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.