The Evolution of Man, V.1. - Ernst Haeckel (learn to read activity book TXT) 📗

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