(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.