absorb from the gut and pour into the +blood-stream the milky food-fluid formed by digestion are distinguished +by the special name of “chyle-vessels.” While the chyle is white on +account of its high proportion of fatty particles, the lymph proper is +colourless. Both chyle and lymph contain the colourless amœboid cells +(leucocytes, Fig. 12) that we also find distributed in the blood as +colourless blood-cells (or “white corpuscles”); but the blood also +contains a much larger quantity of red cells, and these give its +characteristic colour to the blood of the Craniotes (rhodocytes, Fig. +358). The distinction between lymph, chyle, and blood-vessels which is +found in all the Craniotes may be regarded as an outcome of division of +labour between various sections of our originally simple vascular +system. In the Gnathostomes the spleen makes its first appearance, an +organ rich in blood, the chief function of which is the extensive +formation of new colourless and red cells. It is not found in the +Acrania and Cyclostomes, or any of the Invertebrates. It has been +transmitted from the earliest fishes to all the Craniotes. + + +Figs. 367-70. Metamorphosis of the five arterial arches in the human +embryo. Figs. 367–70—Metamorphosis of the five arterial arches in the +human embryo (diagram from _Rathke_). _la_ arterial cone, _1, 2, 3, 4, +5_ first to fifth pair of arteries, _ad_ trunk of aorta, _aw_ roots of +aorta. In Fig. 367 only three, in Fig. 368 all five, of the aortic +arches are given (the dotted ones only are developed). In Fig. 369 the +first two pairs have disappeared again. In Fig. 370 the permanent +trunks of the artery are shown; the dotted parts disappear, _s_ +subclavian artery, _v_ vertebral, _ax_ axillary, _c_ carotid (_c′_ +outer, _c″_ inner carotid), _p_ pulmonary. + + +The heart also, the central organ of circulation in all the Craniotes, +shows an advance in structure in the Cyclostomes. The simple, +spindle-shaped heart-tube, found in the same form in the embryo of all +the Craniotes, is divided into two sections or chambers in the +Cyclostomes, and these are separated by a pair of valves. The hind +section, the auricle, receives the venous blood from the body and +passes it on to the anterior section, the ventricle. From this it is +driven through the trunk of the branchial artery (the foremost section +of the ventral vessel or principal vein) into the gills. + +In the Selachii an arterial cone is developed from the foremost end of +the ventricle, as a special division, cut off by valves. It passes into +the enlarged base of the trunk of the branchial artery (Fig. 363 +_abr_). On each side 5–7 arteries proceed from it. These rise between +the gill-clefts (_s_) on the gill-arches, surround the gullet, and +unite above into a common trunk-aorta, the continuation of which over +the gut corresponds to the dorsal vessel of the worms. As the curved +arteries on the gill-arches spread into a network of respiratory +capillaries, they contain venous blood in their lower part (as arches +of the branchial artery) and arterial blood in the upper part (as +arches of the aorta). The junctures of the various aortic arches on the +right and left are called the roots of the aorta. Of an originally +large number of aortic arches there remain at first six, then (owing to +degeneration of the fifth arch) only five, pairs; and from these five +pairs (Fig. 364) the chief parts of the arterial system develop in all +the higher Vertebrates. + +The appearance of the lungs and the atmospheric respiration connected +therewith, which we first meet in the Dipneusts, is the next important +step in vascular evolution. In the Dipneusts the auricle of the heart +is divided by an incomplete partition into two halves. Only the right +auricle now receives the venous blood from the veins of the body. The +left auricle receives the arterial blood from the pulmonary veins. The +two auricles have a common opening into the simple ventricle, where the +two kinds of blood mix, and are driven through the arterial cone or +bulb into the arterial arches. From the last arterial arches the +pulmonary arteries arise (Fig. 365 _p_). These force a part of the +mixed blood into the lungs, the other part of it going through the +aorta into the body. + + +Fig.371. Heart of a rabbit-embryo, from behind. Fig. 372. Heart of the +same embryo (Fig. 371), from the front. Fig. 371—Heart of a +rabbit-embryo, from behind. _a_ vitelline veins, _b_ auricles of the +heart, _c_ atrium, _d_ ventricle, _e_ arterial bulb, _f_ base of the +three pairs of arterial arches. (From _Bischoff._) Fig. 372—Heart of +the same embryo (Fig. 371), from the front. _v_ vitelline veins, _a_ +auricle, _ca_ auricular canal, _l_ left ventricle, _r_ right ventricle, +_ta_ arterial bulb. (From _Bischoff._) + + +From the Dipneusts upwards we now trace a progressive development of +the vascular system, which ends finally with the loss of branchial +respiration and a complete separation of the two halves of the +circulation. In the Amphibia the partition between the two auricles is +complete. In their earlier stages, as tadpoles (Fig. 262), they have +still the branchial respiration and the circulation of the fishes, and +their heart contains venous blood alone. Afterwards the lungs and +pulmonary vessels are developed, and henceforth the ventricle of the +heart contains mixed blood. In the reptiles the ventricle and its +arterial cone begin to divide into two halves by a longitudinal +partition, and this partition becomes complete in the higher reptiles +and birds on the one hand, and the stem-forms of the mammals on the +other. Henceforth, the right half of the heart contains only venous, +and the left half only arterial, blood, as we find in all birds and +mammals. The right auricle receives its carbonised or venous blood from +the veins of the body, and the right ventricle drives it through the +pulmonary arteries into the lungs. From here the blood returns, as +oxydised or arterial blood, through the pulmonary veins to the left +auricle, and is forced by the left ventricle into the arteries of the +body. Between the pulmonary arteries and veins is the capillary system +of the small or pulmonary circulation. Between the body-arteries and +veins is the capillary system of the large or body-circulation. It is +only in the two highest classes of Vertebrates—the birds and +mammals—that we find a complete division of the circulations. Moreover, +this complete separation has been developed quite independently in the +two classes, as the dissimilar formation of the aortas shows of itself. +In the birds the _right_ half of the fourth arterial arch has become +the permanent arch (Fig. 365). In the mammals this has been developed +from the _left_ half of the same fourth arch (Fig. 366). + + +Fig.373. Heart and head of a dog-embryo, from the front. Fig. 374. +Heart of the same dog-embryo, from behind. Fig. 373—Heart and head of a +dog-embryo, from the front. _a_ fore brain, _b_ eyes, _c_ middle brain, +_d_ primitive lower jaw, _e_ primitive upper jaw, _f_ gill-arches, _g_ +right auricle, _h_ left auricle, _i_ left ventricle, _k_ right +ventricle. (From _Bischoff._) Fig. 374—Heart of the same dog-embryo, +from behind. _a_ inosculation of the vitelline veins, _b_ left auricle, +_c_ right auricle, _d_ auricle, _e_ auricular canal, _f_ left +ventricle, _g_ right ventricle, _h_ arterial bulb. (From _Bischoff._) + + +If we compare the fully-developed arterial system of the various +classes of Craniotes, it shows a good deal of variety, yet it always +proceeds from the same fundamental type. Its development is just the +same in man as in the other mammals; in particular, the modification of +the six pairs of arterial arches is the same in both (Figs. 367–370). +At first there is only a single pair of arches, which +lie on the inner surface of the first pair of gill-arches. Behind this +there then develop a second and third pair of arches (lying on the +inner side of the second and third gill-arches, Fig. 367). Finally, we +get a fourth, fifth, and sixth pair. Of the six primitive arterial +arches of the Amniotes three soon pass away (the first, second, and +fifth); of the remaining three, the third gives the carotids, the +fourth the aortas, and the sixth (number 5 in Figs. 364 and 368) the +pulmonary arteries. + + +Fig.375. Heart of a human embryo, four weeks old. Fig. 376. Heart of a +human embryo, six weeks old, front view. Fig. 377. Heart of a human +embryo, eight weeks old, back view. Fig. 375—Heart of a human embryo, +four weeks old; _1._ front view, _2._ back view, _3._ opened, and upper +half of the atrium removed. _a′_ left auricle, _a″_ right auricle, _v′_ +left ventricle, _v″_ right ventricle, _ao_ arterial bulb, _c_ superior +vena cava (_cd_ right, _cs_ left), _s_ rudiment of the interventricular +wall. (From _Kölliker._) +Fig. 376—Heart of a human embryo, six weeks old, front view. _r_ right +ventricle, _t_ left ventricle, _s_ furrow between ventricles, _ta_ +arterial bulb, _af_ furrow on its surface; to right and left are the +two large auricles. (From _Ecker._) Fig. 377—Heart of a human embryo, +eight weeks old, back view. _a′_ left auricle, _a″_ right auricle, _v′_ +left ventricle, _v″_ right ventricle, _cd_ right superior vena cava, +_ci_ inferior vena cava. (From _Kölliker._) + + +The human heart also develops in just the same way as that of the other +mammals (Fig. 378). We have already seen the first rudiments of its +embryology, which in the main corresponds to its phylogeny (Figs. 201, +202). We saw that the palingenetic form of the heart is a +spindle-shaped thickening of the gut-fibre layer in the ventral wall of +the head-gut. The structure is then hollowed out, forms a simple tube, +detaches from its place of origin, and henceforth lies freely in the +cardiac cavity. Presently the tube bends into the shape of an S, and +turns spirally on an imaginary axis in such a way that the hind part +comes to lie on the dorsal surface of the fore part. The united +vitelline veins open into the posterior end. From the anterior end +spring the aortic arches. + + +Fig.378. Heart of the adult man, fully developed, front view, natural +position. Fig. 378—Heart of the adult man, fully developed, front view, +natural position. _a_ right auricle (underneath it the right +ventricle), _b_ left auricle (under it the left ventricle), _C_ +superior vena cava, _V_ pulmonary veins, _P_ pulmonary artery, _d_ +Botalli’s duct, _A_ aorta. (From _Meyer._) + + +This first structure of the human heart, enclosing a very simple +cavity, corresponds to the tunicate-heart, and is a reproduction of +that of the Prochordonia, but it now divides into two, and subsequently +into three, compartments; this reminds us for a time of the heart of +the Cyclostomes and fishes. The spiral turning and bending of the heart +increases, and at the same time two transverse constrictions appear, +dividing it externally into three sections (Figs. 371, 372). The +foremost section, which is turned towards the ventral side, and from +which the aortic arches rise, reproduces the arterial bulb of the +Selachii. The middle section is a simple ventricle, and the hindmost, +the section turned towards the dorsal side, into which the vitelline +veins inosculate, is a simple auricle (or _atrium_). The latter forms, +like the simple atrium of the fish-heart, a pair of lateral +dilatations, the auricles (Fig. 371 _b_); and the constriction between +the atrium and ventricle is called the auricular canal (Fig. 372 _ca_). +The heart of the human embryo is now a complete fish-heart. + + +In perfect harmony with its phylogeny, the embryonic development of the +human heart shows a gradual transition from the fish-heart, through the +amphibian and reptile, to the mammal form, The most important point in +the transition is the formation of a longitudinal partition—incomplete +at first, but afterwards complete—which separates all three divisions +of the heart into right (venous) and left (arterial) halves (cf. Figs. +373–378). The atrium is separated into a right and left half, each of +which absorbs the corresponding auricle; into the right auricle open +the body-veins (upper and lower vena cava, Figs. 375 _c,_ 377 _c_); the +left auricle receives the pulmonary veins. In the same way a +superficial interventricular furrow is soon seen in the ventricle (Fig. +376 _s_). This is the external sign of the internal partition by which +the ventricle is divided into two—a right venous and left arterial +ventricle. Finally a longitudinal partition is formed in the third +section of the primitive fish-like heart, the arterial bulb, externally +indicated by a longitudinal furrow (Fig. 376 _af_). The cavity of the +bulb is divided into two lateral halves, the pulmonary-artery bulb, +that opens into the right ventricle, and the aorta-bulb, that opens +into the left ventricle. When all the partitions are complete, the +small (pulmonary) circulation is distinguished from the large (body) +circulation; the motive centre of the former is the right half, and +that of the latter the left half, of the heart. + + +Fig.379. Transverse section of the back of the head of a chick-embryo, +forty hours old. Fig. 379—Transverse section of the back of the head of +a chick-embryo, forty hours old. (From _Kölliker._) _m_ medulla +oblongata, _ph_ pharyngeal cavity (head-gut), _h_ horny plate, _h′_ +thicker part of it, from which the auscultory pits afterwards develop, +_hp_ skin-fibre plate, _hh_ cervical cavity (head-cœlom or cardiocœl), +_hzp_ cardiac plate (the outermost mesodermic wall of the heart), +connected by the ventral mesocardium (_uhg_) with the gut-fibre layer +or visceral cœlom-layer (_dfp*prime;_), _Ent_ entoderm, _ihh_ inner +(entodermic?) wall of the heart; the two endothelial cardiac tubes are +still separated by the cenogenetic septum (_s_) of the Amniotes, _g_ +vessels. + + +The heart of all the Vertebrates belongs originally to the hyposoma of +the head, and we accordingly find it in the embryo of man and all the +other Amniotes right in front on the under-side of the head; just as in +the fishes it remains permanently in front of the gullet. It afterwards +descends into the trunk, with the advance in the development of the +neck and breast, and at last reaches the breast, between the two lungs. +At first it lies symmetrically in the middle plane of the body, so that +its long axis corresponds with that of the body. In most of the mammals +it remains permanently in this position. But in the apes the axis +begins to be oblique, and the apex of the heart to move towards the +left side. The displacement is greatest in the anthropoid +apes—chimpanzee, gorilla, and orang—which resemble man in this. + +As the heart of all Vertebrates is originally, in the light of +phylogeny, only a local enlargement of the middle principal vein, it is +in perfect accord with the biogenetic law that its first structure in +the embryo is a simple spindle-shaped tube in the ventral wall of the +head-gut. A thin membrane, standing vertically in the middle plane, the +mesocardium, connects the ventral wall of the head-gut with the lower +head-wall. As the cardiac tube extends and detaches from the gut-wall, +it divides the mesocardium into an upper (dorsal) and lower (ventral) +plate (usually called the _mesocardium anterius_ and _posterius_ in +man, Fig. 379 _uhg_). The +mesocardium divides two lateral cavities, Remak’s “neck-cavities” (Fig. +379 _hh_). These cavities afterwards join and form the simple +pericardial cavity, and are therefore called by Kölliker the “primitive +pericardial cavities.” + +The double cervical cavity of the Amniotes is very interesting, both +from the anatomical and the evolutionary point of view; it corresponds +to a part of the hyposomites of the head of the lower Vertebrates—that +part of the ventral cœlom-pouches which comes next to Van Wijhe’s +“visceral cavities” below. Each of the cavities still communicates +freely behind with the two cœlom-pouches of the trunk; and, just as +these afterwards coalesce into a simple body-cavity (the ventral +mesentery disappearing), we find the same thing happening in the head. +This simple primary pericardial cavity has been well called by +Gegenbaur the “head-cœloma,” and by Hertwig the “pericardial +breast-cavity.” As it now encloses the heart, it may also be called +_cardiocœl._ + + +Fig.380. Frontal section of a human embryo, one-twelfth of an inch long +in the neck. Fig. 380—Frontal section of a human embryo, one-twelfth of +an inch long in the neck; “invented” by _Wilhelm His._ Seen from +ventral side. _mb_ mouth-fissure, surrounded by the branchial +processes, _ab_ bulbus of aorta, _hm_ middle part of ventricle, _hl_ +left lateral part of same, _ho_ auricle, _d_ diaphragm, _vc_ superior +vena cava, _vu_ umbilical vein, _vo_ vitelline space, _lb_ liver, _lg_ +hepatic duct. + + +The cardiocœl, or head-cœlom, is often disproportionately large in the +Amniotes, the simple cardiac tube growing considerably and lying in +several folds. This causes the ventral wall of the amniote embryo, +between the head and the navel, to be pushed outwards as in rupture +(cf. Fig. 180 _h_). A transverse fold of the ventral wall, which +receives all the vein-trunks that open into the heart, grows up from +below between the pericardium and the stomach, and forms a transverse +partition, which is the first structure of the primary diaphragm (Fig. +380 _d_). This important muscular partition, which completely separates +the thoracic and abdominal cavities in the mammals alone, is still very +imperfect here; the two cavities still communicate for a time by two +narrow canals. These canals, which belong to the dorsal part of the +head-cœlom, and which we may call briefly _pleural ducts,_ receive the +two pulmonary sacs, which develop from the hind end of the ventral wall +of the head-gut; they thus become the two pleural cavities. + +The diaphragm makes its first appearance in the class of the Amphibia +(in the salamanders) as an insignificant muscular transverse fold of +the ventral wall, which rises from the fore end of the transverse +abdominal muscle, and grows between the pericardium and the liver. In +the reptiles (tortoises and crocodiles) a later dorsal part is joined +to this earlier ventral part of the rudimentary diaphragm, a pair of +subvertebral muscles rising from the vertebral column and being added +as “columns” to the transverse partition. But it was probably in the +Permian sauro-mammals that the two originally separate parts were +united, and the diaphragm became a complete partition between the +thoracic and abdominal cavities in the mammals; as it considerably +enlarges the chest-cavity when it contracts, it becomes an important +respiratory muscle. The ontogeny of the diaphragm in man and the other +mammals reproduces this phylogenetic process to-day, in accordance with +the biogenetic law; in all the mammals the diaphragm is formed by the +secondary conjunction of the two originally separate structures, the +earlier ventral part and the later dorsal part. + +Sometimes the blending of the two diaphragmatic structures, and +consequently the severance of the one pleural duct from the abdominal +cavity, is not completed in man. This leads to a diaphragmatic rupture +(_hernia diaphragmatica_). The two cavities then remain in +communication by an open pleural duct, and loops of the intestine may +penetrate by this “rupture opening” into the chest-cavity. This is one +of those +fatal mis-growths that show the great part that blind chance has in +organic development. + + +Fig.381. Transverse section of the head of a chick-embryo, thirty-six +hours old. Fig. 381—Transverse section of the head of a chick-embryo, +thirty-six hours old. Underneath the medullary tube the two primitive +aortas (_pa_) can be seen in the head-plates (_s_) at each side of the +chorda. Underneath the gullet (_d_) we see the aorta-end of the heart +(_ae_), _hh_ cervical cavity or head cœlom, _hk_ top of heart, _ks_ +head-sheath, amniotic fold, _h_ horny plate. (From _Remak._ + + +Thus the thoracic cavity of the mammals, with its important contents, +the heart and lungs, belongs originally to the _head-part_ of the +vertebrate body, and its inclusion in the trunk is secondary. This +instructive and very interesting fact is entirely proved by the +concordant evidence of comparative anatomy and ontogeny. The lungs are +outgrowths of the head-gut; the heart develops from its inner wall. The +pleural sacs that enclose the lungs are dorsal parts of the head-cœlom, +originating from the pleuroducts; the pericardium in which the heart +afterwards lies is also double originally, being formed from ventral +halves of the head-cœlom, which only combine at a later stage. When the +lung of the air-breathing Vertebrates issues from the head-cavity and +enters the trunk-cavity, it follows the example of the floating bladder +of the fishes, which also originates from the pharyngeal wall in the +shape of a small pouch-like out-growth, but soon grows so large that, +in order to find room, it has to pass far behind into the trunk-cavity. +To put it more precisely, the lung of the quadrupeds retains this +hereditary growth-process of the fishes; for the hydrostatic floating +bladder of the latter is the air-filled organ from which the +air-breathing organ of the former has been evolved. + + +Fig.382. Transverse section of the cardiac region of the same +chick-embryo (behind the preceding). Fig. 382—Transverse section of the +cardiac region of the same chick-embryo (behind the preceding). In the +cervical cavity (_hh_) the heart (_h_) is still connected by a mesocard +(_hg_) with the gut-fibre layer (_pf_). _d_ gut-gland layer, _up_ +provertebral plates, _jb_ rudimentary auditory vesicle in the horny +plate, _hp_ first rise of the amniotic fold. (From _Remak._) + + +There is an interesting cenogenetic phenomenon in the formation of the +heart of the higher Vertebrates that deserves special notice. In its +earliest form the heart is _double,_ as recent observation has shown, +in all the Amniotes, and the simple spindle-shaped cardiac tube, which +we took as our starting-point, is only formed at a later stage, when +the two lateral tubes move backwards, touch each other, and at last +combine in the middle line. In man, as in the rabbit, the two embryonic +hearts are still far apart at the stage when there are already eight +primitive segments (Fig. 134 _h_). So also the two cœlom-pouches of the +head in which they lie are still separated by a broad space. It is not +until the permanent body of the embryo develops and detaches from the +embryonic vesicle that the separate lateral structures join together, +and finally combine in the middle line. As the median partition between +the right and left cardiocœl disappears, the two cervical cavities +freely communicate (Fig. 381), and form, on the ventral side of the +amniote head, a horseshoe-shaped arch, the points of which advance +backwards into the pleuro-ducts or pleural cavities, and from there +into the two peritoneal sacs of the trunk. But even after the +conjunction of the cervical cavities (Fig. 381) the two cardiac tubes +remain separate at first; and even after they have united a delicate +partition in the middle of the simple endothelial tube (Figs. 379 _s,_ +382 _h_) indicates the original separation. This _cenogenetic_ “primary +cardiac +septum” presently disappears, and has no relation to the subsequent +permanent partition between the halves of the heart, which, as a +heritage from the reptiles, has a great _palingenetic_ importance. + +Thorough opponents of the biogenetic law have laid great stress on +these and similar cenogenetic phenomena, and endeavoured to urge them +as striking disproofs of the law. As in every other instance, careful, +discriminating, comparative-morphological examination converts these +supposed disproofs of evolution into strong arguments in its favour. In +his excellent work, _On the structure of the Heart in the Amphibia_ +(1886), Carl Rabl has shown how easily these curious cenogenetic facts +can be explained by the secondary adaptation of the embryonic structure +to the great extension of the food-yelk. + +The embryology of all the other parts of the vascular system also gives +us abundant and valuable data for the purposes of phylogeny. But as one +needs a thorough knowledge of the intricate structure of the whole +vascular system in man and the other Vertebrates in order to follow +this with profit, we cannot go into it further here. Moreover, many +important features in the ontogeny of the vascular system are still +very obscure and controverted. The characters of the embryonic +circulation of the Amniotes, which we have previously considered +(Chapter XV), are late acquisitions and entirely cenogenetic. (Cf. pp. +170–171; Figs. 198–202.) + + + + +Chapter XXIX. +EVOLUTION OF THE SEXUAL ORGANS + + +If we measure the importance of the systems of organs in the animal +frame according to the richness and variety of their phenomena and the +physiological interest that this implies, we must regard as one of the +principal and most interesting systems the one which we are now going +to examine—the system of the reproductive organs. Just as nutrition is +the first and most urgent condition for the self-maintenance of the +individual organism, so reproduction alone secures the maintenance of +the species—or, rather, the maintenance of the long series of +generations which the totality of the organic stem represents in their +genealogical connection. No individual organism has the prerogative of +immortality. To each is allotted only a brief span of personal +development, an evanescent moment in the million-year course of the +history of life. + +Hence, reproduction and the correlative phenomenon, heredity, have long +been regarded, together with nutrition, as the most important and +fundamental function of living things, and it has been attempted to +distinguish them from “lifeless bodies” on this very score. As a matter +of fact, this division is not so profound and thorough as it seems to +be, and is generally supposed to be. If we examine carefully the nature +of the reproductive process, we soon see that it can be reduced to a +general property that is found in inorganic as well as organic +bodies—growth. Reproduction is a nutrition and growth of the organism +beyond the individual limit, which raises a part of it into the whole. +This is most clearly seen when we study it in the simplest and lowest +organisms, especially the Monera (Figs. 226–228) and the unicellular +Amœbæ (Fig. 17). There the simple individual is a single plastid. As +soon as it has reached a certain limit of size by continuous feeding +and normal growth, it cannot pass it, but divides, by simple cleavage, +into two equal halves. Each of these halves then continues its +independent life, and grows on until it in turn reaches the limit of +growth, and divides. In each of these acts of self-cleavage two new +centres of attraction are formed for the particles of bodies, the +foundations of the two new-formed individuals. There is no such thing +as immortality even in these unicellulars. +The individual as such is annihilated in the act of cleavage (cf. p. +48). + +In many other Protozoa reproduction takes place not by cleavage, but by +budding (gemmation). In this case the growth that determines +reproduction is not total (as in segmentation), but partial. Hence in +gemmation also we may oppose the local growth-product, that becomes a +new individual in the bud, as a child-organism to the parent-organism +from which it is formed. The latter is older and larger than the +former. In cleavage the two products are equal in age and morphological +value. Next to gemmation we have, as other forms of asexual +reproduction, the forming of embryonic buds and the forming of +embryonic cells. But the latter leads us at once to sexual generation, +the distinctive feature of which is the separation of the sexes. I have +dealt fully with these various types of reproduction in my _History of +Creation_ (chap. viii) and my _Wonders of Life_ (chap. xi). + +The earliest ancestors of man and the higher animals had no faculty of +sexual reproduction, but multiplied solely by asexual means—cleavage, +gemmation, or the formation of embryonic buds or cells, as many +Protozoa still do. The differentiation of the sexes came at a later +stage. We see this most plainly in the Protists, in which the union of +two individuals precedes the continuous cleavage of the unicellular +organism (transitory conjugation and permanent copulation of the +Infusoria). We may say that in this case the growth (the condition of +reproduction) is attained by the coalescence of two full-grown cells +into a single, disproportionately large individual. At the same time, +the mixture of the two plastids causes a rejuvenation of the plasm. At +first the copulating cells are quite homogeneous; but natural selection +soon brings about a certain contrast between them—larger female cells +(_macrospores_) and smaller male cells (_microspores_). It must be a +great advantage in the struggle for life for the new individual to have +inherited different qualities from the two cellular parents. The +further advance of this contrast between the generating cells led to +sexual differentiation. One cell became the female ovum +(_macrogonidion_), and the other the male sperm-cell (_microgonidion_). + +The simplest forms of sexual reproduction among the living Metazoa are +seen in the Gastræads p. 233, the lower sponges, the common fresh-water +polyp (_Hydra_), and other Cœlenteria of the lowest rank. Prophysema +(Fig. 234), Olynthus (Fig. 238), Hydra, etc., have very simple tubular +bodies, the thin wall of which consists (as in the original gastrula) +only of the two primary germinal layers. As soon as the body reaches +sexual maturity, a number of the cells in its wall become female ova, +and others male sperm-cells: the former become very large, as they +accumulate a considerable quantity of yelk-granules in their protoplasm +(Fig. 235 _e_); the latter are very small on account of their repeated +cleavage, and change into mobile cone-shaped spermatozoa (Fig. 20). +Both kinds of cells detach from their source of origin, the primary +germinal layers, fall either into the surrounding water or into the +cavity of the gut, and unite there by fusing together. This is the +momentous process of fecundation, which we have examined in Chapter VII +(cf. Figs. 23–29). + +From these simplest forms of sexual propagation, as we can observe them +to-day in the lowest Zoophytes, the Gastræads, Sponges, and Polyps, we +gather most important data. In the first place, we learn that, properly +speaking, nothing is required for sexual reproduction except the fusion +or coalescence of two different cells—a female ovum and male +sperm-cell. All other features, and all the very complex phenomena that +accompany the sexual act in the higher animals, are of a subordinate +and secondary character, and are later additions to this simple, +primary process of copulation and fecundation. But if we bear in mind +how extremely important a part this relation of the two sexes plays in +the whole of organic nature, in the life of plants, of animals, and of +man; how the mutual attraction of the sexes, love, is the mainspring of +the most remarkable processes—in fact, one of the chief mechanical +causes of the highest development of life—we cannot too greatly +emphasise this tracing of love to its source, the attractive force of +two erotic cells. + +Throughout the whole of living nature the greatest effects proceed from +this very small cause. Consider the part that the flowers, the sexual +organs of the flowering plants, play in nature; or the exuberance of +wonderful phenomena that sexual selection produces in animal life; or +the +momentous influence of love in the life of man. In every case the +fusion of two cells is the sole original motive power; in every case +this invisible process profoundly affects the development of the most +varied structures. We may say, indeed, that no other organic process +can be compared to it for a moment in comprehensiveness and intensity +of action. Are not the Semitic myth of Adam and Eve, the old Greek +legend of Paris and Helena, and so many other famous traditions, only +the poetic expression of the vast influence that love and sexual +selection have exercised over the course of history ever since the +differentiation of the sexes? All the other passions that agitate the +heart of man are far outstripped in their joint influence by this +sense-inflaming and mind-benumbing Eros. On the one hand, we look to +love with gratitude as the source of the greatest artistic +achievements—the noblest creations of poetry, plastic art, and music; +we see in it the chief factor in the moral advance of humanity, the +foundation of family life, and therefore of social advance. On the +other hand, we dread it as the devouring flame that brings destruction +on so many, and has caused more misery, vice, and crime than all the +other evils of human life put together. So wonderful is love and so +momentous its influence on the life of the soul, or on the different +functions of the medullary tube, that here more than anywhere else the +“supernatural” result seems to mock any attempt at natural explanation. +Yet comparative evolution leads us clearly and indubitably to the first +source of love—the affinity of two different erotic cells, the +sperm-cell and ovum.[34] + + [34] The sensual perception (probably related to smell) of the two + copulating sex-cells, which causes their mutual attraction, is a + little understood, but very interesting, chemical function of the + cell-soul (cf. p. 58 and _The Riddle of the Universe,_ chap. ix.) + + +The lowest Metazoa throw light on this very simple origin of the +intricate phenomena of reproduction, and they also teach us that the +earliest sexual form was hermaphrodism, and that the separation of the +sexes (by division of labour) is a secondary and later phenomenon. +Hermaphrodism predominates in the most varied groups of the lower +animals; each sexually-mature individual, each person, contains female +and male sexual cells, and is therefore able to fertilise itself and +reproduce. Thus we find ova and sperm-cells in the same individual, not +only in the lowest Zoophytes (Gastræads, Sponges, and many Polyps), but +also in many worms (leeches and earthworms), many of the snails (the +common garden and vineyard snails), all the Tunicates, and many other +invertebrate animals. All man’s earlier invertebrate ancestors, from +the Gastræads up to the Prochordonia, were hermaphrodites; possibly +even the earliest Acrania. We have an instructive proof of this in the +remarkable circumstance that many genera of fishes are still +hermaphrodites, and that it is occasionally found in the higher +Vertebrates of all classes (as atavism). We may conclude from this that +gonochorism (separation of the sexes) was a later stage in our +development. At first, male and female individuals differ only in the +possession of one or other kind of gonads; in other respects they were +identical, as we still find in the Amphioxus and the Cyclostomes. +Afterwards, accessory organs (ducts, etc.) are associated with the +primary sexual glands; and much later again sexual selection has given +rise to the secondary sexual characters—those differences between the +sexes which do not affect the sexual organs themselves, but other parts +of the body (such as the man’s beard or the woman’s breast). + +The third important fact that we learn from the lower Zoophytes relates +to the earliest origin of the two kinds of sexual cells. As in the +Gastræads (the lowest sponges and hydroids), in which we find the first +beginnings of sexual differentiation, the whole body consists merely of +the two primary germinal layers, it follows that the sexual cells also +must have proceeded from the cells of these primary layers, either the +inner or outer, or from both. This simple fact is extremely important, +because the first trace of the ova as well as the spermatozoa is found +in the middle germinal layer or mesoderm in the higher animals, +especially the Vertebrates. This arrangement is a later development +from the preceding (in connection with the secondary formation of the +mesoderm). + +If we trace the phylogeny of the sexual organs in our earliest Metazoa +ancestors, as the comparative anatomy and ontogeny of the lowest +Cœlenteria (_Cnidaria, Platodaria_) exhibit it to us, we find that the +first step in advance is the localisation or concentration of the two +kinds of sexual +cells scattered in the epithelium into definite groups. In the Sponges +and lowest Hydropolyps isolated cells are detached from the cell-strata +of the two primary germinal layers, and become free sexual cells; but +in the Cnidaria and Platodes we find these associated in groups which +we call sexual glands (_gonads_). We can now for the first time speak +of sexual organs in the morphological sense. The female germinative +glands, which in this simplest form are merely groups of homogeneous +cells, are the ovaries (Fig. 241 _c_). The male germinative glands, +which also in their first form consist of a cluster of sperm-cells, are +the testicles (Fig. 241 _h_). In the medusæ, which descend, both +ontogenetically and phylogenetically, from the more simply organised +Polyps, we find these simple sexual glands sometimes as gastric +pouches, sometimes as outgrowths of the radial canals that proceed from +the stomach. Particularly interesting in connection with the question +of the first origin of the gonads are the lowest forms of the Platodes, +the _Cryptocœla_ that have of late been separated as a special class +(_Platodaria_) from the Turbellaria proper (Fig. 239). In these very +primitive Platodes the two pairs of sexual glands are merely two pairs +of rows of differentiated cells in the entodermic wall of the primitive +gut—two median ovaries (_o_) within, and two lateral spermaries (_s_) +without. The mature sexual cells are ejected by the posterior outlets; +the female (_f_) lies in front of the male (_m_). + + +Fig.383. Embryos of Sagitta, in three earlier stages of development. +Fig. 383—Embryos of Sagitta, in three earlier stages of development. +(From _Hertwig._) _A_ gastrula, _B_ cœlomula with open primitive mouth, +_C_ the same primitive mouth closed, _ua_ primitive gut, _bl_ primitive +mouth, _g_ progonidia (hermaphroditic primitive sexual cells), _cs_ +cœlom-pouches, _pm_ parietal layer, _vm_ visceral layer of same, _d_ +permanent gut (enteron), _st_ mouth-pit (stomodæum). + + +In the great majority of the Bilateria or Cœlomaria it is the mesoderm +from which the gonads develop. Probably the first traces of them are +the two large cells that appear at the edge of the primitive mouth +(right and left), as a rule during gastrulation or immediately +afterwards—the important promesoblasts, or “polar cells of the +mesoderm,” or “primitive cells of the middle germinal layer” (p. 194). +In the real Enterocœla, in which the mesoderm appears from the first in +the shape of a couple of cœlom-pouches, these are very probably the +original gonads (p. 194). This is seen very clearly in the arrow-worm +(_Sagitta_). In the gastrula of Sagitta (Fig. 383 _A_) we find at an +early stage a couple of entodermic cells of an unusual size (_g_) at +the base of the primitive gut (_ud_). These primitive sexual cells +(_progonidia_) are symmetrically placed to the right and left of the +middle plane, like the two promesoblasts of the bilateral gastrula of +the Amphioxus (Fig. 38 _p_). A little outwards from them the two cœlom +pouches (_B, cs_) are developed out of the primitive gut, and each +progonidion divides into a male and a female sexual cell (_B, g_). The +two male cells (at first rather the larger) lie close together within, +and are the parent-cells of the testicles (_prospermaria_). The two +female cells lie outwards from these, and are the parent-cells of the +ovary (_protovaria_). Afterwards, when the cœlom-pouches have detached +from the permanent gut (_C, d_) and the primitive mouth (_A, bl_) is +closed, the female cells advance towards the mouth (_C, st_), and the +male towards the rear. The foremost pair of ovaries are then separated +by a transverse partition from the hind pair. Thus the first structures +of the sexual glands of the Sagitta are a couple of hermaphroditic +entodermic cells; each of these divides +into a male and a female cell; and these four cells are the +parent-cells of the four sexual glands. Probably the two promesoblasts +of the Amphioxus-gastrula (Fig. 38) are also hermaphroditic primitive +sexual cells in the same sense, inherited by this earliest vertebrate +from its ancient bilateral gastræad ancestors. + + +Fig.384. A, Part of the kidneys of Bdellostoma. B Portion of same, +highly magnified. Fig. 384—_A,_ Part of the kidneys of Bdellostoma. _a_ +prorenal duct (nephroductus), _b_ segmental or primitive urinary canals +(pronephridia), _c_ renal or Malpighian capsules. _B_ Portion of same, +highly magnified. _c_ renal capsules with the glomerulus, _d_ afferent +artery, _e_ efferent artery. From _Johannes Müller_ (Myxinoides). + + +The sexually-mature Amphioxus is not hermaphroditic, as its nearest +invertebrate relatives, the Tunicates, are, and as the long-extinct +pre-Silurian Primitive Vertebrate (_Prospondylus,_ Figs. 98–102) +probably was. The actual lancelet has gonochoristic structures of a +very interesting kind. As we saw in the anatomy of the Amphioxus, we +find the ovaries of the female and the spermaries of the male in the +shape of twenty to thirty pairs of elliptical or roundish four-cornered +sacs, which lie on either side of the gut on the parietal surface of +the respiratory pore (Fig. 219 _g_). According to the important +discovery of Rückert (1888), the sexual glands of the earliest fishes, +the Selachii, are similarly arranged. They only unite afterwards to +form a pair of simple gonads. These have been transmitted by heredity +to all the rest of the Craniotes. In every case they lie originally on +each side of the mesentery, underneath the chorda, at the bottom of the +body-cavity. The first traces of them are found in the +cœlom-epithelium, at the spot where the skin-fibre layer and gut-fibre +layer meet in the middle of the mesenteric plate (Fig. 93 _mp_). At +this point we observe at an early stage in all craniote embryos a small +string-like cluster of cells, which we may call, with Waldeyer, the +“germ epithelium,” or (in harmony with the other plate-shaped +rudimentary organs) the _sexual plate_ (Fig. 173 _g_). This germinal or +sexual plate is found in the fifth week in the human embryo, in the +shape of a couple of long whitish streaks, on the inner side of the +primitive kidneys (Fig. 183 _t_). The cells of this sexual plate are +distinguished by their cylindrical form and chemical composition from +the rest of the cœlom-cells; they have a different purport from the +flat cells which line the rest of the body-cavity. As the germ +epithelium of the sexual plate becomes thicker, and supporting tissue +grows into it from the mesoderm, it becomes a rudimentary sexual gland. +This ventral gonad then develops into the ovary in the female +Craniotes, and the testicles in the male. + +In the formation of the gonidia or erotic sexual cells and their +conjunction at fecundation we have the sole essential features of +sexual reproduction; but in the great majority of animals we find other +organs taking part in it. The chief of these secondary sexual organs +are the gonoducts, which serve to convey the mature sexual cells out of +the body, and the copulative organs, which bring the fecundating male +sperm into touch with the ovum-bearing female. The latter organs are, +as a rule, only found in the higher animals, and are much less widely +distributed than the gonoducts. But these also are secondary +formations, and are wanting in many animals of the lower groups. + +In the lower animals the mature sexual cells are generally ejected +directly from +the body. Sometimes they pass out immediately through the skin (Hydra +and many hydroids); sometimes they fall into the gastric cavity, and +are evacuated by the mouth (gastræads, sponges, many medusæ, and +corals); sometimes they fall into the body-cavity, and are ejected by a +special pore (_porus genitalis_) in the ventral wall. The latter +procedure is found in many of the worms, and also in the lowest +Vertebrates. Amphioxus has the peculiar feature that the mature sexual +products fall first into the mantle-cavity; from there they are either +evacuated by the respiratory pore, or else they pass through the +gill-clefts into the branchial gut, and so out by the mouth (p. 185). +In the Cyclostomes they fall into the body-cavity, and are ejected by a +genital pore in its wall; so also in some of the fishes. From these we +gather the features of our earlier ancestors in this respect. On the +other hand, in all the higher and most of the lower Vertebrates (and +most of the higher Invertebrates) we find in both sexes special tubular +passages of the sexual gland, which are called “gonoducts.” In the +female they conduct the ova from the ovary, and so are called +“oviducts,” or “Fallopian tubes.” In the male they convey the +spermatozoa from the testicles, and are called “spermaducts,” or _vasa +deferentia._ + + +Fig.385. Transverse section of the embryonic shield of a chick, +forty-two hours old. Fig. 385—Transverse section of the embryonic +shield of a chick, forty-two hours old. (From _Kölliker._) _mr_ +medullary tube, _ch_ chorda, _h_ horny plate (skin-sense layer), _ung_ +nephroduct, _vw_ episomites (dorsal primitive segments), _hp_ +skin-fibre layer (parietal layer of the hyposomites), _dfp_ gut-fibre +layer (visceral layer of hyposomites), _ao_ aorta, _g_ vessels. (Cf. +transverse section of duck-embryo, Fig. 152.) + + +The original and genetic relation of these two kinds of ducts is just +the same in man as in the rest of the higher Vertebrates, and quite +different from what we find in most of the Invertebrates. In the +latter, as a rule, the gonoducts develop directly from the embryonic +glands or from the outer skin; but in the Vertebrates an independent +organic system is employed to convey the sexual products, and this had +originally a totally different function—namely, the system of urinary +organs. These organs have primarily the sole duty of removing unusable +matter from the body in a fluid form. Their liquid excretory product, +the urine, is either evacuated directly through the skin or through the +last section of the gut. It is only at a later stage that the tubular +urinary passages also convey the sexual products from the body. In this +way they become “urogenital ducts.” This remarkable secondary +conjunction of the urinary and sexual organs into a common urogenital +system is very characteristic of the Gnathostomes, the six higher +classes of Vertebrates. It is wanting in the lower classes. In order to +appreciate it fully, we must give a comparative glance at the structure +of the urinary organs. + +The renal or urinary system is one of the oldest and most important +systems of organs in the differentiated animal body, as I have pointed +out on several previous occasions (cf. Chapter XVII). We find it not +only in the higher stems, but also very generally distributed in the +earlier group of the Vermalia. Here we meet it in the lowest worms, the +Rotatoria (Gastrotricha, Fig. 242), and in the instructive stem of the +Platodes. It consists of a pair of simple or branching canals, which +are lined with one layer of cells, absorb unusable juices from the +tissue, and eject them by an outlet in the outer skin (Fig. 240 _nm_). +Not only the free-living Turbellaria, but also the parasitic Suctoria, +and even the still more degenerate tapeworms, which have lost their +alimentary canal in consequence of their parasitic life, are equipped +with these renal canals +or nephridia. In the first embryonic structure they are merely a pair +of simple cutaneous glands, or depressions in the ectoderm. They are +generally described as excretory organs in the worms, but formerly +often as “water vessels.” They may be conceived as largely-developed +tubular cutaneous glands, formed by invagination of the cutaneous +layer. According to another view, they owe their origin to a later +rupture of the body-cavity outwards. In most of the Vermalia each +nephridium has an inner opening (with cilia) into the body-cavity and +an outer one on the epidermis. + + +Fig.386. Rudimentary primitive kidneys of a dog-embryo. Fig. +386—Rudimentary primitive kidneys of a dog-embryo. The hind end of the +embryonic body is seen from the ventral side and covered with the +visceral layer of the yelk-sac, which is torn away and folded down in +front in order to show the nephroducts with the primitive urinary +canals (_a_). _b_ primitive vertebræ, _c_ spinal cord, _d_ entrance +into the pelvic-gut cavity. (From _Bischoff._) + + +Fig. 387. Primitive kidneys of a human embryo. Fig. 387—Primitive +kidneys of a human embryo. _u_ the urinary canals of the primitive +kidneys, _w_ Wolffian duct, _w′_ uppermost end of the same (Morgagni’s +hydatid), _m_ Mullerian duct. _m′_ uppermost end of same (Fallopian +hydatid), _g_ gonad (sexual gland). (From _Kobelt._) + + +In these lowest, unsegmented worms, and in the unsegmented Molluscs, +there is only one pair of renal canals. They are more numerous in the +higher Articulates. In the Annelids, the body of which is composed of a +large number of joints, there is a pair of these pronephridia in each +segment (hence they are called segmental canals or organs). Even here +they are still simple tubes; on account of their coiled or looped form +they are often called “looped canals.” In most of the Annelids, and +many of the Vermalia, we can distinguish three sections in the +nephridium—an outer muscular duct, a glandular middle part, and an +inner part that opens by a ciliated funnel into the body-cavity. This +opening is furnished with whirling cilia, and can, therefore, take up +the juices to be excreted directly from the body-cavity and convey them +from the body. But in these worms the sexual cells, which develop in +very primitive form on the inner surface of the body-cavity, also fall +into it when mature, and are sucked up by the funnel-shaped inner +ciliated openings of the renal canals, and ejected with the urine. Thus +the urine-forming looped canals, or pronephridia, serve as oviducts in +the female Annelids and as spermaducts in the male. + +The renal system of the Vertebrates is similar to, yet materially +different from, these segmental canals of the Annelids. The peculiar +development of it and its relations to the sexual organs are among the +most difficult problems in the morphology of our stem. If we examine +briefly the vertebrate renal system from the phylogenetic point of +view, as confirmed by recent discoveries, we may distinguish three +forms of it: (1) Fore-kidneys or head-kidneys (_pronephros_); (2) +primitive or middle kidneys (mesonephros); (3) permanent kidneys +(_metanephros_). These three systems of kidneys are not fundamentally +and completely distinct, as earlier students (such as Semper) wrongly +supposed; they represent three different generations of one and the +same excretory apparatus; they correspond to three phylogenetic stages, +and succeed each other in the stem-history of the Vertebrates in such +wise that each younger and more advanced generation develops farther +behind in the body, and replaces the older and less advanced generation +that preceded it in time and space. The _fore kidneys,_ first +accurately described by Wilhelm Müller in 1875 in the Cyclostomes and +Ichthyoda, form the sole excretory organ of the Acrania (Amphioxus); +they continue in the Cyclostomes and some of the fishes, but are found +only in slight traces and for a time in the embryos of the six other +classes of Vertebrates. The _primitive kidneys_ are first found in the +Cyclostomes, behind the fore kidneys; they have been transmitted from +the Selachii to all the Gnathostomes. In the _Anamnia_ they act +permanently as urinary glands; in the _Amniotes_ their anterior part +(“germinal kidneys”) changes into organs of the sexual apparatus, while +the third generation develops from the end of their posterior part +(“urinal kidneys”)—the characteristic after or permanent kidneys of the +three higher classes of Vertebrates. The order in which the three renal +systems succeed each other in the embryo of man and the higher +Vertebrates corresponds to their phylogenetic succession in the history +of our stem, and, consequently, in the natural classification of the +Vertebrates. + + +Fig.388. Pig-embryo, three-fifths of an inch long, seen from the +ventral side. Fig. 388—Pig-embryo, three-fifths of an inch long, seen +from the ventral side. _a_ fore leg, _z_ hind leg, _b_ ventral wall, +_r_ sexual prominence, _w_ nephroduct, _n_ primitive kidneys, _n1_ +their inner part. (From _Oscar Schultze._) + + +Fig. 389. Human embryo of the fifth week, two-fifths of an inch long, +seen from the ventral side. Fig. 389—Human embryo of the fifth week, +two-fifths of an inch long, seen from the ventral side (the anterior +ventral wall, _b,_ is removed, the body-cavity, _c,_ opened). _d_ gut +(cut off), _f_ frontal process, _g_ cerebrum, _m_ middle brain, _e_ +after brain, _h_ heart, _k_ first gill-cleft, _l_ pulmonary sac, _n_ +primitive kidneys, _r_ sexual region, _p_ phallus (sexual prominences), +_s_ tail. (From _Kollmann._) + + +As in the morphology of any other system of organs, so in the case of +the urinary and sexual organs the Amphioxus is the real typical +primitive Vertebrate; it affords the key to the mysteries of the +structure of man and the higher Vertebrates. The kidneys of the +Amphioxus—first discovered by Boveri in 1890—are typical “fore +kidneys,” composed of a double row of short segmental canals (Fig. 217 +_x_). The inner aperture of these pronephridia opens into the +mesodermic body-cavity (the middle part of the cœloma, _B_); the +external aperture into the ectodermic mantle or peribranchial cavity +(_C_). Their position, their +structure, and their relation to the branchial vessel make it clear +that these segmental pronephridia correspond to the rudimentary fore +kidneys of the Craniotes. The mantle-cavity into which they open seems +to correspond to the prorenal duct of the latter. + + +Fig.390, 391, 392. Primitive kidneys and rudimentary sexual organs. +Figs. 390, 391, 392—Primitive kidneys and rudimentary sexual organs. +Figs. 390 and 391 of Amphibia (frog-larvæ); Fig. 390 earlier, 391 later +stage. Fig. 392 of a mammal (ox-embryo). _u_ primitive kidney, _k_ +sexual gland (rudiment of testicle and ovary). The primary nephroduct +(_ug_ in Fig. 390) divides (in Figs. 391 and 392) into the two +secondary nephroducts—the Mullerian (_m_) and Wolffian (_ug′_) ducts, +joined together behind in the genital cord (_g_). _l_ ligament of the +primitive kidneys. (From Gegenbaur.) + + +Fig.393, 394. Urinary and sexual organs of an Amphibian (water +salamander or Triton). Fig. 393 of a female, 394 of a male. Figs. 393, +394—Urinary and sexual organs of an Amphibian (water salamander or +Triton). Fig. 393 of a female, 394 of a male. _r_ primitive kidney, +_ov_ ovary, _od_ oviduct and _c_ Rathke’s duct, both developed from the +Müllerian duct, _u_ primitive ureter (also acting as spermaduct [_ve_] +in the male, opening below into the Wolffian duct [_u_ apostrophe]), +_ms_ mesovarium. (From _Gegenbaur._) + + +The next higher Vertebrates, the Cyclostomes, yield some very +interesting data. Both orders of this class, the hags and lampreys, +have still the fore kidneys inherited from the Acrania—the former +permanently, the latter in their earlier stages. Behind these the +primitive kidneys soon develop, and in a very characteristic form. The +remarkable structure of the mesonephros of the Cyclostomes, discovered +by Johannes Müller, explains the intricate formation of the kidneys in +the higher Vertebrates. We find in the hag-fishes (_Bdellostoma_) a +long tube, the prorenal duct (_nephroductus,_ Fig. 384 _a_). This opens +with its anterior end into the cœloma by a ciliated aperture, and +externally with its posterior end by an outlet in the skin. Inside it +open a large number of small transverse canals (“segmental or primitive +urinary canals,” _b_). Each of these terminates blindly in a vesicular +capsule (_c_), and this encloses a coil of blood-vessel (_glomerulus,_ +an arterial network, Fig. 384 _B, c_). Afferent branches of arteries +conduct arterial blood into the coiled branches of the glomerulus +(_d_), and efferent arterial branches conduct it away from the net +(_c_). The primitive renal canals (mesonephridia) are distinguished by +this net-formation from their predecessors. + +In the Selachii also we find a longitudinal row of segmental canals on +each side, which open outwards into the primitive renal ducts +(_nephrotomes,_ p. 149. The segmental canals (a pair in each segment of +the middle part of the body) open internally by a ciliated funnel into +the body-cavity. From the posterior group of these organs a compact +primitive kidney is formed, the anterior group taking part in the +construction of the sexual organs. + +In the same simple form that remains +throughout life in the Myxinoides and partly in the Selachii we find +the primitive kidney first developing in the embryo of man and the +higher Craniotes (Figs. 386, 387). Of the two parts that compose the +comb-shaped primitive kidney the longitudinal channel, or nephroduct, +is always the first to appear; afterwards the transverse “canals,” the +excreting nephridia, are formed in the mesoderm; and after this again +the Malpighian capsules with their arterial coils are associated with +these as cœlous outgrowths. The primitive renal duct, which appears +first, is found in all craniote embryos at the early stage in which the +differentiation of the medullary tube takes place in the ectoderm, the +severance of the chorda from the visceral layer in the entoderm, and +the first trace of the cœlom-pouches arises between the limiting layers +(Fig. 385). The nephroduct (_ung_) is seen on each side, directly under +the horny plate, in the shape of a long, thin, thread-like string of +cells. It presently hollows out and becomes a canal, running straight +from front to back, and clearly showing in the transverse section of +the embryo its original position in the space between horny plate +(_h_), primitive segments (_uw_), and lateral plates (_hpl_). As the +originally very short urinary canals lengthen and multiply, each of the +two primitive kidneys assumes the form of a half-feathered leaf (Fig. +387). The lines of the leaf are represented by the urinary canals +(_u_), and the rib by the outlying nephroduct (_w_). At the inner edge +of the primitive kidneys the rudiment of the ventral sexual gland (_g_) +can now be seen as a body of some size. The hindermost end of the +nephroduct opens right behind into the last section of the rectum, thus +making a cloaca of it. However, this opening of the nephroducts into +the intestine must be regarded as a secondary formation. Originally +they open, as the Cyclostomes clearly show, quite independently of the +gut, in the external skin of the abdomen. + + +Fig.395. Primitive kidneys and germinal glands of a human embryo, three +inches in length (beginning of the sixth week). Fig. 395—Primitive +kidneys and germinal glands of a human embryo, three inches in length +(beginning of the sixth week), magnified. _k_ germinal gland, _u_ +primitive kidney, _z_ diaphragmatic ligament of same, _w_ Wolffian duct +(opened on the right), _g_ directing ligament (gubernaculum), _a_ +allantoic duct. (From _Kollmann._) + + +In the Myxinoides the primitive kidneys retain this simple comb-shaped +structure, and a part of it is preserved in the Selachii; but in all +the other Craniotes it is only found for a short time in the embryo, as +an ontogenetic reproduction of the earlier phylogenetic structure. In +these the primitive kidney soon assumes the form (by the rapid growth, +lengthening, increase, and serpentining of the urinary canals) of a +large compact gland, of a long, oval or spindle-shaped character, which +passes through the greater part of the embryonic body-cavity (Figs. 183 +_m,_ 184 _m,_ 388 _n_). It lies near the middle line, directly under +the primitive vertebral column, and reaches from the cardiac region to +the cloaca. The right and left kidneys are parallel to each other, +quite close together, and only separated by the mesentery—the thin +narrow layer that attaches the middle gut to the under surface of the +vertebral column. The passage of each primitive kidney, the nephroduct, +runs towards the back on the lower and outer side of the gland, and +opens in the cloaca, close to the starting-point of the allantois; it +afterwards opens into the allantois itself. + +The primitive or primordial kidneys of the amniote embryo were formerly +called the “Wolffian bodies,” and sometimes “Oken’s bodies.” They act +for a time as +kidneys, absorbing unusable juices from the embryonic body and +conducting them to the cloaca—afterwards to the allantois. There the +primitive urine accumulates, and thus the allantois acts as bladder or +urinary sac in the embryos of man and the other Amniotes. It has, +however, no genetic connection with the primitive kidneys, but is a +pouch-like growth from the anterior wall of the rectum (Fig. 147 _u_). +Thus it is a product of the visceral layer, whereas the primitive +kidneys are a product of the middle layer. Phylogenetically we must +suppose that the allantois originated as a pouch-like growth from the +cloaca-wall in consequence of the expansion caused by the urine +accumulated in it and excreted by the kidneys. It is originally a blind +sac of the rectum. The real bladder of the vertebrate certainly made +its first appearance among the Dipneusts (in Lepidosiren), and has been +transmitted from them to the Amphibia, and from these to the Amniotes. +In the embryo of the latter it protrudes far out of the not yet closed +ventral wall. It is true that many of the fishes also have a “bladder.” +But this is merely a local enlargement of the lower section of the +nephroducts, and so totally different in origin and composition from +the real bladder. The two structures can be compared from the +physiological point of view, and so are _analogous,_ as they have the +same function; but not from the morphological point of view, and are +therefore not _homologous._ The false bladder of the fishes is a +mesodermic product of the nephroducts; the true bladder of the +Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of the +rectum. + + +Figs. 396-398. Urinary and sexual organs of ox-embryos. Figs. +396–398—Urinary and sexual organs of ox-embryos. Fig. 396, female +embryo one and a half inches long; Fig. 397, male embryo, one and a +half inches long. Fig. 398 female embryo two and a half inches long. +_w_ primitive kidney, _wg_ Wolffian duct, _m_ Müllerian duct, _m′_ +upper end of same (opened at _t_), _i_ lower and thicker part of same +(rudiment of uterus), _g_ genital cord, _h_ testicle, (_h′,_ lower and +_h″,_ upper testicular ligament), _o_ ovary, _o′_ lower ovarian +ligament, _i_ inguinal ligament of primitive kidney, _d_ diaphragmatic +ligament of primitive kidney, _nn_ accessory kidneys, _n_ permanent +kidneys, under them the S-shaped ureters, between these the rectum, _v_ +bladder, _a_ umbilical artery. (From _Kölliker._) + + +In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, +Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower +stage of development to this extent, that the primitive kidneys +(_protonephri_) act permanently as urinary glands. This is only so as a +passing phase of the early embryonic life in the three higher classes +of Vertebrates, the Amniotes. In these the permanent or after or +secondary (really _tertiary_) kidneys (_renes_ or _metanephri_) that +are distinctive of these three classes soon make their appearance. They +represent the third and last generation of the vertebrate kidneys. The +permanent kidneys do not arise (as was long supposed) as independent +glands from the alimentary tube, but from the last section of the +primitive kidneys and the nephroduct. Here a simple tube, the secondary +renal duct, develops, near the point of its entry into the cloaca; and +this tube grows considerably forward. With its blind upper or anterior +end is connected a glandular renal growth, that owes its origin to a +differentiation of the last part of the primitive kidneys. This +rudiment of the +permanent kidneys consists of coiled urinary canals with Malpighian +capsules and vascular coils (without ciliated funnels), of the same +structure as the segmental mesonephridia of the primitive kidneys. The +further growth of these metanephridia gives rise to the compact +permanent kidneys, which have the familiar bean-shape in man and most +of the higher mammals, but consist of a number of separate folds in the +lower mammals, birds, and reptiles. As the permanent kidneys grow +rapidly and advance forward, their passage, the ureter, detaches +altogether from its birth-place, the posterior end of the nephroduct; +it passes to the posterior surface of the allantois. At first in the +oldest Amniotes this ureter opens into the cloaca together with the +last section of the nephroduct, but afterwards separately from this, +and finally into the permanent bladder apart from the rectum +altogether. The bladder originates from the hindmost and lowest part of +the allantoic pedicle (_urachus_), which enlarges in spindle shape +before the entry into the cloaca. The anterior or upper part of the +pedicle, which runs to the navel in the ventral wall of the embryo, +atrophies subsequently, and only a useless string-like relic of it is +left as a rudimentary organ; that is the single vesico-umbilical +ligament. To the right and left of it in the adult male are a couple of +other rudimentary organs, the lateral vesico-umbilical ligaments. These +are the degenerate string-like relics of the earlier umbilical +arteries. + +Though in man and all the other Amniotes the primitive kidneys are thus +early replaced by the permanent kidneys, and these alone then act as +urinary organs, all the parts of the former are by no means lost. The +nephroducts become very important physiologically by being converted +into the passages of the sexual glands. In all the Gnathostomes—or all +the Vertebrates from the fishes up to man—a second similar canal +develops beside the nephroduct at an early stage of embryonic +evolution. The latter is usually called the Müllerian duct, after its +discoverer, Johannes Müller, while the former is called the Wolffian +duct. The origin of the Müllerian duct is still obscure; comparative +anatomy and ontogeny seem to indicate that it originates by +differentiation from the Wolffian duct. Perhaps it would be best to +say: “The original primary nephroduct divides by differentiation (or +longitudinal cleavage) into two secondary nephroducts, the Wolffian and +the Müllerian ducts.” The latter (Fig. 387 _m_) lies just on the inner +side of the former (Fig. 387 _w_). Both open behind into the cloaca. + + +Fig.399. Female sexual organs of a Monotreme (Ornithorhynchus, Fig. +269). Fig. 399—Female sexual organs of a Monotreme (_Ornithorhynchus,_ +Fig. 269). _o_ ovaries, _t_ oviducts, _u_ womb, _sug_ urogenital sinus; +at _u′_ is the outlet of the two wombs, and between them the bladder +(_vu_). _cl_ cloaca. (From _Gegenbaur._) + + +However uncertain the origin of the nephroduct and its two products, +the Müllerian and the Wolffian ducts, may be, its later development is +clear enough. In all the Gnathostomes the Wolffian duct is converted +into the spermaduct, and the Müllerian duct into the oviduct. Only one +of them is retained in each sex; the other either disappears +altogether, or only leaves relics in the shape of rudimentary organs. +In the male sex, in which the two Wolffian ducts become the +spermaducts, we often find traces of the Müllerian ducts, which I have +called “Rathke’s canals” (Fig. 394 _c_). In the female sex, in which +the two Müllerian ducts form the oviducts, there are relics of the +Wolffian ducts, which are called “the ducts of Gaertner.” + +We obtain the most interesting information with regard to this +remarkable evolution of the nephroducts and their association with the +sexual glands from the Amphibia (Figs. 390–395). The first structure of +the nephroduct and its differentiation into Müllerian and Wolffian +ducts are just the same in both sexes in the Amphibia, as in the mammal +embryos (Figs. 392, 396). In the female Amphibia +the Müllerian duct develops on either side into a large oviduct (Fig. +393 _od_), while the Wolffian duct acts permanently as ureter (_u_). In +the male Amphibia the Müllerian duct only remains as a rudimentary +organ without any functional significance, as Rathke’s canal (Fig. 394 +_c_); the Wolffian duct serves also as ureter, but at the same time as +spermaduct, the sperm-canals (_ve_) that proceed from the testicles +(_t_) entering the fore part of the primitive kidneys and combining +there with the urinary canals. + + +Figs. 400, 401. Original position of the sexual glands in the ventral +cavity of the human embryo (three months old). Figs. 400, 401—Original +position of the sexual glands in the ventral cavity of the human embryo +(three months old). Fig. 400, male. _h_ testicles, _gh_ conducting +ligament of the testicles, _wg_ spermaduct, _h_ bladder, _uh_ inferior +vena cava, _nn_ accessory kidneys, _n_ kidneys. Fig. 401, female. _r_ +round maternal ligament (underneath it the bladder, over it the +ovaries). _r′_ kidneys, _s_ accessory kidneys, _c_ cæcum, _o_ small +reticle, _om_ large reticle (stomach between the two), _l_ spleen. +(From _Kölliker._) + + +In the mammals these permanent amphibian features are only seen as +brief phases of the earlier period of embryonic development (Fig. 392). +Here the primitive kidneys, which act as excretory organs of urine +throughout life in the amnion-less Vertebrates, are replaced in the +mammals by the permanent kidneys. The real primitive kidneys disappear +for the most part at an early stage of development, and only small +relics of them remain. In the male mammal the _epididymis_ develops +from the uppermost part of the primitive kidney; in the female a +useless rudimentary organ, the _epovarium,_ is formed from the same +part. The atrophied relic of the former is known as the _paradidymis,_ +that of the latter as the _parovarium._ + + +Fig.402. Urogenital system of a human embryo of three inches in length. +Fig. 402—Urogenital system of a human embryo of three inches in length. +_h_ testicles, _wg_ spermaducts, _gh_ conducting ligament, _p_ +processus vaginalis, _b_ bladder, _au_ umbilical arteries, _m_ +mesorchium, _d_ intestine, _u_ ureter, _n_ kidney, _nn_ accessory +kidney. (From _Kollman._) + + +The Müllerian ducts undergo very important changes in the female +mammal. The oviducts proper are developed only from their upper part; +the lower part dilates into a spindle-shaped tube with thick muscular +wall, in which the impregnated ovum develops into the embryo. This is +the womb (_uterus_). At first the two wombs (Fig. 399 _u_) are +completely separate, and open into the cloaca on either side of the +bladder (_vu_), as is still the case in the lowest living mammals, the +Monotremes. But in the Marsupials a communication is opened between the +two Müllerian ducts, and in the Placentals they combine below with the +rudimentary Wolffian ducts to form a single “genital cord.” The +original independence of the two wombs and the vaginal canals formed +from their lower ends are retained in many of the lower Placentals, but +in the higher they gradually blend and form a single organ. The +conjunction proceeds from below (or behind) upwards (or forwards). In +many of the Rodents (such as the rabbit and squirrel) two separate +wombs still open into the simple and single vaginal canal; but in +others, and in the Carnivora, Cetacea, and Ungulates, the lower halves +of the wombs have already fused into a single piece, though the upper +halves (or “horns”) are still separate (“two-horned” womb, _uteris +bicornis_). In the bats and lemurs the “horns” are +very short, and the lower common part is longer. Finally, in the apes +and in man the blending of the two halves is complete, and there is +only the one simple, pear-shaped uterine pouch, into which the oviducts +open on each side. This simple uterus is a late evolutionary product, +and is found _only_ in the ape and man. + + +Figs. 403-406. Origin of human ova in the female ovary. Figs. +403–406—Origin of human ova in the female ovary. Fig. 403. Vertical +section of the ovary of a new-born female infant, _a_ ovarian +epithelium, _b_ rudimentary string of ova, _c_ young ova in the +epithelium, _d_ long string of ova with follicle-formation (Pflüger’s +tube), _e_ group of young follicles, _f_ isolated young follicle, _g_ +blood-vessels in connective tissue (stroma) of the ovary. In the +strings the young ova are distinguished by their considerable size from +the surrounding follicle-cells. (From _Waldeyer._) Fig. 404—Two young +Graafian follicles, isolated. In _1_ the follicle-cells still form a +simple, and in _2_ a double, stratum round the young ovum; in _2_ they +are beginning to form the ovolemma or the zona pellucida (_a_). Figs. +405 and 406—Two older Graafian follicles, in which fluid is beginning +to accumulate inside the eccentrically thickened epithelial mass of the +follicle-cells (Fig. 405 with little, 406 with much, follicle-water). +_ei_ the young ovum, with embryonic vesicle and spot, _zp_ ovolemma or +zona pellucida, _dp_ discus proligerus, formed of an accumulation of +follicle-cells, which surround the ovum, _ff_ follicle-liquid (_liquor +folliculi_), gathered inside the stratified follicle-epithelium (_fe_), +_fk_ connective-tissue fibrous capsule of the Graafian follicle (_theca +folliculi_). + + +In the male mammals there is the same fusion of the Müllerian and +Wolffian ducts at their lower ends. Here again they form a single +genital cord (Fig. 397 _g_), and this opens similarly into the +original urogenital sinus, which develops from the lowest section of +the bladder (_v_). But while in the male mammal the Wolffian ducts +develop into the permanent spermaducts, there are only rudimentary +relics left of the Müllerian ducts. The most notable of these is the +“male womb” (_uterus masculinus_), which originates from the lowest +fused part of the ducts, and corresponds to the female uterus. It is a +small, flask-shaped vesicle without any physiological significance, +which opens into the ureter between the two spermaducts and the +prostate folds (_vesicula prostatica_). + + +Fig.407. A ripe human Graafian follicle. Fig. 407—A ripe human Graafian +follicle. _a_ the mature ovum, _b_ the surrounding follicle-cells, _c_ +the epithelial cells of the follicle, _d_ the fibrous membrane of the +follicle, _e_ its outer surface. + + +The internal sexual organs of the mammals undergo very distinctive +changes of position. At first the germinal glands of both sexes lie +deep inside the ventral cavity, at the inner edge of the primitive +kidneys (Figs. 386 _g_, 392 _k_), attached to the vertebral column by a +short mesentery (_mesorchium_ in the male, _mesovarium_ in the female). +But this primary arrangement is retained permanently only in the +Monotremes (and the lower Vertebrates). In all other mammals (both +Marsupials and Placentals) they leave their original cradle and travel +more or less far down (or behind), following the direction of a +ligament that goes from the primitive kidneys to the inguinal region of +the ventral wall. This is the inguinal ligament of the primitive +kidneys, known in the male as the Hunterian ligament (Fig. 400 _gh_), +and in the female as the “round maternal ligament” (Fig. 401 _r_). In +woman the ovaries travel more or less towards the small pelvis, or +enter into it altogether. In the male the testicles pass out of the +ventral cavity, and penetrate by the inguinal canal into a sac-shaped +fold of the outer skin. When the right and left folds (“sexual +swellings”) join together they form the _scrotum._ The various mammals +bring before us the successive stages of this displacement. In the +elephant and the whale the testicles descend very little, and remain +underneath the kidneys. In many of the rodents and carnassia they enter +the inguinal canal. In most of the higher mammals they pass through +this into the scrotum. As a rule, the inguinal canal closes up. When it +remains open the testicles may periodically pass into the scrotum, and +withdraw into the ventral cavity again in time of rut (as in many of +the marsupials, rodents, bats, etc.). + +The structure of the external sexual organs, the copulative organs that +convey the fecundating sperm from the male to the female organism in +the act of copulation, is also peculiar to the mammals. There are no +organs of this character in most of the other Vertebrates. In those +that live in water (such as the Acrania and Cyclostomes, and most of +the fishes) the ova and sperm-cells are simply ejected into the water, +where their conjunction and fertilisation are left to chance. But in +many of the fishes and amphibia, which are viviparous, there is a +direct conveyance of the male sperm into the female body; and this is +the case with all the Amniotes (reptiles, birds, and mammals). In these +the urinary and sexual organs always open originally into the last +section of the rectum, which thus forms a cloaca +(p. 249). Among the mammals this arrangement is permanent only in the +Monotremes, which take their name from it (Fig. 399 _cl_). In all the +other mammals a frontal partition is developed in the cloaca (in the +human embryo about the beginning of the third month), and this divides +it into two cavities. The anterior cavity receives the urogenital +canal, and is the sole outlet of the urine and the sexual products; the +hind or anus-cavity passes the excrements only. + +Even before this partition has been formed in the Marsupials and +Placentals, we see the first trace of the external sexual organs. First +a conical protuberance rises at the anterior border of the +cloaca-outlet—the sexual prominence (_phallus,_ Fig. 402 _A, e, B, e_). +At the tip it is swollen in the shape of a club (“acorn” _glans_). On +its under side there is a furrow, the sexual groove (_sulcus genitalis, +f_), and on each side of this a fold of skin, the “sexual pad” (_torus +genitalis, h l_). The sexual protuberance or phallus is the chief organ +of the sexual sense (p. 282); the sexual nerves spread on it, and these +are the principal organs of the specific sexual sensation. As erectile +bodies (_corpora cavernosa_) are developed in the male phallus by +peculiar modifications of the blood-vessels, it becomes capable of +erecting periodically on a strong accession of blood, becoming stiff, +so as to penetrate into the female vagina and thus effect copulation. +In the male the phallus becomes the penis; in the female it becomes the +much smaller clitoris; this is only found to be very large in certain +apes (_Ateles_). A prepuce (“foreskin”) is developed in both sexes as a +protecting fold on the anterior surface of the phallus. + + +Fig.408. The human ovum after issuing from the Graafian follicle, +surrounded by the clinging cells of the discus proligerus (in two +radiating crowns). Fig. 408—The human ovum after issuing from the +Graafian follicle, surrounded by the clinging cells of the _discus +proligerus_ (in two radiating crowns). _z_ ovolemma (zona pellucida, +with radial porous canals), _p_ cytosoma (protoplasm of the cell-body, +darker within, lighter without), _k_ nucleus of the ovum (embryonic +vesicle). (From _Nagel._) (Cf. Figs. 1 and 14.) + + +The external sexual member (_phallus_) is found at various stages of +development within the mammal class, both in regard to size and shape, +and the differentiation and structure of its various parts; this +applies especially to the terminal part of the phallus, the glans, both +the larger _glans penis_ of the male and the smaller _glans clitoridis_ +of the female. The part of the cloaca from the upper wall of which it +forms belongs to the _proctodæum,_ the ectodermic invagination of the +rectum (p. 311); hence its epithelial covering can develop the same +horny growths as the corneous layer of the epidermis. Thus the glans, +which is quite smooth in man and the higher apes, is covered with +spines in many of the lower apes and in the cat, and in many of the +rodents with hairs (marmot) or scales (guinea-pig) or solid horny warts +(beaver). Many of the Ungulates have a free conical projection on the +glans, and in many of the Ruminants this “phallus-tentacle” grows into +a long cone, bent hook-wise at the base (as in the goat, antelope, +gazelle, etc.). The different forms of the phallus are connected with +variations in the structure and distribution of the sensory +corpuscles—_i.e._ the real organs of the sexual sense, which develop in +certain papillæ of the corium of the phallus, and have been evolved +from ordinary tactile corpuscles of the corium by erotic adaptation (p. +282). + + +The formation of the _corpora cavernosa,_ which cause the stiffness of +the phallus and its capability of penetrating the vagina, by certain +special structures of their spongy vascular spaces, also shows a good +deal of variety within the vertebrate stem. This stiffness is increased +in many orders of mammals (especially the carnassia and rodents) by the +ossification of a part of the fibrous body (_corpus fibrosum_). This +penis-bone (_os priapi_) is very large in the badger and dog, and bent +like a hook in the marten; it is also very large in some of the lower +apes, and protrudes far out into the glans. It is wanting in most of +the anthropoid apes; it seems to have been lost in their case (and in +man) by atrophy. + +The sexual groove on the under side of the phallus receives in the male +the mouth of the urogenital canal, and is changed into a continuation +of this, becoming a closed canal by the juncture of its parallel edges, +the male urethra. In the female this only takes place in a few cases +(some of the lemurs, rodents, and moles); as a rule, the groove remains +open, and the borders of this “vestibule of the vagina” develop into +the smaller labia (_nymphæ_). The large labia of the female develop +from the sexual pads (_tori genitales_), the two parallel folds of the +skin that are found on each side of the genital groove. They join +together in the male, and form the closed scrotum. These striking +differences between the two sexes cannot yet be detected in the human +embryo of the ninth week. We begin to trace them in the tenth week of +development, and they are accentuated in proportion as the difference +of the sexes develops. + +Sometimes the normal juncture of the two sexual pads in the male fails +to take place, and the sexual groove may also remain open +(_hypospadia_). In these cases the external male genitals resemble the +female, and they are often wrongly regarded as cases of hermaphrodism. +Other malformations of various kinds are not infrequently found in the +human external sexual organs, and some of them have a great +morphological interest. The reverse of hypospadia, in which the penis +is split open below, is seen in _epispadia,_ in which the urethra is +open above. In this case the urogenital canal opens above at the dorsal +root of the penis; in the former case down below. These and similar +obstructions interfere with a man’s generative power, and thus +prejudicially affect his whole development. They clearly prove that our +history is not guided by a “kind Providence,” but left to the play of +blind chance. + +We must carefully distinguish the rarer cases of real hermaphrodism +from the preceding. This is only found when the essential organs of +reproduction, the genital glands of both kinds, are united in one +individual. In these cases either an ovary is developed on the right +and a testicle on the left (or _vice versa_); or else there are +testicles and ovaries on both sides, some more and others less +developed. As hermaphrodism was probably the original arrangement in +all the Vertebrates, and the division of the sexes only followed by +later differentiation of this, these curious cases offer no theoretical +difficulty. But they are rarely found in man and the higher mammals. On +the other hand, we constantly find the original hermaphrodism in some +of the lower Vertebrates, such as the Myxinoides, many fishes of the +perch-type (_serranus_), and some of the Amphibia (ringed snake, toad). +In these cases the male often has a rudimentary ovary at the fore end +of the testicle; and the female sometimes has a rudimentary, inactive +testicle. In the carp also and some other fishes this is found +occasionally. We have already seen how traces of the earlier +hemaphrodism can be traced in the passages of the Amphibia. + +Man has faithfully preserved the main features of his stem-history in +the ontogeny of his urinary and sexual organs. We can follow their +development step by step in the human embryo in the same advancing +gradation that is presented to us by the comparison of the urogenital +organs in the Acrania, Cyclostomes; Fishes, Amphibia, Reptiles, and +then (within the mammal series) in the Monotremes, Marsupials, and the +various Placentals. All the peculiarities of urogenital structure that +distinguish the mammals from the rest of the Vertebrates are found in +man; and in all special structural features he resembles the apes, +particularly the anthropoid apes. In proof of the fact that the special +features of the mammals have been inherited by man, I will, in +conclusion, point out the identical way in which the ova are formed in +the ovary. In all the mammals the mature ova are contained in special +capsules, which are known as the _Graafian_ +_follicles,_ after their discoverer, Roger de Graaf (1677). They were +formerly supposed to be the ova themselves; but Baer discovered the ova +within the follicles (p. 16). Each follicle (Fig. 407) consists of a +round fibrous capsule (_d_), which contains fluid and is lined with +several strata of cells (_c_). The layer is thickened like a knob at +one point (_b_); this ovum-capsule encloses the ovum proper (_a_). The +mammal ovary is originally a very simple oval body (Fig. 387 _g_), +formed only of connective tissue and blood-vessels, covered with a +layer of cells, the ovarian epithelium or the female germ epithelium. +From this germ epithelium strings of cells grow out into the connective +tissue or “stroma” of the ovary (Fig. 403 _b_). Some of the cells of +these strings (or Pflüger’s tubes) grow larger and become ova +(primitive ova, _c_); but the great majority remain small, and form a +protective and nutritive stratum of cells round each ovum—the +“follicle-epithelium” (_e_). + +The follicle-epithelium of the mammal has at first one stratum (Fig. +404 _1_), but afterwards several (_2_). It is true that in all the +other Vertebrates the ova are enclosed in a membrane, or “follicle,” +that consists of smaller cells. But it is only in the mammals that +fluid accumulates between the growing follicle-cells, and distends the +follicle into a large round capsule, on the inside wall of which the +ovum lies, at one side (Figs. 405, 406). There again, as in the whole +of his morphology, man proves indubitably his descent from the mammals. + +In the lower Vertebrates the formation of ova in the germ-epithelium of +the ovary continues throughout life; but in the higher it is restricted +to the earlier stages, or even to the period of embryonic development. +In man it seems to cease in the first year; in the second year we find +no new-formed ova or chains of ova (Pflüger’s tubes). However, the +number of ova in the two ovaries is very large in the young girl; there +are calculated to be 72,000 in the sexually-mature maiden. In the +production of the ova men resemble most of the anthropoid apes. + +Generally speaking, the natural history of the human sexual organs is +one of those parts of anthropology that furnish the most convincing +proofs of the animal origin of the human race. Any man who is +acquainted with the facts and impartially weighs them will conclude +from them alone that we have been evolved from the lower Vertebrates. +The larger and the detailed structure, the action, and the +embryological development of the sexual organs are just the same in man +as in the apes. This applies equally to the male and the female, the +internal and the external organs. The differences we find in this +respect between man and the anthropoid apes are much slighter than the +differences between the various species of apes. But all the apes have +certainly a common origin, and have been evolved from a long-extinct +early-Tertiary stem-form, which we must trace to a branch of the +lemurs. If we had this unknown pithecoid stem-form before us, we should +certainly put it in the order of the true apes in the primate system; +but within this order we cannot, for the anatomic and ontogenetic +reasons we have seen, separate man from the group of the anthropoid +apes. Here again, therefore, on the ground of the +pithecometra-principle, comparative anatomy and ontogeny teach with +full confidence the descent of man from the ape. + + + + +Chapter XXX. +RESULTS OF ANTHROPOGENY + + +Now that we have traversed the wonderful region of human embryology and +are familiar with the principal parts of it, it will be well to look +back on the way we have come, and forward to the further path to truth +to which it has led us. We started from the simplest facts of ontogeny, +or the development of the individual—from observations that we can +repeat and verify by microscopic and anatomic study at any moment. The +first and most important of these facts is that every man, like every +other animal, begins his existence as a simple cell. This round ovum +has the same characteristic form and origin as the ovum of any other +mammal. From it is developed in the same manner in all the Placentals, +by repeated cleavage, a multicellular blastula. This is converted into +a gastrula, and this in turn into a blastocystis (or embryonic +vesicle). The two strata of cells that compose its wall are the primary +germinal layers, the skin-layer (ectoderm), and gut-layer (entoderm). +This two-layered embryonic form is the ontogenetic reproduction of the +extremely important phylogenetic stem-form of all the Metazoa, which we +have called the Gastræa. As the human embryo passes through the +gastrula-form like that of all the other Metazoa, we can trace its +phylogenetic origin to the Gastræa. + +As we continued to follow the embryonic development of the two-layered +structure, we saw that first a third, or middle layer (mesoderm), +appears between the two primary layers; when this divides into two, we +have the four secondary germinal layers. These have just the same +composition and genetic significance in man as in all the other +Vertebrates. From the skin-sense layer are developed the epidermis, the +central nervous system, and the chief part of the sense-organs. The +skin-fibre layer forms the corium and the motor organs—the skeleton and +the muscular system. From the gut-fibre layer are developed the +vascular system, the muscular wall of the gut, and the sexual glands. +Finally, the gut-gland layer only forms the epithelium, or the inner +cellular stratum of the mucous membrane of the alimentary canal and +glands (lungs, liver, etc.). + +The manner in which these different systems of organs arise from the +secondary germinal layers is essentially the same from the start in man +as in all the other Vertebrates. We saw, in studying the embryonic +development of each organ, that the human embryo follows the special +lines of differentiation and construction that are only found otherwise +in the Vertebrates. Within the limits of this vast stem we have +followed, step by step, the development both of the body as a whole and +of its various parts. This higher development follows in the human +embryo the form that is peculiar to the mammals. Finally, we saw that, +even within the limits of this class, the various phylogenetic stages +that we distinguish in a natural classification of the mammals +correspond to the ontogenetic stages that the human embryo passes +through in the course of its evolution. We were thus in a position to +determine precisely the position of man in this class, and so to +establish his relationship to the different orders of mammals. + +The line of argument we followed in this explanation of the ontogenetic +facts was simply a consistent application of the biogenetic law. In +this we have throughout taken strict account of the distinction between +palingenetic and cenogenetic phenomena. Palingenesis (or “synoptic +development”) alone enables us to draw conclusions from the observed +embryonic form to the stem-form preserved by heredity. Such inference +becomes more or less precarious when there has been cenogenesis, or +disturbance of development, owing to fresh adaptations. We cannot +understand embryonic development unless we appreciate this very +important distinction. Here we stand at the very limit that separates +the older and the new science or philosophy of nature. The whole of the +results of recent morphological research compel us irresistibly +to recognise the biogenetic law and its far-reaching consequences. +These are, it is true, irreconcilable with the legends and doctrines of +former days, that have been impressed on us by religious education. But +without the _biogenetic law,_ without the distinction between +_palingenesis_ and _cenogenesis,_ and without the theory of _evolution_ +on which we base it, it is quite impossible to understand the facts of +organic development; without them we cannot cast the faintest gleam of +explanation over this marvellous field of phenomena. But when we +recognise the causal correlation of ontogeny and phylogeny expressed in +this law, the wonderful facts of embryology are susceptible of a very +simple explanation; they are found to be the necessary mechanical +effects of the evolution of the stem, determined by the laws of +heredity and adaptation. The correlative action of these laws under the +universal influence of the struggle for existence, or—as we may say in +a word, with Darwin—“natural selection,” is entirely adequate to +explain the whole process of embryology in the light of phylogeny. It +is the chief merit of Darwin that he explained by his theory of +selection the correlation of the laws of heredity and adaptation that +Lamarck had recognised, and pointed out the true way to reach a causal +interpretation of evolution. + +The phenomenon that it is most imperative to recognise in this +connection is the inheritance of functional variations. Jean Lamarck +was the first to appreciate its fundamental importance in 1809, and we +may therefore justly give the name of Lamarckism to the theory of +descent he based on it. Hence the radical opponents of the latter have +very properly directed their attacks chiefly against the former. One of +the most distinguished and most narrow-minded of these opponents, +Wilhelm His, affirms very positively that “characteristics acquired in +the life of the individual are not inherited.” + +The inheritance of acquired characters is denied, not only by thorough +opponents of evolution, but even by scientists who admit it and have +contributed a good deal to its establishment, especially Weismann, +Galton, Ray Lankester, etc. Since 1884 the chief opponent has been +August Weismann, who has rendered the greatest service in the +development of Darwin’s theory of selection. In his work on _The +Continuity of the Germ-plasm,_ and in his recent excellent _Lectures on +the Theory of Descent_ (1902), he has with great success advanced the +opinion that “only those characters can be transmitted to subsequent +generations that were contained in rudimentary form in the embryo.” +However, this germ-plasm theory, with its attempt to explain heredity, +is merely a “provisional molecular hypothesis”; it is one of those +metaphysical speculations that attribute the evolutionary phenomena +exclusively to internal causes, and regard the influence of the +environment as insignificant. Herbert Spencer, Theodor Eimer, Lester +Ward, Hering, and Zehnder have pointed out the untenable consequences +of this position. I have given my view of it in the tenth edition of +the _History of Creation_ (pp. 192, 203). I hold, with Lamarck and +Darwin, that the hereditary transmission of acquired characters is one +of the most important phenomena in biology, and is proved by thousands +of morphological and physiological experiences. It is an indispensable +foundation of the theory of evolution. + +Of the many and weighty arguments for the truth of this conception of +evolution I will for the moment merely point to the invaluable evidence +of dysteleology, the science of rudimentary organs. We cannot insist +too often or too strongly on the great morphological significance of +these remarkable organs, which are completely useless from the +physiological point of view. We find some of these useless parts, +inherited from our lower vertebrate ancestors, in every system of +organs in man and the higher Vertebrates. Thus we find at once on the +skin a scanty and rudimentary coat of hair, only fully developed on the +head, under the shoulders, and at a few other parts of the body. The +short hairs on the greater part of the body are quite useless and +devoid of physiological value; they are the last relic of the thicker +hairy coat of our simian ancestors. The sensory apparatus presents a +series of most remarkable rudimentary organs. We have seen that the +whole of the shell of the external ear, with its cartilages, muscles, +and skin, is in man a useless appendage, and has not the physiological +importance that was formerly ascribed to it. It is the degenerate +remainder of the pointed, freely moving, and more advanced mammal ear, +the muscles of which we still have, but cannot work them. We found at +the +inner corner of our eye a small, curious, semi-lunar fold that is of no +use whatever to us, and is only interesting as the last relic of the +nictitating membrane, the third, inner eye-lid that had a distinct +physiological purpose in the ancient sharks, and still has in many of +the Amniotes. + +The motor apparatus, in both the skeleton and muscular systems, +provides a number of interesting dysteleological arguments. I need only +recall the projecting tail of the human embryo, with its rudimentary +caudal vertebræ and muscles; this is totally useless in man, but very +interesting as the degenerate relic of the long tail of our simian +ancestors. From these we have also inherited various bony processes and +muscles, which were very useful to them in climbing trees, but are +useless to us. At various points of the skin we have cutaneous muscles +which we never use—remnants of a strongly-developed cutaneous muscle in +our lower mammal ancestors. This “panniculus carnosus” had the function +of contracting and creasing the skin to chase away the flies, as we see +every day in the horse. Another relic in us of this large cutaneous +muscle is the frontal muscle, by which we knit our forehead and raise +our eye-brows; but there is another considerable relic of it, the large +cutaneous muscle in the neck (_platysma myoides_), over which we have +no voluntary control. + +Not only in the systems of animal organs, but also in the vegetal +apparatus, we find a number of rudimentary organs, many of which we +have already noticed. In the alimentary apparatus there are the +thymus-gland and the thyroid gland, the seat of goitre and the relic of +a ciliated groove that the Tunicates and Acrania still have in the +gill-pannier; there is also the vermiform appendix to the cæcum. In the +vascular system we have a number of useless cords which represent +relics of atrophied vessels that were once active as blood-canals—the +_ductus Botalli_ between the pulmonary artery and the aorta, the +_ductus venosus Arantii_ between the portal vein and the vena cava, and +many others. The many rudimentary organs in the urinary and sexual +apparatus are particularly interesting. These are generally developed +in one sex and rudimentary in the other. Thus the spermaducts are +formed from the Wolffian ducts in the male, whereas in the female we +have merely rudimentary traces of them in Gaertner’s canals. On the +other hand, in the female the oviducts and womb are developed from the +Mullerian ducts, while in the male only the lowest ends of them remain +as the “male womb” (_vesicula prostatica_). Again, the male has in his +nipples and mammary glands the rudiments of organs that are usually +active only in the female. + +A careful anatomic study of the human frame would disclose to us +numbers of other rudimentary organs, and these can only be explained on +the theory of evolution. Robert Wiedersheim has collected a large +number of them in his work on _The Human Frame as a Witness to its +Past._ They are some of the weightiest proofs of the truth of the +mechanical conception and the strongest disproofs of the teleological +view. If, as the latter demands, man or any other organism had been +designed and fitted for his life-purposes from the start and brought +into being by a creative act, the existence of these rudimentary organs +would be an insoluble enigma; it would be impossible to understand why +the Creator had put this useless burden on his creatures to walk a path +that is in itself by no means easy. But the theory of evolution gives +the simplest possible explanation of them. It says: The rudimentary +organs are parts of the body that have fallen into disuse in the course +of centuries; they had definite functions in our animal ancestors, but +have lost their physiological significance. On account of fresh +adaptations they have become superfluous, but are transmitted from +generation to generation by heredity, and gradually atrophy. + +We have inherited not only these rudimentary parts, but all the organs +of our body, from the mammals—proximately from the apes. The human body +does not contain a single organ that has not been inherited from the +apes. In fact, with the aid of our biogenetic law we can trace the +origin of our various systems of organs much further, down to the +lowest stages of our ancestry. We can say, for instance, that we have +inherited the oldest organs of the body, the external skin and the +internal coat of the alimentary system, from the Gastræads; the nervous +and muscular systems from the Platodes; the vascular system, the +body-cavity, and the blood from the Vermalia; the chorda and the +branchial gut from the Prochordonia; +the articulation of the body from the Acrania; the primitive skull and +the higher sense-organs from the Cyclostomes; the limbs and jaws from +the Selachii; the five-toed foot from the Amphibia; the palate from the +Reptiles; the hairy coat, the mammary glands, and the external sexual +organs from the Pro-mammals. When we formulated “the law of the +ontogenetic connection of systematically related forms,” and determined +the relative age of organs, we saw how it was possible to draw +phylogenetic conclusions from the ontogenetic succession of systems of +organs. + +With the aid of this important law and of comparative anatomy we were +also enabled to determine “man’s place in nature,” or, as we put it, +assign to man his position in the classification of the animal kingdom. +In recent zoological classification the animal world is divided into +twelve stems or phyla, and these are broadly sub-divided into about +sixty classes, and these classes into at least 300 orders. In his whole +organisation man is most certainly, in the first place, a member of one +of these stems, the vertebrate stem; secondly, a member of one +particular class in this stem, the Mammals; and thirdly, of one +particular order, the order of Primates. He has all the characteristics +that distinguish the Vertebrates from the other eleven animal stems, +the Mammals from the other sixty classes, and the Primates from the 300 +other orders of the animal kingdom. We may turn and twist as we like, +but we cannot get over this fact of anatomy and classification. Of late +years this fact has given rise to a good deal of discussion, and +especially of controversy as to the particular anatomic relationship of +man to the apes. The most curious opinions have been advanced on this +“ape-question,” or “pithecoid-theory.” It is as well, therefore, to go +into it once more and distinguish the essential from the unessential. +(Cf. pp. 261–5.) + +We start from the undisputed fact that man is in any case—whether we +accept or reject his special blood-relationship to the apes—a true +mammal; in fact, a placental mammal. This fundamental fact can be +proved so easily at any moment from comparative anatomy that it has +been universally admitted since the separation of the Placentals from +the lower mammals (Marsupials and Monotremes). But for every consistent +subscriber to the theory of evolution it must follow at once that man +descends from a common stem-form with all the other Placentals, the +stem-ancestor of the Placentals, just as we must admit a common +mesozoic ancestor of all the mammals. This is, however, to settle +decisively the great and burning question of man’s place in nature, +whether or no we go on to admit a nearer or more distant relationship +to the apes. Whether man is or is not a member of the ape-order (or, if +you prefer, the primate-order.) in the phylogenetic sense, in any case +his direct blood-relationship to the rest of the mammals, and +especially the Placentals, is established. It is possible that the +affinities of the various orders of mammals to each other are different +from what we hypothetically assume to-day. But, in any case, the common +descent of man and all the other mammals from one stem-form is beyond +question. This long-extinct Promammal was probably evolved from +Proreptiles during the Triassic period, and must certainly be regarded +as the monotreme and oviparous ancestor of _all_ the mammals. + +If we hold firmly to this fundamental and most important thesis, we +shall see the “ape-question” in a very different light from that in +which it is usually regarded. Little reflection is then needed to see +that it is not nearly so important as it is said to be. The origin of +the human race from a series of mammal ancestors, and the historic +evolution of these from an earlier series of lower vertebrate +ancestors, together with all the weighty conclusions that every +thoughtful man deduces therefrom, remain untouched; so far as these are +concerned, it is immaterial whether we regard true “apes” as our +nearest ancestors or not. But as it has become the fashion to lay the +chief stress in the whole question of man’s origin on the “descent from +the apes,” I am compelled to return to it once more, and recall the +facts of comparative anatomy and ontogeny that give a decisive answer +to this “ape-question.” + +The shortest way to attain our purpose is that followed by Huxley in +1863 in his able work, which I have already often quoted, _Man’s Place +in Nature_—the way of comparative anatomy and ontogeny. We have to +compare impartially all man’s organs with the same organs in the higher +apes, and then to examine if the differences between the two are +greater +than the corresponding differences between the higher and the lower +apes. The indubitable and incontestable result of this +comparative-anatomical study, conducted with the greatest care and +impartiality, was the pithecometra-principle, which we have called the +Huxleian law in honour of its formulator—namely, that the differences +in organisation between man and the most advanced apes we know are much +slighter than the corresponding differences in organisation between the +higher and lower apes. We may even give a more precise formula to this +law, by excluding the Platyrrhines or American apes as distant +relatives, and restricting the comparison to the narrower family-circle +of the Catarrhines, the apes of the Old World. Within the limits of +this small group of mammals we found the structural differences between +the lower and higher catarrhine apes—for instance, the baboon and the +gorilla—to be much greater than the differences between the anthropoid +apes and man. If we now turn to ontogeny, and find, according to our +“law of the ontogenetic connection of systematically related forms,” +that the embryos of the anthropoid apes and man retain their +resemblance for a longer time than the embryos of the highest and the +lowest apes, we are forced, whether we like it or no, to recognise our +descent from the order of apes. We can assuredly construct an +approximate picture in the imagination of the form of our early +Tertiary ancestors from the foregoing facts of comparative anatomy; +however we may frame this in detail, it will be the picture of a true +ape, and a distinct catarrhine ape. This has been shown so well by +Huxley (1863) that the recent attacks of Klaatsch, Virchow, and other +anthropologists, have completely failed (cf. pp.263–264). All the +structural characters that distinguish the Catarrhines from the +Platyrrhines are found in man. Hence in the genealogy of the mammals we +must derive man immediately from the catarrhine group, and locate the +origin of the human race in the Old World. Only the early root-form +from which both descended was common to them. + +It is, therefore, established beyond question for all impartial +scientific inquiry that the human race comes directly from the apes of +the Old World; but, at the same time, I repeat that this is not so +important in connection with the main question of the origin of man as +is commonly supposed. Even if we entirely ignore it, all that we have +learned from the zoological facts of comparative anatomy and ontogeny +as to the placental character of man remains untouched. These prove +beyond all doubt the common descent of man and all the rest of the +mammals. Further, the main question is not in the least affected if it +is said: “It is true that man is a mammal; but he has diverged at the +very root of the class from all the other mammals, and has no closer +relationship to any living group of mammals.” The affinity is more or +less close in any case, if we examine the relation of the mammal class +to the sixty other classes of the animal world. Quite certainly the +whole of the mammals, including man, have had a common origin; and it +is equally certain that their common stem-forms were gradually evolved +from a long series of lower Vertebrates. + +The resistance to the theory of a descent from the apes is clearly due +in most men to feeling rather than to reason. They shrink from the +notion of such an origin just because they see in the ape organism a +caricature of man, a distorted and unattractive image of themselves, +because it hurts man’s æsthetic complacency and self-ennoblement. It is +more flattering to think we have descended from some lofty and god-like +being; and so, from the earliest times, human vanity has been pleased +to believe in our origin from gods or demi-gods. The Church, with that +sophistic reversal of ideas of which it is a master, has succeeded in +representing this ridiculous piece of vanity as “Christian humility”; +and the very men who reject with horror the notion of an animal origin, +and count themselves “children of God,” love to prate of their “humble +sense of servitude.” In most of the sermons that have poured out from +pulpit and altar against the doctrine of evolution human vanity and +conceit have been a conspicuous element; and, although we have +inherited this very characteristic weakness from the apes, we must +admit that we have developed it to a higher degree, which is entirely +repudiated by sound and normal intelligence. We are greatly amused at +all the childish follies that the ridiculous pride of ancestry has +maintained from the Middle Ages to our own time; yet there is a large +amount of this empty feeling in +most men. Just as most people much prefer to trace their family back to +some degenerate baron or some famous prince rather than to an unknown +peasant, so most men would rather have as parent of the race a sinful +and fallen Adam than an advancing, and vigorous ape. It is a matter of +taste, and to that extent we cannot quarrel over these genealogical +tendencies. Personally, the notion of ascent is more congenial to me +than that of descent. It seems to me a finer thing to be the advanced +offspring of a simian ancestor, that has developed progressively from +the lower mammals in the struggle for life, than the degenerate +descendant of a god-like being, made from a clod, and fallen for his +sins, and an Eve created from one of his ribs. Speaking of the rib, I +may add to what I have said about the development of the skeleton, that +the number of ribs is just the same in man and woman. In both of them +the ribs are formed from the middle germinal layer, and are, from the +phylogenetic point of view, lower or ventral vertebral arches. + +But it is said: “That is all very well, as far as the human body is +concerned; on the facts quoted it is impossible to doubt that it has +really and gradually been evolved from the long ancestral series of the +Vertebrates. But it is quite another thing as regards man’s mind, or +soul; this cannot possibly have been developed from the +vertebrate-soul.”[35] Let us see if we cannot meet this grave stricture +from the well-known facts of comparative anatomy, physiology, and +embryology. It will be best to begin with a comparative study of the +souls of various groups of Vertebrates. Here we find such an enormous +variety of vertebrate souls that, at first sight, it seems quite +impossible to trace them all to a common “Primitive Vertebrate.” Think +of the tiny Amphioxus, with no real brain but a simple medullary tube, +and its whole psychic life at the very lowest stage among the +Vertebrates. The following group of the Cyclostomes are still very +limited, though they have a brain. When we pass on to the fishes, we +find their intelligence remaining at a very low level. We do not see +any material advance in mental development until we go on to the +Amphibia and Reptiles. There is still greater advance when we come to +the Mammals, though even here the minds of the Monotremes and of the +stupid Marsupials remain at a low stage. But when we rise from these to +the Placentals we find within this one vast group such a number of +important stages of differentiation and progress that the psychic +differences between the least intelligent (such as the sloths and +armadillos) and the most intelligent Placentals (such as the dogs and +apes) are much greater than the psychic differences between the lowest +Placentals and the Marsupials or Monotremes. Most certainly the +differences are far greater than the differences in mental power +between the dog, the ape, and man. Yet all these animals are +genetically-related members of a single natural class. + + [35] The English reader will recognise here the curious position of + Dr. Wallace and of the late Dr. Mivart.—Translator. + + +We see this to a still more astonishing extent in the comparative +psychology of another class of animals, that is especially interesting +for many reasons—the insect class. It is well known that we find in +many insects a degree of intelligence that is found in man alone among +the Vertebrates. Everybody knows of the famous communities and states +of bees and ants, and of the very remarkable social arrangements in +them, such as we find among the more advanced races of men, but among +no other group of animals. I need only mention the social organisation +and government of the monarchic bees and the republican ants, and their +division into different conditions—queen, drone-nobles, workers, +educators, soldiers, etc. One of the most remarkable phenomena in this +very interesting province is the cattle-keeping of the ants, which rear +plant-lice as milch-cows and regularly extract their honeyed juice. +Still more remarkable is the slave-holding of the large red ants, which +steal the young of the small black ants and bring them up as slaves. It +has long been known that these political and social arrangements of the +ants are due to the deliberate cooperation of the countless citizens, +and that they understand each other. A number of recent observers, +especially Fritz Müller, Sir J. Lubbock (Lord Avebury), and August +Forel, have put the astonishing degree of intelligence of these tiny +Articulates beyond question. + +Now, compare with these the mental life of many of the lower, +especially the parasitic insects, as Darwin did. There is, for +instance, the cochineal insect +(_Coccus_), which, in its adult state, has a motionless, shield-shaped +body, attached to the leaves of plants. Its feet are atrophied. Its +snout is sunk in the tissue of the plants of which it absorbs the sap. +The whole psychic life of these inert female parasites consists in the +pleasure they experience from sucking the sap of the plant and in +sexual intercourse with the males. It is the same with the maggot-like +females of the fan-fly (_Strepsitera_), which spend their lives +parasitically and immovably, without wings or feet, in the abdomen of +wasps. There is no question here of higher psychic action. If we +compare these sluggish parasites with the intelligent and active ants, +we must admit that the psychic differences between them are much +greater than the psychic differences between the lowest and highest +mammals, between the Monotremes, Marsupials, and armadillos on the one +hand, and the dog, ape, or man on the other. Yet all these insects +belong to the same class of Articulates, just as all the mammals belong +to one and the same class. And just as every consistent evolutionist +must admit a common stem-form for all these insects, so he must also +for all the mammals. + +If we now turn from the comparative study of psychic life in different +animals to the question of the organs of this function, we receive the +answer that in all the higher animals they are always bound up with +certain groups of cells, the ganglionic cells or neurona that compose +the nervous system. All scientists without exception are agreed that +the central nervous system is the organ of psychic life in the animal, +and it is possible to prove this experimentally at any moment. When we +partially or wholly destroy the central nervous system, we extinguish +in the same proportion, partially or wholly, the “soul” or psychic +activity of the animal. We have, therefore, to examine the features of +the psychic organ in man. The reader already knows the incontestable +answer to this question. Man’s psychic organ is, in structure and +origin, just the same organ as in all the other Vertebrates. It +originates in the shape of a simple medullary tube from the outer +membrane of the embryo—the skin-sense layer. The simple cerebral +vesicle that is formed by the expansion of the head-part of this +medullary tube divides by transverse constrictions into five, and these +pass through more or less the same stages of construction in the human +embryo as in the rest of the mammals. As these are undoubtedly of a +common origin, their brain and spinal cord must also have a common +origin. + +Physiology teaches us further, on the ground of observation and +experiment, that the relation of the “soul” to its organ, the brain and +spinal cord, is just the same in man as in the other mammals. The one +cannot act at all without the other; it is just as much bound up with +it as muscular movement is with the muscles. It can only develop in +connection with it. If we are evolutionists at all, and grant the +causal connection of ontogenesis and phylogenesis, we are forced to +admit this thesis: The human soul or psyche, as a function of the +medullary tube, has developed along with it; and just as brain and +spinal cord now develop from the simple medullary tube in every human +individual, so the human mind or the psychic life of the whole human +race has been gradually evolved from the lower vertebrate soul. Just as +to-day the intricate structure of the brain proceeds step by step from +the same rudiment in every human individual—the same five cerebral +vesicles—as in all the other Craniotes; so the human soul has been +gradually developed in the course of millions of years from a long +series of craniote-souls. Finally, just as to-day in every human embryo +the various parts of the brain differentiate after the special type of +the ape-brain, so the human psyche has proceeded historically from the +ape-soul. + +It is true that this Monistic conception is rejected with horror by +most men, and the Dualistic idea, which denies the inseparable +connection of brain and mind, and regards body and soul as two totally +different things, is still popular. But how can we reconcile this view +with the known facts of evolution? It meets with difficulties equally +great and insuperable in embryology and in phylogeny. If we suppose +with the majority of men that the soul is an independent entity, which +has nothing to do with the body originally, but merely inhabits it for +a time, and gives expression to its experiences through the brain just +as the pianist does through his instrument, we must assign a point in +human embryology at which the soul enters into the brain; and at death +again we must assign a moment at which it abandons the body. As, +further, each human individual has inherited certain +personal features from each parent, we must suppose that in the act of +conception pieces were detached from their souls and transferred to the +embryo. A piece of the paternal soul goes with-the spermatozoon, and a +piece of the mother’s soul remains in the ovum. At the moment of +conception, when portions of the two nuclei of the copulating cells +join together to form the nucleus of the stem-cell, the accompanying +fragments of the immaterial souls must also be supposed to coalesce. + +On this Dualistic view the phenomena of psychic development are totally +incomprehensible. Everybody knows that the new-born child has no +consciousness, no knowledge of itself and the surrounding world. Every +parent who has impartially followed the mental development of his +children will find it impossible to deny that it is a case of +biological evolutionary processes. Just as all other functions of the +body develop in connection with their organs, so the soul does in +connection with the brain. This gradual unfolding of the soul of the +child is, in fact, so wonderful and glorious a phenomenon that every +mother or father who has eyes to observe is never tired of +contemplating it. It is only our manuals of psychology that know +nothing of this development; we are almost tempted to think sometimes +that their authors can never have had children themselves. The human +soul, as described in most of our psychological works, is merely the +soul of a learned philosopher, who has read a good many books, but +knows nothing of evolution, and never even reflects that his own soul +has had a development. + +When these Dualistic philosophers are consistent they must assign a +moment in the phylogeny of the human soul at which it was first +“introduced” into man’s vertebrate body. Hence, at the time when the +human body was evolved from the anthropoid body of the ape (probably in +the Tertiary period), a specific human psychic element—or, as people +love to say, “a spark of divinity”—must have been suddenly infused or +breathed into the anthropoid brain, and been associated with the +ape-soul already present in it. I need not insist on the enormous +theoretical difficulties of this idea. I will only point out that this +“spark of divinity,” which is supposed to distinguish the soul of man +from that of the other animals, must be itself capable of development, +and has, as a matter of fact, progressively developed in the course of +human history. As a rule, reason is taken to be this “spark of +divinity,” and is supposed to be an exclusive possession of humanity. +But comparative psychology shows us that it is quite impossible to set +up this barrier between man and the brute. Either we take the word +“reason” in the wider sense, and then it is found in the higher mammals +(ape, dog, elephant, horse) just as well as in most men; or else in the +narrower sense, and then it is lacking in most men just as much as in +the majority of animals. On the whole, we may still say of man’s reason +what Goethe’s Mephistopheles said:— + +Life somewhat better might content him +But for the gleam of heavenly light that Thou hast given him. +He calls it reason; thence his power’s increased +To be still beastlier than any beast. + + +If, then, we must reject these popular and, in some respects, agreeable +Dualistic theories as untenable, because inconsistent with the genetic +facts, there remains only the opposite or Monistic conception, +according to which the human soul is, like any other animal soul, a +function of the central nervous system, and develops in inseparable +connection therewith. We see this _ontogenetically_ in every child. The +biogenetic law compels us to affirm it _phylogenetically._ Just as in +every human embryo the skin-sense layer gives rise to the medullary +tube, from the anterior end of which the five cerebral vesicles of the +Craniotes are developed, and from these the mammal brain (first with +the characters of the lower, then with those of the higher mammals); +and as the whole of this ontogenetic process is only a brief, +hereditary reproduction of the same process in the phylogenesis of the +Vertebrates; so the wonderful spiritual life of the human race through +many thousands of years has been evolved step by step from the lowly +psychic life of the lower Vertebrates, and the development of every +child-soul is only a brief repetition of that long and complex +phylogenetic process. From all these facts sound reason must conclude +that the still prevalent belief in the immortality of the soul is an +untenable superstition. I have shown its inconsistency with modern +science in the eleventh chapter of _The Riddle of the Universe._ + +Here it may also be well to point out +the great importance of anthropogeny, in the light of the biogenetic +law, for the purposes of philosophy. The speculative philosophers who +take cognizance of these ontogenetic facts, and explain them (in +accordance with the law) phylogenetically, will advance the great +questions of philosophy far more than the most distinguished thinkers +of all ages have yet succeeded in doing. Most certainly every clear and +consistent thinker must derive from the facts of comparative anatomy +and ontogeny we have adduced a number of suggestive ideas that cannot +fail to have an influence on the progress of philosophy. Nor can it be +doubted that the candid statement and impartial appreciation of these +facts will lead to the decisive triumph of the philosophic tendency +that we call “Monistic” or “Mechanical,” as opposed to the “Dualistic” +or “Teleological,” on which most of the ancient, medieval, and modern +systems of philosophy are based. The Monistic or Mechanical philosophy +affirms that all the phenomena of human life and of the rest of nature +are ruled by fixed and unalterable laws; that there is everywhere a +necessary causal connection of phenomena; and that, therefore, the +whole knowable universe is a harmonious unity, a _monon._ It says, +further, that all phenomena are due solely to mechanical or efficient +causes, not to final causes. It does not admit free-will in the +ordinary sense of the word. In the light of the Monistic philosophy the +phenomena that we are wont to regard as the freest and most +independent, the expressions of the human will, are subject just as +much to rigid laws as any other natural phenomenon. As a matter of +fact, impartial and thorough examination of our “free” volitions shows +that they are never really free, but always determined by antecedent +factors that can be traced to either heredity or adaptation. We cannot, +therefore, admit the conventional distinction between nature and +spirit. There is spirit everywhere in nature, and we know of no spirit +outside of nature. Hence, also, the common antithesis of natural +science and mental or moral science is untenable. Every science, as +such, is both natural and mental. That is a firm principle of Monism, +which, on its religious side, we may also denominate Pantheism. Man is +not above, but in, nature. + +It is true that the opponents of evolution love to misrepresent the +Monistic philosophy based on it as “Materialism,” and confuse the +philosophic tendency of this name with a wholly unconnected and +despicable moral materialism. Strictly speaking, it would be just as +proper to call our system Spiritualism as Materialism. The real +Materialistic philosophy affirms that the phenomena of life are, like +all other phenomena, effects or products of matter. The opposite +extreme, the Spiritualistic philosophy, says, on the contrary, that +matter is a product of energy, and that all material forms are produced +by free and independent forces. Thus, according to one-sided +Materialism, the matter is antecedent to the living force; according to +the equally one-sided view of the Spiritist, it is the reverse. Both +views are Dualistic, and, in my opinion, both are false. For us the +antithesis disappears in the Monistic philosophy, which knows neither +matter without force nor force without matter. It is only necessary to +reflect for some time over the question from the strictly scientific +point of view to see that it is impossible to form a clear idea of +either hypothesis. As Goethe said, “Matter can never exist or act +without spirit, nor spirit without matter.” + +The human “spirit” or “soul” is merely a force or form of energy, +inseparably bound up with the material sub-stratum of the body. The +thinking force of the mind is just as much connected with the +structural elements of the brain as the motor force of the muscles with +their structural elements. Our mental powers are functions of the brain +as much as any other force is a function of a material body. We know of +no matter that is devoid of force, and no forces that are not bound up +with matter. When the forces enter into the phenomenon as movements we +call them living or active forces; when they are in a state of rest or +equilibrium we call them latent or potential. This applies equally to +inorganic and organic bodies. The magnet that attracts iron filings, +the powder that explodes, the steam that drives the locomotive, are +living inorganics; they act by living force as much as the sensitive +Mimosa does when it contracts its leaves at touch, or the venerable +Amphioxus that buries itself in the sand of the sea, or man when he +thinks. Only in the latter cases the combinations of the different +forces that appear as “movement” in the +phenomenon are much more intricate and difficult to analyse than in the +former. + +Our study has led us to the conclusion that in the whole evolution of +man, in his embryology and in his phylogeny, there are no living forces +at work other than those of the rest of organic and inorganic nature. +All the forces that are operative in it could be reduced in the +ultimate analysis to growth, the fundamental evolutionary function that +brings about the forms of both the organic and the inorganic. But +growth itself depends on the attraction and repulsion of homogeneous +and heterogeneous particles. Seventy-five years ago Carl Ernst von Baer +summed up the general result of his classic studies of animal +development in the sentence: “The evolution of the individual is the +history of the growth of individuality in every respect.” And if we go +deeper to the root of this law of growth, we find that in the long run +it can always be reduced to that attraction and repulsion of animated +atoms which Empedocles called the “love and hatred” of the elements. + +Thus the evolution of man is directed by the same “eternal, iron laws” +as the development of any other body. These laws always lead us back to +the same simple principles, the elementary principles of physics and +chemistry. The various phenomena of nature only differ in the degree of +complexity in which the different forces work together. Each single +process of adaptation and heredity in the stem-history of our ancestors +is in itself a very complex physiological phenomenon. Far more +intricate are the processes of human embryology; in these are condensed +and comprised thousands of the phylogenetic processes. + +In my _General Morphology,_ which appeared in 1866, I made the first +attempt to apply the theory of evolution, as reformed by Darwin, to the +whole province of biology, and especially to provide with its +assistance a mechanical foundation for the science of organic forms. +The intimate relations that exist between all parts of organic science, +especially the direct causal nexus between the two sections of +evolution—ontogeny and phylogeny—were explained in that work for the +first time by transformism, and were interpreted philosophically in the +light of the theory of descent. The anthropological part of the +_General Morphology_ (Book vii) contains the first attempt to determine +the series of man’s ancestors (vol. ii, p. 428). However imperfect this +attempt was, it provided a starting-point for further investigation. In +the thirty-seven years that have since elapsed the biological horizon +has been enormously widened; our empirical acquisitions in +paleontology, comparative anatomy, and ontogeny have grown to an +astonishing extent, thanks to the united efforts of a number of able +workers and the employment of better methods. Many important biological +questions that then appeared to be obscure enigmas seem to be entirely +settled. Darwinism arose like the dawn of a new day of clear Monistic +science after the dark night of mystic dogmatism, and we can say now, +proudly and gladly, that there is daylight in our field of inquiry. + +Philosophers and others, who are equally ignorant of the empirical +sources of our evidence and the phylogenetic methods of utilising it, +have even lately claimed that in the matter of constructing our +genealogical tree nothing more has been done than the discovery of a +“gallery of ancestors,” such as we find in the mansions of the +nobility. This would be quite true if the genealogy given in the second +part of this work were merely the juxtaposition of a series of animal +forms, of which we gathered the genetic connection from their external +physiognomic resemblances. As we have sufficiently proved already, it +is for us a question of a totally different thing—of the morphological +and historical proof of the phylogenetic connection of these ancestors +on the basis of their identity in internal structure and embryonic +development; and I think I have sufficiently shown in the first part of +this work how far this is calculated to reveal to us their inner nature +and its historical development. I see the essence of its significance +precisely in the proof of historical connection. I am one of those +scientists who believe in a real “natural history,” and who think as +much of an historical knowledge of the past as of an exact +investigation of the present. The incalculable value of the historical +consciousness cannot be sufficiently emphasised at a time when +historical research is ignored and neglected, and when an “exact” +school, as dogmatic as it is narrow, would substitute for it physical +experiments and mathematical formulæ. Historical knowledge cannot be +replaced by any other branch of science. + + +It is clear that the prejudices that stand in the way of a general +recognition of this “natural anthropogeny” are still very great; +otherwise the long struggle of philosophic systems would have ended in +favour of Monism. But we may confidently expect that a more general +acquaintance with the genetic facts will gradually destroy these +prejudices, and lead to the triumph of the natural conception of “man’s +place in nature.” When we hear it said, in face of this expectation, +that this would lead to retrogression in the intellectual and moral +development of mankind, I cannot refrain from saying that, in my +opinion, it will be just the reverse; that it will promote to an +enormous extent the advance of the human mind. All progress in our +knowledge of truth means an advance in the higher cultivation of the +human intelligence; and all progress in its application to practical +life implies a corresponding improvement of morality. The worst enemies +of the human race—ignorance and superstition—can only be vanquished by +truth and reason. In any case, I hope and desire to have convinced the +reader of these chapters that the true scientific comprehension of the +human frame can only be attained in the way that we recognise to be the +sole sound and effective one in organic science generally—namely, the +way of Evolution. + + + + +INDEX + + +A + +Abiogenesis, 26 + +_Accipenser_, 234 + +Abortive ova, 55 + +Achromatin, 42 + +Achromin, 42 + +Acœla, 221 + +Acoustic nerve, the, 289, 290 + +Acquired characters, inheritance of, 349 + +Acrania, the, 182 + +Acroganglion, the, 268, 275 + +Adam’s apple, the, 184 + +Adapida, 257 + +Adaptation, 3, 5, 27 + +After-birth, the, 167 + +Agassiz, L., 34 + +Age of life, 200 + +Alimentary canal, evolution of the, 13, 14, 133, 308–17 +— — structure of the, 169, 308–10 + +Allantoic circulation, the, 171 + +Allantois, development of the, 166 + +Allmann, 20 + +_Amblystoma,_ 243 + +Amitotic cleavage, 40 + +Ammoconida, 217 + +_Ammolynthus,_ 217 + +Amnion, the, 115 +— formation of the, 134, 244 + +Amniotic fluid, the, 134 + +Amœba, the, 47–9, 210 + +Amphibia, the, 239 + +_Amphichœrus,_ 221 + +Amphigastrula, 80 + +Amphioxus, the, 105, 181–95 +— circulation of the, 184 +— cœlomation of the, 95 +— embryology of the, 191–95 +— structure of the, 183–88 + +Amphirhina, 230 + +Anamnia, the, 115 + +Anatomy, comparative, 208 + +Animalculists, 12 + +Animal layer, the, 16 + +Annelids, the, 142, 219 + +Annelid theory, the, 142 + +Anomodontia, 246 + +Ant, intelligence of the, 353 + +_Anthropithecus,_ 174, 262 + +Anthropogeny, 1 + +Anthropoid apes, the, 166, 173, 262 + +Anthropology, 1, 35 + +Anthropozoic period, 203 + +Antimera, 107 + +Anura, 243 + +Anus, the, 317 + +Anus, formation of the, 139 + +Aorta, the, 327 +— development of the, 170 + +Ape and man, 157, 164, 261, 307, 351 + +Ape-man, the, 263 + +Apes, the, 257–60 + +_Aphanocapsa,_ 210 + +_Aphanostomum,_ 221 + +Appendicaria, 197 + +Appendix vermiformis, the, 32 + +Aquatic life, early prevalence of, 235 + +Ararat, Mount, 24 + +Archenteron, 64, 74 + +Archeolithic age, 203 + +Archicaryon, 55 + +Archicrania, 230 + +Archigastrula, 65, 193 + +_Archiprimas,_ 263 + +Arctopitheca, 261 + +Area, the germinative, 121 + +Aristotle, 9 + +Arm, structure of the, 306 + +Arrow-worm, the, 191 + +Arterial arches, the, 325–26 +— cone, the, 324 + +Arteries, evolution of the, 170, 323–24 + +Articulates, the, 142, 219 +— skeleton of the, 294 + +Articulation, 141–42 + +Aryo-Romanic languages, the, 203 + +Ascidia, the, 181, 188–90 +— embryology of the, 196–98 + +Ascula, 217 + +Asexual reproduction, 51 + +Atlas, the, 247 + +Atrium, the, 183, 185 +— (heart), the, 326 + +Auditory nerve, the, 289, 290 + +Auricles of the heart, 325 + +_Autolemures,_ 257 + +Axolotl, the, 243 + +B + +Bacteria, 38, 210 + +Baer, K. E. von, 15–17 + +Balanoglossus, 226 + +Balfour, F., 21 + +Batrachia, 241 + +_Bdellostoma Stouti,_ 78 + +Bee, generation of the, 9 + +Beyschlag, W., on evolution, 50 + +Bilateral symmetry, 66 +— — origin of, 221 + +Bimana, 258 + +Biogenetic law, the, 2, 21, 23, 179, 349 + +Biogeny, 2 + +Bionomy, 33 + +Bird, evolution of the, 245 +— ovum of the, 44–6, 80–1 + +Bischoff, W., 17 + +Bladder, evolution of the, 244, 339 + +Blastæa, the, 206, 213 + +Blastocœl, the, 62, 74 + +Blastocrene, the, 99 + +Blastocystis, the, 62, 119, 120 + +Blastoderm, the, 62 + +Blastodermic vesicle, the, 119 + +Blastoporus, the, 64 + +Blastosphere, the, 62, 119 + +Blastula, the, 62, 74 +— the mammal, 119 + +Blood, importance of the, 318 +— recent experiments in mixture of, 172 +— structure of the, 319 + +Blood-cells, the, 319 + +Blood-vessels, the, 318–25 +— development of the, 168 +— of the vertebrate, 110 +— origin of the, 320–21 + +Boniface VIII, Bull of, 10 + +Bonnet, 13 + +Borneo nosed-ape, the, 164 + +Boveri, Theodor, 185 + +Brachytarsi, 257 + +Brain and mind, 278, 354–56 +— evolution of the, 8, 275–80 +— in the fish, 276 +— in the lower animals, 275 +— structure of the, 273–74 + +Branchial arches, evolution of the, 303 +— cavity, the, 183, 189 +— system, the, 110 + +Branchiotomes, 149 + +Breasts, the, 113 + +Bulbilla, 184 + +C + +_Calamichthys,_ 234 + +_Calcolynthus,_ 217 + +Capillaries, the, 323 + +Caracoideum, the, 249 + +Carboniferous strata, 202 + +_Carcharodon,_ 234 + +Cardiac cavity, the, 170 + +Cardiocœl, the, 328 + +Caryobasis, 38, 54 + +Caryokinesis, 42 + +Caryolymph, 38, 54 + +Caryolyses, 42 + +Caryon, 37 + +Caryoplasm, 37 + +Catallacta, 213 + +Catarrhinæ, the, 173, 261 + +Catastrophic theory, the, 24 + +Caudate cells, 53 + +Cell, life of the, 41–3 +— nature of the, 36–7 +— size of the, 38 + +Cell theory, the, 18, 36 + +Cenogenesis, 4 + +Cenogenetic structures, 4 + +Cenozoic period, the, 203 + +Central body, the, 38, 42 + +Central nervous system, the, 273 + +Centrolecithal ova, 68 + +Centrosoma, the, 38, 42 + +Ceratodus, the, 76, 237 + +Cerebellum, the, 274 + +Cerebral vesicles, evolution of the, 276 + +Cerebrum, the, 273 + +_Cestracion Japonicus,_ 75, 79 + +Chætognatha, 94 + +Chick, importance of the, in embryology, 11, 16 + +Child, mind of the, 8, 355 + +Chimpanzee, the, 174, 262 + +_Chiromys,_ 257 + +Chiroptera, 258 + +_Chirotherium,_ 239 + +Chondylarthra, 257 + +Chorda, the, 17, 95, 107, 183 +— evolution of the, 296 + +_Chordæa,_ the, 97 + +Chordalemma, the, 296 + +Chordaria, 97 + +Chordula, the, 3, 96, 191 + +Choriata, the, 166 + +Chorion, the, 119 +— development of the, 165–6 +— frondosum, 255 +— læve, 255 + +Choroid coat, the, 286 + +Chorology, 33 + +Chromacea, 209 + +Chromatin, 42 + +Chroococcacea, 210 + +_Chroococcus,_ the, 210 + +Church, opposition of, to science in Middle Ages, 10 + +Chyle, 318 + +Chyle-vessels, 324 + +Cicatricula, the, 45, 81 + +Ciliated cells, 53, 193 + +Cinghalese gynecomast, 114 + +Circulation in the lancelet, 184 + +Circulatory system, evolution of the, 321–25 +— — structure of the, 318 + +Classification, 103 +— evolutionary value of, 33 + +Clitoris, the, 345 + +Cloaca, the, 249, 317 + +Cnidaria, 217 + +Coccyx, the, 295 + +Cochineal insect, the, 354 + +Cochlea, the, 289 + +Cœcilia, 241 + +Cœcum, the, 310, 317 + +Cœlenterata, 20, 91, 93, 104 + +Cœlenteria, 221 + +Cœloma, the, 21, 64, 91 + +Cœlomæa, the, 98 + +Cœlomaria, 21, 91, 104, 221 + +Cœlomation, 93–4 + +Cœlom-theory, the, 21, 93 + +Cœlomula, the, 98 + +Colon, the, 310, 317 + +Comparative anatomy, 31 + +Conception, nature of, 51 + +Conjunctiva, the, 286 + +_Conocyema,_ 215 + +_Convoluta,_ 221 + +Copelata, the, 197 + +Copulative organs, evolution of the, 344–45 + +Corium, the, 108, 268 + +Cornea, the, 286 + +Corpora cavernosa, the, 345, 346 + +Corpora quadrigemina, 274 + +Corpora striata, 274 + +Corpus callosum, the, 274 + +Corpus vitreum, the, 285 + +Corpuscles of the blood, 319 + +Craniology, 303 + +Craniota, the, 182, 229 + +Cranium, the, 299 + +Creation, 23–4 + +Cretaceous strata, 202 + +Crossopterygii, 234 + +Crustacea, the, 142, 219 + +Cryptocœla, 221 + +Cryptorchism, 114 + +Crystalline lens, the, 285 +— — development of the, 287 + +Cutaneous glands, 268 + +Cuttlefish, embryology of the, 9 + +Cuvier, G., 17, 24 + +Cyanophycea, 209 + +Cyclostoma, the, 188, 230–32 +— ova of the, 75 + +Cyemaria, 214 + +Cynopitheca, 262 + +_Cynthia,_ 191, 196 + +Cytoblastus, the, 37 + +Cytodes, 40 + +Cytoplasm, 37, 38 + +Cytosoma, 37 + +Cytula, the, 54 + +D + +Dalton, 15 + +Darwin, C., 2, 5, 23, 28–9 + +Darwin, E., 28 + +Darwinism, 5, 28 + +Decidua, the, 167 + +Deciduata, 255 + +Deduction, nature of, 208 + +Degeneration theory, the, 219 + +Dentition of the ape and man, 259 + +Depula, 62 + +_Descent of Man,_ 30 + +Design in organisms, 33 + +Deutoplasm, 44 + +Devonian strata, 202 + +Diaphragm, the, 309 +— evolution of the, 328 + +_Dicyema,_ 215 + +Dicyemida, 215 + +Didelphia, 248 + +Digonopora, 223 + +Dinosauria, 202 + +Dipneumones, 238 + +Dipneusta, 235–38 +— ova of the, 75 + +Dipnoa, 236 + +Directive bodies, 54 + +Discoblastic ova, 68 + +Discoplacenta, 255 + +_Dissatyrus,_ 174 + +Dissection, medieval decrees against, 10 + +Dohrn, Anton, 219 + +Döllinger, 15 + +Dorsal furrow, the, 125 +— shield, the, 123 +— zone, the, 129 + +_Dromatherium,_ 248 + +Dualism, 6 + +Dubois, Eugen, 263 + +_Ductus Botalli,_ the, 350 + +_Ductus venosus Arantii,_ 350 + +Duodenum, the, 309, 317 + +Duration of embryonic development, 199 +— of man’s history, 199 + +Dysteleology, 32 +— proofs of, 349 + +E + +Ear, evolution of the, 288–92 +— structure of the, 288 +— uselessness of the external, 32 + +Ear-bones, the, 289 + +Earth, age of the, 200–201 + +_Echidna hystrix,_ 249 + +Ectoblast, 20, 64 + +Ectoderm, the, 20, 64 + +Edentata, 250 + +Efficient causes, 6 + +Egg of the bird, 44–6, 81 +— or the chick, priority of the, 211 + +Elasmobranchs, the, 79 + +Embryo, human, development of the, 158 + +Embryology, 2 +— evolutionary value of, 34 + +Embryonic development, duration of, 199 +— disk, the, 121–22 +— spot, the, 125 + +Encephalon, the, 273 + +Endoblast, 20, 64 + +Endothelia, 321 + +Enterocœla, 93, 223 + +Enteropneusta, 226 + +Entoderm, the, 20, 64 + +Eocene strata, 203 + +Eopitheca, 259 + +Epiblast, 20, 64 + +Epidermis, the, 108, 268 + +Epididymis, the, 342 + +Epigastrula, 80 + +Epigenesis, 11, 13 + +Epiglottis, the, 309 + +Epiphysis, the, 108 + +Episoma, 129 + +Episomites, 130, 194 + +Epispadia, 346 + +Epithelia, 37 + +Epitheria, 243, 253 + +Epovarium, the, 342 + +Equilibrium, sense of, 291 + +Esthonychida, 257 + +Eustachian tube, the, 289 + +Eutheria, 253 + +Eve, 12 + +Evolution theory, the, 11, 208 +— inductive nature of, 30 + +Eye, evolution of the, 285–88 +— structure of the, 285 + +Eyelid, the third, 32 + +Eyelids, evolution of the, 288 + +F + +Fabricius ab Aquapendente, 10 + +Face, embryonic development of the, 284 + +Fat glands in the skin, 269 + +Feathers, evolution of, 270 + +Fertilisation, 51 +— place of, 119 + +Fin, evolution of the, 239, 304 + +Final causes, 6 + +Flagellate cells, 193 + +Floating bladder, the, 233, 241 +— — evolution of the, 314 + +Fœtal circulation, 170–71 + +Food-yelk, the, 67, + +Foot, evolution of the, 241, 304–6 +— of the ape and man, 258–59 + +Fore brain, the, 278 + +Fore kidneys, the, 336, 337 + +Fossiliferous strata, list of, 201 + +Fossils, 180 +— scarcity of, 208 + +Free will, 356 + +Friedenthal, experiments of, 172 + +Frog, the, 241–42 +— ova of the, 71–2 + +Frontonia, 224 + +Function and structure, 7 + +Furcation of ova, 72 + +G + +Gaertner’s duct, 341, 350 + +Ganglia, commencement of, 268 + +Ganglionic cell, the, 39 + +Ganoids, 233, 234 + +Gastræa, the, 3, 20, 206 +— formation of the, 213 + +Gastræa theory, the, 20, 64, 69 + +Gastræads, 69, 214 + +Gastremaria, 214 + +Gastrocystis, the, 62, 119, 120 + +_Gastrophysema,_ 215 + +Gastrotricha, 224 + +Gastrula, the, 3, 20, 62 + +Gastrulation, 62 + +Gegenbaur, Carl, 220 +— on evolution, 32 +— on the skull, 300–1 + +Gemmation, 331 + +_General Morphology,_ 8, 29 + +_Genesis,_ 23 + +Genital pore, the, 335 + +Geological evolution, length of, 200 +— periods, 201 + +Geology, methods of, 180 +— rise of, 24 + +Germ-plasm, theory of, 349 + +Germinal disk, 46, 81 +— layers, the, 14, 16 +— — scheme of the, 92 +— spot, the, 44 +— vesicle, the, 43, 54 + +Germinative area, the, 121 + +Giant gorilla, the, 176 + +Gibbon, the, 173, 262 + +Gill-clefts and arches, 110 +— formation of the, 151–52, 303 + +Gill-crate, the, 183, 189 + +Gills, disappearance of the, 244 + +Glœocapsa, 210 + +Gnathostoma, 230, 232 + +Goethe as an evolutionist, 27, 299 + +Goitre, 110 + +Gonads, the, 111 +— formation of the, 149–50 + +Gonidia, 334 + +Gonochorism, beginning of, 322 + +Gonoducts, 335 + +Gonotomes, 146, 149 + +Goodsir, 189 + +Gorilla, the, 174, 176, 262 + +Graafian follicles, the, 17, 119, 347 + +Gregarinæ, 211 + +Gullet-ganglion, the, 190 + +Gut, evolution of the, 310–17 + +_Gyrini,_ 242 + +Gynecomastism, 114 + +H + +Hag-fish, the, 188 + +Hair, evolution of the, 270 +— on the human embryo and infant, 271 + +Hair, restriction of, by sexual selection, 271 + +_Haliphysema,_ 215 + +Halisauria, 202 + +Haller, Albrecht, 12 + +_Halosphæra viridis,_ 213 + +Hand, evolution of the, 250, 304–6 +— of the ape and man, 258 + +Hapalidæ, 261 + +Harderian gland, the, 288 + +Hare-lip, 284 + +Harrison, Granville, 161 + +Hartmann, 262 + +Harvey, 10 + +Hatschek, 192 + +Hatteria, 243, 246 + +Head-cavity, the, 138 + +Head-plates, the, 149 + +Heart, development of the, 7, 10, 111, 151, 170, 322, 324–27 +— of the ascidia, 190 +— position of the, 327 + +Helmholtz, 207 + +Helminthes, 223 + +Hepatic gut, the, 109, 316 + +Heredity, nature of, 3, 5, 27, 56–7, 349 + +Hermaphrodism, 9, 23, 114, 218, 322, 346 + +Hertwig, 21 + +Hesperopitheca, 259 + +His, W., 19 + +Histogeny, 18, 19 + +_History of Creation,_ 6, 30 + +Holoblastic ova, 67, 71, 77 + +_Homœosaurus,_ 244, 246 + +Homology of the germinal layers, 20 + +Hoof, evolution of the, 270 + +Hunterian ligament, the, 344 + +Huxleian law, the, 171, 257, 262 + +Huxley, T. H., 7, 20, 29 + +Hydra, the, 69, 217 + +Hydrostatic apparatus in the fish, 315 + +_Hylobates,_ 173, 262 + +_Hylodes Martinicensis,_ 241 + +Hyoid bone, the, 299 + +Hypermastism, 113 + +Hyperthelism, 113 + +Hypoblast, 20, 64 + +Hypobranchial groove, the, 110, 184, 226, 316 + +Hypodermis, the, 268 + +Hypopsodina, 257 + +Hyposoma, the, 129 + +Hyposomites, 130, 194 + +Hypospadia, 346 + +I + +Ichthydina, 224 + +_Ichthyophis glutinosa,_ 80 + +Ictopsida, 257 + +Ileum, the, 310 + +Immortality, Aristotle on, 10 + +Immortality of the soul, 58 + +Impregnation-rise, the, 55 + +Indecidua, 255 + +Indo-Germanic languages, 203 + +Induction and deduction, 31, 208 + +Inheritance of acquired characters, 349 + +Insects, intelligence of, 353 + +Interamniotic cavity, the, 165 + +Intestines, the, 309, 316–17 + +Invagination, 62 + +Iris, the, 286 + +J + +_Jacchus,_ 261 + +Java, ape-man of, 263, 264 + +Jaws, evolution of the, 301 + +Jurassic strata, 202 + +K + +Kant, dualism of, 25 + +Kelvin, Lord, on the origin of life, 207 + +Kidneys, the, 111 +— formation of the, 150–51, 336–42 + +Klaatsch, 262 + +Kölliker, 21 + +Kowalevsky, 191 + +L + +Labia, the, 346 + +Labyrinth, the, 290 + +Lachrymal glands, 269 + +Lamarck, J., 23, 25–7 +— theories of, 26, 349 + +Lamprey, the, 230 +— ova of the, 75 + +Lancelet, the, 60, 181–95 +— description of the, 105 + +Languages, evolution of, 203 + +Lanugo of the embryo, 271 + +Larynx, the, 309 +— evolution of the, 314 + +Latebra, the, 45 + +Lateral plates, the, 129 + +Laurentian strata, 201 + +Lecithoma, the, 117 + +Leg, evolution of the, 304 +— structure of the, 306 + +Lemuravida, 257 + +Lemurogona, 257 + +Lemurs, the, 257 + +_Lepidosiren,_ 257 + +Leucocytes, 319 + +Life, age of, 200 + +Limbs, evolution of the, 152, 239, 304 + +Limiting furrow, the, 133 + +Linin, 42 + +Liver, the, 309, 317 + +Long-nosed ape, the, 164 + +Love, importance of in nature, 332 + +Lungs, the, 110 +— evolution of the, 241, 314–15 + +Lyell, Sir C., 24 + +Lymphatic vessels, the, 318 + +Lymph-cells, the, 319 + +M + +Macrogonidion, 331 + +Macrospores, 331 + +_Magosphæra planula,_ 213 + +Male womb, the, 344, 350 + +Mallochorion, the, 166 + +Mallotheria, 257 + +Malpighian capsules, 339, 341 + +Mammal, characters of the, 112 +— gastrulation of the, 84 + +Mammals, unity of the, 247–48 + +Mammary glands, the, 113, 269 + +Man and the ape, relation of, 262, 351 +— origin of, 29 + +_Man’s Place in Nature,_ 7, 29, 351 + +Mantle, the, 189 + +Mantle-folds, the, 185 + +Marsupials, the, 250–52 +— ova of the, 85 + +Materialism, 356 + +Mathematical method, the, 30 + +Mechanical causes, 6 +— embryology, 8, 19, 22 + +Meckel’s cartilage, 304 + +_Medulla capitis,_ the, 273 +— _oblongata,_ the, 274 +— _spinalis,_ the, 273 + +Medullary groove, the, 125 +— tube, the, 107, 128 +— — formation of the, 131, 133, 227, 267, 276 + +Mehnert, E., on the biogenetic law, 5 + +Meroblastic ova, 67, 71, 78 + +Merocytes, 68, 321 + +Mesentery, the, 98, 109, 310, 316 + +Mesocardium, the, 327 + +Mesoderm, the, 20, 64, 90, 93 + +Mesogastria, 215 + +Mesonephridia, the, 338 + +Mesonephros, the, 336 + +Mesorchium, the, 344 + +Mesovarium, the, 344 + +Mesozoic period, the, 202 + +Metogaster, the, 64 + +Metagastrula, the, 67 + +Metamerism, 142 + +Metanephridia, the, 341 + +Metanephros, the, 336 + +Metaplasm, 39 + +Metastoma, 64, 222 + +Metatheria, 248 + +Metazoa, 20, 62 + +Metovum, the, 81 + +Microgonidian, 331 + +Microspores, 331 + +Middle ear, the, 291 + +Migration, effect of, 33 + +Milk, secretion of the, 269 + +Mind, evolution of, 353–54 +— in the lower animals, 353 + +Miocene strata, 203 + +Mitosis, 40, 41 + +Monera, 40, 206, 209 + +Monism, 6, 356 + +Monodelphia, 248 + +Monogonopora, 223 + +Monopneumones, 238 + +Monotremes, 118, 249 +— ova of the, 84 + +_Monoxenia Darwinii,_ 60 + +Morea, the, 212 + +Morphology, 2, 27 + +Morula, the, 62, 212 + +Motor-germinative layer, the, 19 + +Mouth, development of the, 124, 139 +— structure of the, 308 + +Mucous layer, the, 16 + +Müllerian duct, the, 341 + +Muscle-layer, the, 16 + +Muscles, evolution of the, 307 +— of the ear, rudimentary, 292 + +Myotomes, 108, 146 + +Myxinoides, the, 188, 230 + +N + +Nails, evolution of the, 270 + +Nasal pits, 284 + +Natural philosophy, 25 +— selection, 26, 28, 349 + +Navel, the, 117, 134 + +Necrolemurs, 257 + +Nectocystis, the, 314 + +Nemertina, 224–26 + +Nephroduct, evolution of the, 338–39 + +Nephrotomes, 149, 338 + +Nerve-cell, the, 39 + +Nerves, animals without, 267 + +Nervous system, evolution of the, 7, 267 + +Neurenteric canal, the, 127 + +Nictitating membrane, the, 32, 286, 288 + +Nose, the, in man and the ape, 164 +— development of the, 282–85 +— structure of the, 283 + +Notochorda, the, 107 + +Nuclein, 37 + +Nucleolinus, 44 + +Nucleolus, the, 38, 44, 54 + +Nucleus of the cell, 37 + +O + +Œsophagus, the, 309, 316 + +Oken, 5, 27, 300 + +Oken’s bodies, 339 + +Oligocene strata, 203 + +_Olynthus,_ 217 + +On the generation of animals, 9 + +Ontogeny, 2, 23 +— defective evidence of, 208 + +Opaque area, the, 122 + +Opossum, the, 252 +— ova of the, 85 + +Optic nerve, the, 287 + +Optic thalami, 274 +— vesicles, 286 + +Orang, the, 174, 262 + +Ornithodelphia, 248 + +_Ornithorhyncus,_ 85, 249 + +Ornithostoma, 249 + +Ossicles of the ear, 289 + +Otoliths, 289 + +Ova, number of, 347 +— of the lancelet, 192 + +Ovaries, evolution of the, 333–34 + +Oviduct, origin of the, 335, 342 + +Ovolemma, the, 44 + +Ovulists, 12 + +Ovum, discovery of the, 16 +— nature of the, 40, +— size of the, 44 + +P + +Pachylemurs, the, 257 + +Pacinian corpuscles, 282 + +Paleontology, 2 +— evolutionary evidence of, 31 +— incompleteness of, 208 +— rise of, 24 + +Paleozoic age, the, 202 + +Palingenesis, 4 + +Palingenetic structures, 4 + +_Palæhatteria,_ 244, 246 + +_Panniculus carnosus,_ the, 350 + +Paradidymis, the, 342 + +Parietal zone, the, 129 + +Parthenogenesis, 9, 13 + +Pastrana, Miss Julia, 164 + +Pedimana, 252 + +Pellucid area, the, 122 + +Pelvic cavity, the, 138 + +_Pemmatodiscus gastrulaceus,_ 215 + +Penis-bone, the, 346 + +Penis, varieties of the, 345 + +Peramelida, 254 + +Periblastic ova, 68 + +Peribranchial cavity, the, 185, 190 + +Pericardial cavity, the, 328 + +Perichorda, the, 108, 183 +— formation of the, 136 + +Perigastrula, 89 + +Permian strata, 202 + +Petromyzontes, the, 188, 230 + +Phagocytes, 49, 320 + +Pharyngeal ganglion, the, 275 + +Pharynx, the, 309 + +Philology, comparison with, 203 + +_Philosophie Zoologique,_ 25 + +Philosophy and evolution, 6 + +Phycochromacea, 209 + +Phylogeny, 2, 23 + +Physemaria, 214 + +Physiology, backwardness of, 7 + +Phytomonera, 209 + +Pineal eye, the, 108 + +Pinna, the, 291 + +_Pithecanthropus,_ 263, 264 + +Pithecometra-principle, the, 171 + +Placenta, the, 166, 253–54 + +Placentals, the, 166 +— characters of the, 253 +— gastrulation of the, 86 + +Planocytes, 49, 320 + +Plant-louse, parthenogenesis of the, 13 + +Planula, the, 89 + +Plasma-products, 38, 39 + +Plasson, 40, 59 + +Plastids, 36, 40, 209 + +Plastidules, 59 + +Platodaria, 221 + +Platodes, the, 221 + +Platyrrhinæ, 261 + +Pleuracanthida, 234 + +Pleural ducts, 328 + +Pliocene strata, 203 + +Polar cells, 54 + +Polyspermism, 58 + +Preformation theory, the, 11 + +Primary period, the, 202 + +Primates, the, 157, 257–60 + +_Primatoid,_ 263 + +Primitive groove, the, 69, 82, 124, 125 +— gut, the, 20, 63, 214 +— kidneys, the, 111, 337 +— mouth, the, 20, 63 +— segments, 143 +— streak, the, 100, 122 +— vertebræ, 144, 195, 206, 229 + +Primordial period, the, 201 + +Prochordata, 192 + +Prochordonia, the, 192, 218 + +Prochoriata, 253 + +Prochorion, the, 44, 119 + +_Proctodæum,_ the, 345 + +_Procytella primordialis,_ 210 + +Prodidelphia, 256 + +Progaster, the, 20, 63 + +Progonidia, 333 + +Promammalia, 247 + +Pronephridia, the, 151 + +Pronucleus femininus, 54 +— masculinus, 54 + +Properistoma, 69 + +Prorenal canals of the lancelet, 186 +— duct, the, 132, 139, 186 +— — evolution of the, 338 + +Proselachii, 234 + +Prosimiæ, the, 257 + +Prospermaria, 333 + +_Prospondylus,_ 105, 229 + +Prostoma, 20, 63, 222 + +Protamniotes, 243–44 + +Protamœba, 210 + +Proterosaurus, the, 202, 244 + +Protists, 36, 38 + +Protonephros, 111, 336 + +Protophyta, 210 + +Protoplasm, 37, 209 + +_Protopterus,_ 238 + +Prototheria, 248 + +Protovertebræ, 142, 144 + +Protozoa, 20, 210 + +Provertebral cavity, the, 148 +— plates, the, 136, 144 + +Pseudocœla, 93, 221 + +Pseudopodia, 48 + +Pseudova, 13 + +Psychic life, evolution of the, 8 + +Psychology, 8 + +Pterosauria, 202 + +Pylorus, the, 309 + +Q + +Quadratum, the, 247 + +Quadrumana, 258 + +Quaternary period, 203 + +R + +Rabbit, ova of the, 86–7 + +Radiates, the, 103 + +Rathke’s canals, 341 + +Rectum, the, 317 + +Regner de Graaf, 119 + +Renal system, evolution of the, 335–42 + +Reproduction, nature of, 330–31 + +Reptiles, 245–47 + +Respiratory organs, evolution of the, 314–15 +— pore, the, 183, 189 + +Retina, the, 286 + +Rhabdocœla, 222 + +Rhodocytes, 319 + +_Rhopalura,_ 215 + +Rhyncocephala, 243 + +Ribs, the, 295 +— number of the, 353 + +Rudimentary ear-muscles, 292 +— organs, 32 +— — list of, 349–50 +— toes, 306 + +S + +Sacculus, the, 289 + +_Sagitta,_ 65, 66, 191 +— cœlomation of, 93 + +Salamander, the, 241 +— ova of the, 74 + +Sandal-shape of embryo, 128–29 + +_Satyrus,_ 174, 262 + +Sauromammalia, 246 + +Sauropsida, 245 + +Scatulation theory, the, 12 + +Schizomycetes, 210 + +Schleiden, M., 18, 36 + +Schwann, T., 18, 36 + +Sclerotic coat, the, 286 + +Sclerotomes, 108, 143, 148 + +Scrotum, the, 344 + +_Scyllium,_ nose of the, 283 + +Sea-squirt, the, 181, 188–90 + +Secondary period, the, 202 + +Segmentation, 60, 141–42 + +Segmentation-cells, 54 + +Segmentation-sphere, the, 17 + +Selachii, 223 +— skull of the, 301 + +Selection, theory of, 28 + +Selenka, 166, 168 + +Semnopitheci, 262 + +Sense-organs, evolution of the, 151, 280 +— number of the, 281 +— origin of the, 281 + +Sensory nerves, 279 + +Serocœlom, the, 165 + +Serous layer, the, 16 + +Sex-organs, early vertebrate form of the, 111 +— evolution of the, 333 + +Sexual reproduction, simplest forms of, 331 +— selection, 30, 271–72 + +Shark, the, 233 +— nose of the, 283 +— ova of the, 75 +— placenta of the, 9 +— skull of the, 301 + +Shoulder-blade, the, 306 + +Sickle-groove, the, 82, 121 + +Sieve-membrane, the, 167 + +Silurian strata, 202 + +Simiæ, the, 257–60 + +Siphonophoræ, embryology of the, 21 + +Skeleton, structure of the, 294 + +Skeleton-plate, the, 148 + +Skin, the, 151 +— evolution of, 266–69 +— function of the, 269 + +Skin-layer, the, 16 + +Skull, evolution of the, 149, 299–303 +— structure of the, 299 +— vertebral theory of the, 300 + +Smell, the sense of, 282 + +Soul, evolution of the, 353–56 +— nature of the, 58, 356 +— phylogeny of the, 8 +— seat of the, 278 + +Sound, sensations of, 289–90 + +Sozobranchia, 242 + +Space, sense of, 291 + +Species, nature of the, 23, 34 + +Speech, evolution of, 264 + +Spermaducts, 335, 342 + +Spermaries, evolution of the, 333–34 + +Spermatozoon, the, 52–3 +— discovery of the, 12, 53 + +Spinal cord, development of the, 8 +— structure of the, 273 + +Spirema, the, 42 + +Spiritualism, 356 + +Spleen, the, 318 + +Spondyli, 142 + +Sponges, classification of the, 34 +— ova of the, 49 + +Spontaneous generation, 26, 206 + +Stegocephala, 239 + +Stem-cell, the, 54 + +Stem-zone, the, 129 + +Stomach, evolution of the, 311–14, 316 +— structure of the human, 309 + +Strata, thickness of, 200–201 + +Struggle for life, the, 28 + +Subcutis, the, 268 + +Sweat glands, 269 + +T + +Tactile corpuscles, 268, 282 + +Tadpole, the, 242 + +Tail, evolution of the, 242–43 +— rudimentary, in man, 159, 295, 350 + +Tailed men, 160–61 + +Taste, the sense of, 282 + +Teeth, evolution of the, 314 +— of the ape and man, 259 + +Teleostei, 234 + +Telolecithal ova, 67, 68 + +Temperature, sense of, 282 + +Terrestrial life, beginning of, 235 + +Tertiary period, the, 203 + +_Theoria generationis,_ the, 13 + +Theories, value of, 181 + +Theromorpha, 246 + +Third eyelid, the, 286, 288 + +Thyroid gland, the, 110, 184, 315 + +Time-variations in ontogeny, 5 + +Tissues, primary and secondary, 37 + +Toad, the, 241 + +Tocosauria, 246 + +Toes, number of the, 240 + +_Tori genitales,_ the, 346 + +Touch, the sense of, 282 + +Tracheata, 142, 219 + +Tread, the, 45, 81 + +Tree-frog, the, 241 + +Triassic strata, 202 + +_Triton tæniatus,_ 74 + +Troglodytes, 174 + +Tunicates, the, 189 + +Turbellaria, 222 + +Turbinated bones, the, 283 + +Tympanic cavity, the, 288 + +U + +Umbilical, cord, the, 117 +— vesicle, the, 138 + +Unicellular ancestor of all animals, 47 +— animals, 38, 47 + +Urachus, the, 317, 341 + +Urinary system, evolution of the, 335–42 + +Urogenital ducts, 335 + +_Uterus masculinus,_ the, 344, 350 + +Utriculus, the, 289 + +V + +_Vasa deferentia,_ 335 + +Vascular layer, the, 16, 168 +— system, evolution of the, 321–25 +— — structure of the, 318 + +Vegetative layer, the, 16 + +Veins, evolution of the, 323–24 + +Ventral pedicle, the, 166 + +Ventricles of the heart, 325 + +Vermalia, 220, 223 + +Vermiform appendage, the, 32, 310, 317 + +Vertebræ, 142, 294 + +Vertebræa, 105 + +Vertebral arch, the, 148, 295 +— column, the, 144 +— — evolution of the, 296 +— — structure of the, 294 + +Vertebrates, character of the, 104–10 +— descent of the, 219–20 + +Vertebration, 142 + +Vesico-umbilical ligament, the, 341 + +_Vesicula prostatica,_ the, 344, 350 + +Villi of the chorion, 165 + +Virchow, R., 35 +— on the ape-man, 303 +— on the evolution of man, 264 + +Virgin-birth, 9, 13 + +Vitalism, 6 + +Vitelline duct, the, 138 + +Volvocina, 213 + +W + +Wallace, A. R., 29 + +Water, organic importance of, 200 + +Water vessels, 336 + +Weismann’s theories, 349 + +Wolff, C. F., 13 + +Wolffian bodies, 339 + +Wolffian duct, the, 341 + +Womb, evolution of the, 342–43 + +Y + +Yelk, the, 43, 45, 67 + +Yelk-sac, the, 117, 134 + +Z + +Zona pellucida, the, 44 + +Zonoplacenta, 255 + +Zoomonera, 209 + +Zoophytes, 20, 64, 104 + + + + +End of the Project Gutenberg EBook of The Evolution of Man, by Ernst Haeckel + +*** END OF THIS PROJECT GUTENBERG EBOOK THE EVOLUTION OF MAN *** + +***** This file should be named 8700-0.txt or 8700-0.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/8/7/0/8700/ + +Produced by Produced by Sue Asscher and Derek Thompson + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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