The Evolution of Man Volume I Part 18

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 part.i.tion 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 cla.s.sification. 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 pa.s.s 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 pa.s.s 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 pa.s.s 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 pa.s.s 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 albima.n.u.s), 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 cla.s.sifiers 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 circ.u.mstances have a high phylogenetic value. In conjunction with the other facts, it affords a striking confirmation of our biogenetic law.