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 vertebrae in the vertebral column (seven cervical, twelve dorsal, five lumbar, five sacral, and four caudal). To these we must add at least nine head-vertebrae, 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).

(FIGURES 1.158 TO 1.160. Embryo of the amphioxus, twenty four hours old, with eight somites. (From Hatschek.) Figures 1.158 and 1.159 lateral view (from left). Figure 1.160 seen from back. In Figure 1.158 only the outlines of the eight primitive segments are indicated, in Figure 1.159 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 coelom-pouches have hardly grown out of the primitive gut (Figure 1.156 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 coelom-pouches begins to divide into a series of pieces by new transverse folds (Figure 1.157). The foremost of these primitive segments (us1) is the first and oldest; in Figures 1.124 and 1.157 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 (Figures 1.159 to 1.160).

(FIGURES 1.161 AND 1.162. Transverse section of shark-embryos (through the region of the kidneys). (From Wijhe and Hertwig.) In Figure 1.162 the dorsal segment-cavities (h) are already separated from the body-cavity (lh), but they are connected a little earlier (Figure 1.161), 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 (mk1 parietal, mk2 visceral), ik inner germ-layer (gut-gland layer).)

This typical articulation of the two coelom-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; Figure 1.82 d, left half of the figure). Afterwards the two halves completely separate, a lateral longitudinal fold cutting between them (mk, right half of Figure 1.82). The dorsal segments (sd) provide the muscles of the trunk the whole length of the body (1.159): 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 metacoel, or the simple permanent body-cavity.

The articulation proceeds in substantially the same way in the other vertebrates, the craniota, starting from the coelom-pouches. But whereas in the former case there is first a transverse division of the coelom-sacs (by vertical folds) and then the dorso-ventral division, the procedure is reversed in the craniota; in their case each of the long coelom-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 2.21).

(FIGURE 1.163. 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 (Figures 1.161 and 1.162). The careful studies of Ruckert, 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 myocoels, h) are still connected with the ventral body-cavity (lh; Figure 1.161). In Figure 1.162, 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 coelom-pouches and the rise of the primitive segments from their dorsal half (cf. Figure 1.91, A, B, C). A horizontal longitudinal section of the salamander-embryo (Figure 1.163) 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.

(FIGURE 1.164. The third cervical vertebra (human).

FIGURE 1.165. The sixth dorsal vertebra (human).

FIGURE 1.166. 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 coelom-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 (Figures 1.94 to 1.97). And when the articulation of the somites begins in the sole-shaped embryonic shield, and a couple of protovertebrae are developed in succession, constantly increasing in number towards the rear, these cube-shaped somites (formerly called protovertebrae, or primitive vertebrae) have the appearance of solid dice, made up of mesodermic cells (Figure 1.93). Nevertheless, there is for a time a ventral cavity, or provertebral cavity, even in these solid "protovertebrae" (Figure 1.143 uwh). This vesicular condition of the provertebra is of the greatest phylogenetic interest; we must, according to the coelom theory, regard it as an hereditary reproduction of the hollow dorsal somites of the amphioxus (Figures 1.156 to 1.160) and the lower vertebrates (Figures 1.161 to 1.163). 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.

(FIGURE 1.167. Head of a shark embryo (Pristiurus), one-third of an inch long, magnified twenty times. (From Parker.) Seen from the ventral side.)

The innermost median part of the primitive segment plates, which lies immediately on the chorda (Figure 1.145 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 provertebrae 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 (Figure 1.137 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 (Figures 1.164 to 1.166). The upper or dorsal half of this bony ring, the vertebral arch (Figure 1.145 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.

(FIGURES 1.168 AND 1.169. Head of a chick embryo, of the third day. Figure 1.168 from the front, Figure 1.169 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 Kolliker.))

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" (Figure 1.148 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.

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 vertebrae, 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).

(FIGURE 1.170. 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 apostrophe, f double apostrophe, 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.))

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 Ruckert, 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.

(FIGURE 1.171. Human embryo of the fourth week (twenty-six days old), one-fourth of an inch in length magnified twenty times, showing: point of development of the hind-leg, umbilical cord (underneath it the tail, bent upwards), trigeminal nerve V Trigeminus, optic-muscle nerve III Oculo-motorius, rolling muscle nerve IV Trochlearis, rudiment of ear (labyrinthic vesicles), pneumogastric nerve X Vagus, terminal nerve XI Accessorius, hypoglossal nerve XII Hypoglossus, first spinal nerve, point of development of arm (or fore-leg), true spinal nerve. (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).)

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.

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 medusae, 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 (Figure 1.102 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 Figure 1.161 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 coelom-pouches grows out of the dorsal wall of the gut, on either side of the chorda. In the brief period during which these segmental coelom-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.

(FIGURE 1.172. 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.)

FIGURE 1.173. Transverse section of the pelvic region and hind legs of a chick-embryo of the fourth day, magnified about forty times. 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 coeloma. (From Waldeyer.))

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 (Figures 1.167 to 1.170 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 (Figure 1.98 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 (Figure 1.170 d, f, f apostrophe, f double apostrophe). In some of the fishes (selachii) and in the cyclostoma we find six or seven of them permanently.

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 (Figures 1.161 and 1.162 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 (Figures 1.170 and 1.171). 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 (Figure 1.169 n). The organ of sight, the eye, is found at the side of the head, also in the shape of a depression (Figures 1.169 l and 1.170 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 (Figure 1.169 g). As yet we can see nothing of the later elaborate structure of these organs, nor of the characteristic build of the face.

(FIGURE 1.174. 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.))

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.

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 (Figures 1.172 and 1.173). 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.

(FIGURE 1.175. Human embryo, five weeks old, half an inch long, seen from the right, magnified ten times. (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 Figure 1.174. 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 (Figure 1.175).

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.

(FIGURES 1.176 TO 1.178. Embryos of the bat (Vespertilio murinus) at three different stages. (From Oscar Schultze.) Figure 1.176: Rudimentary limbs (v fore-leg, h hind-leg). l lenticular depression, r olfactory pit, ok upper jaw, uk lower jaw, k2, k3, k4 first, second and third gill-arches, a amnion, n umbilical vessel, d yelk-sac. Figure 1.177: Rudiment of flying membrane, membranous fold between fore and hind leg. n umbilical vessel, o ear-opening, f flying membrane. Figure 1.178: 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.* (* 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 1.15. FOETAL 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, Figure 1.14) 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. Linne (1735) placed him in this class with the apes, in one and the same order (primates), in his Systema Naturae. 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.

(FIGURE 1.179. Human embryos from the second to the fifteenth week, natural size, 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.

(FIGURE 1.180. 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 gastraea. 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 1.1). Thus the apparent exceptions to the law can always be traced to cenogenesis.

(FIGURE 1.181. Human embryo of the middle of the fifth week, one-third of an inch long. (From Rabl.) Letters as in Figure 1.180, 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 (Figures 1.1 and 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 foetus 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 (Figure 1.133). 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 coelom-pouches proceeds in the same way as in the other mammals (cf. Figures 1.96 and 1.97). 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 (Figure 1.179, 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.

(FIGURE 1.182. 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.)

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 (Figure 1.179 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 (Figures 1.180, 1.181 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 (Figure 1.179 V.; Figure 1.181). 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 (Figure 1.182) 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) vertebrae 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.

(FIGURE 1.183. 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 double apostrophe, third gill-arch, ov heart (o right, o apostrophe, left auricle; v right, v apostrophe, left ventricle), b origin of the aorta, f liver (u umbilical vein), e gut (with vitelline artery, cut off at a apostrophe), j apostrophe, 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 apostrophe, hind-leg. (From Coste.)

FIGURE 1.184. Human embryo, five weeks old, opened from the ventral side (as in Figure 1.183). Breast and belly-wall and liver are removed. 3 outer nasal process, 4 upper jaw, 5 lower jaw, z tongue, v right, v apostrophe, left ventricle of heart, o apostrophe, left auricle, b origin of aorta, b apostrophe, b double apostrophe, b triple apostrophe, first, second, and third aorta-arches, c, c apostrophe, c double apostrophe, 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 apostrophe, 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 fur Anthropologie, Band 15), and critically examined. These atavistic human tails are often mobile; sometimes they contain only muscles and fat, sometimes also rudiments of caudal vertebrae. 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.

(FIGURE 1.185. The head of Miss Julia Pastrana. (From a photograph by Hintze.)

FIGURE 1.186. Human ovum of twelve to thirteen days (?). (From Allen Thomson.) 1. Not opened, natural size. 2. Opened and magnified. Within the outer chorion the tiny curved foetus lies on the large embryonic vesicle, to the left above.

FIGURE 1.187. Human ovum of ten days. (From Allen Thomson.) Natural size, opened; the small foetus in the right half, above.

FIGURE 1.188. Human foetus of ten days, taken from the preceding ovum, magnified ten times, 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.

FIGURE 1.189. Human ovum of twenty to twenty-two days. (From Allen Thomson.) Natural size, opened. The chorion forms a spacious vesicle, to the inner wall of which the small foetus (to the right above) is attached by a short umbilical cord.

FIGURE 1.190. Human foetus 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 fur Anthropologie, Band 15 page 129).

When we open a human embryo of one month (Figure 1.183), 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 (Figure 1.183 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.

(FIGURE 1.191. 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 (Figure 1.190) 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.

FIGURE 1.192. Human embryo of the fourth week, one-third of an inch long, lying in the dissected chorion.

FIGURE 1.193. Human embryo of the fourth week, with its membranes, like Figure 1.192, 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. Figure 1.179 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 foetus 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 foetal 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 2.23.)

(FIGURE 1.194. 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, Figure 1.185), 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 foetus 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.

(FIGURE 1.195. Diagram of the embryonic organs of the mammal (foetal 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 pericoelom or serocoelom (inter-amniotic cavity), sz serolemma (or serous membrane), pc prochorion (with villi).)

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 (Figures 1.1 and 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 foetus and its appendages as a broad, completely closed sac; the space between the two, filled with clear watery fluid, is the serocoelom, 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 (Figures 1.186, 1.191 and 1.198 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 (Figures 1.186 to 1.194).

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 (Figures 1.186 to 1.188). 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 foetus 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 foetus (Figure 1.194). 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 (Figure 1.147 r, u, Figure 1.195 ALC}.

(FIGURE 1.196. 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 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.

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 (Figure 1.196 al). As the large and blood-filled vascular network of the foetal 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 foetal body which is characteristic of the placentalia (or choriata). We shall return afterwards to the closer consideration of this (cf. Chapter 2.23).

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 (Figure 1.197) 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.

(FIGURE 1.197. Male embryo of the Siamang-gibbon (Hylobates siamanga) of Sumatra, two-thirds natural size; 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.)

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 (Figure 1.198). 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 (Figure 1.196 du, Figure 1.199 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, Figure 1.196 ds, Figure 1.199 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 foetal placenta. The internal or false decidua (interna or reflexa, Figure 1.196 dr, Figure 1.199 f) is that part of the mucous lining of the womb which encloses the remaining surface of the ovum, the smooth chorion (chorion laeve), 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 foetus (like the anmnion) to the end.

The peculiar anatomic features that characterise the human foetal 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.)

(FIGURE 1.198. Frontal section of the pregnant human womb, showing: end of the decidua, uterine cavity, chorion (laeve), amniotic cavity, foetal placenta, oviduct, spongy decidua serotina, umbilical vesicle, amnion, decidua reflexa, decidua vera, muscular wall of the uterus, mouth of the uterus. (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).

FIGURE 1.199. Human foetus, twelve weeks old, with its membranes. Natural size. 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.)

FIGURE 1.200. Mature human foetus (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.))

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.

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.

(FIGURE 1.201. 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.)

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.

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 (Figure 1.201 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 foetal 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 (Figure 1.183 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 (Figures 1.183 n and 1.184 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 (Figure 1.196 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.

(FIGURE 1.202. Boat-shaped embryo of the dog, from the ventral side, magnified about ten times. 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 protovertebrae, 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.

(FIGURE 1.203. Lar or white-handed gibbon (Hylobates lar or albimanus), from the Indian mainland (From Brehm.)

FIGURE 1.204. Young orang (Satyrus orang), asleep.)

The existing anthropoid apes are only a small remnant of a large family of eastern apes (or Catarrhinae), 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, Figure 1.203); 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 (chapter 11). 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.

(FIGURE 1.205. 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, Dissatyrus (orang-bentang, Figure 1.205), 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).

(FIGURE 1.206. 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 Figure 1.207) in the structure of the head, the colouring, and the absence of hair in parts.)

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 (Figure 1.206), and the gorilla-like A. mafuca differ very strikingly from the ordinary Anthropithecus niger (Figure 1.207), 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.

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 Figure 1.208), not only by its unusual size and strength, but also by a special formation of the skull. This giant gorilla (Gorilla gigas, Figure 1.209) 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.

(FIGURE 1.207. Female chimpanzee (Anthropithecus niger). (From Brehm.)

FIGURE 1.208. Female gorilla. (From Brehm.)

FIGURE 1.209. Male giant-gorilla (Gorilla gigas), from Yaunde, in the interior of the Cameroons. killed by H. Paschen, stuffed by Umlauff.)

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 2.23. 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 foetal 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.

THE END