THE EVOLUTION OF MAN

A POPULAR SCIENTIFIC STUDY

BY

ERNST HAECKEL

VOLUME 2.

HUMAN STEM-HISTORY, OR PHYLOGENY.



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. 1911.



CONTENTS OF VOLUME 2.

LIST OF ILLUSTRATIONS.

INDEX.

CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT.

CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT.

CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM.

CHAPTER 2.19. OUR PROTIST ANCESTORS.

CHAPTER 2.20. OUR WORM-LIKE ANCESTORS.

CHAPTER 2.21. OUR FISH-LIKE ANCESTORS.

CHAPTER 2.22. OUR FIVE-TOED ANCESTORS.

CHAPTER 2.23. OUR APE ANCESTORS.

CHAPTER 2.24. EVOLUTION OF THE NERVOUS SYSTEM.

CHAPTER 2.25. EVOLUTION OF THE SENSE-ORGANS.

CHAPTER 2.26. EVOLUTION OF THE ORGANS OF MOVEMENT.

CHAPTER 2.27. EVOLUTION OF THE ALIMENTARY SYSTEM.

CHAPTER 2.28. EVOLUTION OF THE VASCULAR SYSTEM.

CHAPTER 2.29. EVOLUTION OF THE SEXUAL ORGANS.

CHAPTER 2.30. RESULTS OF ANTHROPOGENY.

LIST OF ILLUSTRATIONS.

FIGURE 2.210. THE LANCELET.

FIGURE 2.211. SECTION OF THE HEAD OF THE LANCELET.

FIGURE 2.212. SECTION OF AN AMPHIOXUS-LARVA.

FIGURE 2.213. DIAGRAM OF PRECEDING.

FIGURE 2.214. SECTION OF A YOUNG AMPHIOXUS.

FIGURE 2.215. DIAGRAM OF A YOUNG AMPHIOXUS.

FIGURE 2.216. TRANSVERSE SECTION OF LANCELET.

FIGURE 2.217. SECTION THROUGH THE MIDDLE OF THE LANCELET.

FIGURE 2.218. SECTION OF A PRIMITIVE-FISH EMBRYO.

FIGURE 2.219. SECTION OF THE HEAD OF THE LANCELET.

FIGURES 2.220 AND 2.221. ORGANISATION OF AN ASCIDIA.

FIGURES 2.222 TO 2.224. SECTIONS OF YOUNG AMPHIOXUS-LARVAE.

FIGURE 2.225. AN APPENDICARIA.

FIGURE 2.226. Chroococcus minor.

FIGURE 2.227. Aphanocapsa primordialis.

FIGURE 2.228. PROTAMOEBA.

FIGURE 2.229. ORIGINAL OVUM-CLEAVAGE.

FIGURE 2.230. MORULA.

FIGURES 2.231 AND 2.232. Magosphaera planula.

FIGURE 2.233. MODERN GASTRAEADS.

FIGURES 2.234 AND 2.235. Prophysema primordiale.

FIGURES 2.236 AND 2.237. Ascula of Gastrophysema.

FIGURE 2.238. Olynthus.

FIGURE 2.239. Aphanostomum Langii.

FIGURES 2.240 AND 2.241. A TURBELLARIAN.

FIGURES 2.242 AND 2.243. Chaetonotus.

FIGURE 2.244. A NEMERTINE WORM.

FIGURE 2.245. AN ENTEROPNEUST.

FIGURE 2.246. SECTION OF THE BRANCHIAL GUT.

FIGURE 2.247. THE MARINE LAMPREY.

FIGURE 2.248. FOSSIL PRIMITIVE FISH.

FIGURE 2.249. EMBRYO OF A SHARK.

FIGURE 2.250. MAN-EATING SHARK.

FIGURE 2.251. FOSSIL ANGEL-SHARK.

FIGURE 2.252. TOOTH OF A GIGANTIC SHARK.

FIGURES 2.253 TO 2.255. CROSSOPTERYGII.

FIGURE 2.256. FOSSIL DIPNEUST.

FIGURE 2.257. THE AUSTRALIAN DIPNEUST.

FIGURES 2.258 AND 2.259. YOUNG CERATODUS.

FIGURE 2.260. FOSSIL AMPHIBIAN.

FIGURE 2.261. LARVA OF THE SPOTTED SALAMANDER.

FIGURE 2.262. LARVA OF COMMON FROG.

FIGURE 2.263. FOSSIL MAILED AMPHIBIAN.

FIGURE 2.264. THE NEW ZEALAND LIZARD.

FIGURE 2.265. Homoeosaurus pulchellus.

FIGURE 2.266. SKULL OF A PERMIAN LIZARD.

FIGURE 2.267. SKULL OF A THEROMORPHUM.

FIGURE 2.268. LOWER JAW OF A PRIMITIVE MAMMAL.

FIGURES 2.269 AND 2.270. THE ORNITHORHYNCUS.

FIGURE 2.271. LOWER JAW OF A PROMAMMAL.

FIGURE 2.272. THE CRAB-EATING OPOSSUM.

FIGURE 2.273. FOETAL MEMBRANES OF THE HUMAN EMBRYO.

FIGURE 2.274. SKULL OF A FOSSIL LEMUR.

FIGURE 2.275. THE SLENDER LORI.

FIGURE 2.276. THE WHITE-NOSED APE.

FIGURE 2.277. THE DRILL-BABOON.

FIGURES 2.278 TO 2.282. SKELETONS OF MAN AND THE ANTHROPOID APES.

FIGURE 2.283. SKULL OF THE JAVA APE-MAN.

FIGURE 2.284. SECTION OF THE HUMAN SKIN.

FIGURE 2.285. EPIDERMIC CELLS.

FIGURE 2.286. RUDIMENTARY LACHRYMAL GLANDS.

FIGURE 2.287. THE FEMALE BREAST.

FIGURE 2.288. MAMMARY GLAND OF A NEW-BORN INFANT.

FIGURE 2.289. EMBRYO OF A BEAR.

FIGURE 2.290. HUMAN EMBRYO.

FIGURE 2.291. CENTRAL MARROW OF A HUMAN EMBRYO.

FIGURES 2.292 AND 2.293. THE HUMAN BRAIN.

FIGURES 2.294 TO 2.296. CENTRAL MARROW OF HUMAN EMBRYO.

FIGURE 2.297. HEAD OF A CHICK EMBRYO.

FIGURE 2.298. BRAIN OF THREE CRANIOTE EMBRYOS.

FIGURE 2.299. BRAIN OF A SHARK.

FIGURE 2.300. BRAIN AND SPINAL CORD OF A FROG.

FIGURE 2.301. BRAIN OF AN OX-EMBRYO.

FIGURES 2.302 AND 2.303. BRAIN OF A HUMAN EMBRYO.

FIGURE 2.304. BRAIN OF THE RABBIT.

FIGURE 2.305. HEAD OF A SHARK.

FIGURES 2.306 TO 2.310. HEADS OF CHICK-EMBRYOS.

FIGURE 2.311. SECTION OF MOUTH OF HUMAN EMBRYO.

FIGURE 2.312. DIAGRAM OF MOUTH-NOSE CAVITY.

FIGURES 2.313 AND 2.314. HEADS OF HUMAN EMBRYOS.

FIGURES 2.315 AND 2.316. FACE OF HUMAN EMBRYO.

FIGURE 2.317. THE HUMAN EYE.

FIGURE 2.318. EYE OF THE CHICK EMBRYO.

FIGURE 2.319. SECTION OF EYE OF A HUMAN EMBRYO.

FIGURE 2.320. THE HUMAN EAR.

FIGURE 2.321. THE BONY LABYRINTH.

FIGURE 2.322. DEVELOPMENT OF THE LABYRINTH.

FIGURE 2.323. PRIMITIVE SKULL OF HUMAN EMBRYO.

FIGURE 2.324. RUDIMENTARY MUSCLES OF THE EAR.

FIGURES 2.325 AND 2.326. THE HUMAN SKELETON.

FIGURE 2.327. THE HUMAN VERTEBRAL COLUMN.

FIGURE 2.328. PIECE OF THE DORSAL CORD.

FIGURES 2.329 AND 2.330. DORSAL VERTEBRAE.

FIGURE 2.331. INTERVERTEBRAL DISK.

FIGURE 2.332. HUMAN SKULL.

FIGURE 2.333. SKULL OF NEW-BORN CHILD.

FIGURE 2.334. HEAD-SKELETON OF A PRIMITIVE FISH.

FIGURE 2.335. SKULLS OF NINE PRIMATES.

FIGURES 2.336 TO 2.338. EVOLUTION OF THE FIN.

FIGURE 2.339. SKELETON OF THE FORE-LEG OF AN AMPHIBIAN.

FIGURE 2.340. SKELETON OF GORILLA'S HAND.

FIGURE 2.341. SKELETON OF HUMAN HAND.

FIGURE 2.342. SKELETON OF HAND OF SIX MAMMALS.

FIGURES 2.343 TO 2.345. ARM AND HAND OF THREE ANTHROPOIDS.

FIGURE 2.346. SECTION OF FISH'S TAIL.

FIGURE 2.347. HUMAN SKELETON.

FIGURE 2.348. SKELETON OF THE GIANT GORILLA.

FIGURE 2.349. THE HUMAN STOMACH.

FIGURE 2.350. SECTION OF THE HEAD OF A RABBIT-EMBRYO.

FIGURE 2.351. SHARK'S TEETH.

FIGURE 2.352. GUT OF A HUMAN EMBRYO.

FIGURES 2.353 AND 2.354. GUT OF A DOG EMBRYO.

FIGURES 2.355 AND 2.356. SECTIONS OF HEAD OF LAMPREY.

FIGURE 2.357. VISCERA OF A HUMAN EMBRYO.

FIGURE 2.358. RED BLOOD-CELLS.

FIGURE 2.359. VASCULAR TISSUE.

FIGURE 2.360. SECTION OF TRUNK OF A CHICK-EMBRYO.

FIGURE 2.361. MEROCYTES.

FIGURE 2.362. VASCULAR SYSTEM OF AN ANNELID.

FIGURE 2.363. HEAD OF A FISH-EMBRYO.

FIGURES 2.364 TO 2.370. THE FIVE ARTERIAL ARCHES.

FIGURES 2.371 AND 2.372. HEART OF A RABBIT-EMBRYO.

FIGURES 2.373 AND 2.374. HEART OF A DOG-EMBRYO.

FIGURES 2.375 TO 2.377. HEART OF A HUMAN EMBRYO.

FIGURE 2.378. HEART OF ADULT MAN.

FIGURE 2.379. SECTION OF HEAD OF A CHICK-EMBRYO.

FIGURE 2.380. SECTION OF A HUMAN EMBRYO.

FIGURES 2.381 AND 2.382. SECTIONS OF A CHICK-EMBRYO.

FIGURE 2.383. EMBRYOS OF SAGITTA.

FIGURE 2.384. KIDNEYS OF BDELLOSTOMA.

FIGURE 2.385. SECTION OF EMBRYONIC SHIELD.

FIGURES 2.386 AND 2.387. PRIMITIVE KIDNEYS.

FIGURE 2.388. PIG-EMBRYO.

FIGURE 2.389. HUMAN EMBRYO.

FIGURES 2.390 TO 2.392. RUDIMENTARY KIDNEYS AND SEXUAL ORGANS.

FIGURES 2.393 AND 2.394. URINARY AND SEXUAL ORGANS OF SALAMANDER.

FIGURE 2.395. PRIMITIVE KIDNEYS OF HUMAN EMBRYO.

FIGURES 2.396 TO 2.398. URINARY ORGANS OF OX-EMBRYOS.

FIGURE 2.399. SEXUAL ORGANS OF WATER-MOLE.

FIGURES 2.400 AND 2.401. ORIGINAL POSITION OF SEXUAL GLANDS.

FIGURE 2.402. UROGENITAL SYSTEM OF HUMAN EMBRYO.

FIGURE 2.403. SECTION OF OVARY.

FIGURES 2.404 TO 2.406. GRAAFIAN FOLLICLES.

FIGURE 2.407. A RIPE GRAAFIAN FOLLICLE.

FIGURE 2.408. THE HUMAN OVUM.



CHAPTER 2.16. 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 larvae, 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 2.18).

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.* (* See the ample monograph by Arthur Willey, Amphioxus and the Ancestry of the Vertebrates; Boston, 1894.) 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.

(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural size, 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.

FIGURE 2.211. 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 coeloma (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 coeloma (subchordal body-cavity), si principal (or subintestinal) vein, sk perichorda (skeletal layer).)

Johannes Muller 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 vertebrae 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.

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 vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even after the formation of the primitive vertebrae has begun, the segmented foetus 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 foetus, 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 (Figure 2.210) 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 (Figure 2.210 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 (Figure 2.211). 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, Figure 2.210 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 (Figure 2.210 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, Figure 2.210 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 larvae 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.

(FIGURE 2.212. Transverse section of an Amphioxus-larva, with five gill-clefts, through the middle of the body.

FIGURE 2.213. Diagram of the preceding. (From Hatschek.) A epidermis, B medullary tube, C chorda, C1 inner chorda-sheath, D visceral epithelium, E sub-intestinal vein. 1 cutis, 2 muscle-plate (myotome), 3 skeletal plate (sclerotome), 4 coeloseptum (partition between dorsal and ventral coeloma), 5 skin-fibre layer, 6 gut-fibre layer, I myocoel (dorsal body-cavity), II splanchnocoel (ventral body-cavity).)

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 (Figure 2.210 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.

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 (Figure 2.210 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 (Figure 2.214 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 (Figures 1.210 o and 2.212 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.

(FIGURE 2.214. Transverse section of a young Amphioxus, immediately after metamorphosis, through the hindermost third (between the atrium-cavity and the anus).

FIGURE 2.215. 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 myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.)

The coeloma 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 1.10), in these the two coelom-pouches are divided at an early stage by transverse constrictions into a double row of primitive segments (Figure 1.124), and each of these subdivides, by a frontal or lateral constriction, into an upper (dorsal) and lower (ventral) pouch.

These important structures are seen very clearly in the trunk of the amphioxus (the latter third, Figures 2.212 to 2.215), but it is otherwise in the head, the foremost third (Figure 2.216). 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. Figure 1.81). The branchial gut lies free in a spacious cavity filled with water, which was wrongly thought formerly to be the body-cavity (Figure 2.216 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 (M1, 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 (Figure 2.210 c).

On the inner surface of these mantle-folds (M1), 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 (Figure 2.219 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.

(FIGURE 2.216. 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 (M1) and to the chorda-sheath. N medullary tube, Ch chorda, Lh body-cavity, A atrium, L upper wall of same, E1 inner wall, E2 outer wall, Lh1 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 (Figure 2.217 x). They are short segmented canals; corresponding to the primitive kidneys of the other vertebrates (Figure 2.218 B). Their internal aperture (Figure 2.217 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 (Figure 2.218 B). The prorenal duct of the latter (Figure 2.218 C) corresponds to the branchial cavity or atrium of the former (Figure 2.217 C).

(FIGURE 2.217. 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 gonocoel), x pronephridium, B its coelom-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.

FIGURE 2.218. 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 1.11.)

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.

(FIGURE 2.219. 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 coeloma (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 Ascidiae 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 Ascidiae 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 Ascidiae 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 Ascidiae 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."

(FIGURE 2.220. 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.)

FIGURE 2.221. Organisation of an Ascidia (as in Figure 2.220, seen from the left). sb branchial sac, v stomach, i small intestine, c heart, t testicle, vd sperm-duct, o ovary, o apostrophe 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.))

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 (Figure 2.220 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 apostrophe). 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 (Figure 2.220 a), not directly outwards, but first into the mantle cavity; from this the excrements are ejected by a common outlet (a apostrophe) 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 Ascidiae 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 coeloma, 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.

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 Ascidiae are hermaphrodites. Each individual has a male and a female gland, and so is able to fertilise itself. The ripe ova (Figure 2.221 o apostrophe) 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 Ascidiae 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 2.17. 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 Ascidiae. We cannot say that the vertebrates are descended from the Ascidiae—and still less the reverse—but we can say that of all the invertebrates it is the Tunicates, and, within this group, the Ascidiae, 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 (Chapter 1.7). The active elements of the male sperm, the cone-shaped spermatozoa, are similar to those of most other animals (cf. Figure 1.20). 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, Figure 1.2). 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 foetus attains the familiar blastula shape and forms a hollow ball, the wall of which consists of a single stratum of cells (Figure 1.38 A to 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 (Figure 1.38 D). This pit becomes deeper and deeper (Figure 1.38 E and 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 (Figure 1.38 E). From hemispherical the body soon becomes almost spherical once more, and then oval, the internal cavity enlarging considerably and its mouth growing narrower (Figure 2.213). 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 (Figures 1.29 to 1.35).

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, Figure 1.38 g, 1.35 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 coelom-pouches, etc. (Figures 1.35 and 1.36 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 (Figure 1.36). 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 (Figure 1.39). In the middle of the dorsal surface a shallow longitudinal groove or furrow is formed (Figure 1.79), 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 (Figures 1.83 m and 1.84 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 foetal membranes.

Simultaneously with the construction of the medullary tube we have in the Amphioxus-embryo the formation of the chorda, the coelom-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 (Figure 1.83 p). They indicate the original starting-point of the two coelom-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 coelom-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 (Figure 1.82). 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 vertebrae"). 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 (metacoel); in the lower part they remain separate, and afterwards form the segmental gonads.

In the middle, between the two lateral coelom-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 (Figures 1.83 and 1.84 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:—

1. underneath, at the ventral side, the permanent alimentary canal or permanent gut;

2. above, at the dorsal side, the axial rod or chorda; and

3. the two coelom-sacs, which immediately sub-divide into two structures:—

3A. above, on the dorsal side, the episomites, the double row of primitive or muscular segments; and

3B. 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 larvae 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.

(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae (diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222 there is free communication from without with the gut-cavity (D) through the gill-clefts (K). In Figure 2.223 the lateral folds of the body-wall, or the gill-covers, which grow downwards, are formed. In Figure 2.224 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, E1 the same as visceral epithelium of the mantle-cavity, E2 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" (Figures 1.98 to 1.102). 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 (Figure 2.216 U) in the lateral walls of the mantle-cavity, in the gill-covers or mantle-folds (Figure 2.224 U), there is one present in the beginning (Figure 2.224 Lh). The sexual cells are formed below, at the bottom of this continuation (Figure 2.224 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 Ascidiae that show most clearly the relationship of the vertebrates to the invertebrates, and the larvae 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 Ascidiae 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 Ascidiae 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 coelom-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 larvae 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.

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.

(FIGURE 2.225. 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.)

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, Figure 2.225). 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-larvae, 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 (Figure 2.225 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 2.18. 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 coelomula, 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 foetal 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.