Human ovum of twelve to thirteen days. Fig. 186—Human ovum of twelve to thirteen days (?). (From _Allen Thomson._) 1. Not opened. 2. Opened and magnified. Within the outer chorion the tiny curved fœtus lies on the large embryonic vesicle, to the left above. Fig.187. Human ovum of ten days. Fig. 188. Human foetus of ten days, taken from the preceding ovum, magnified. Fig. 187—Human ovum of ten days. (From _Allen Thomson._) Opened; the small fœtus in the right half, above. Fig. 188—Human fœtus of ten days, taken from the preceding ovum, magnified, _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. Fig.189. Human ovum of twenty to twenty-two days. Fig. 190. Human foetus of twenty to twenty-two days, taken from the preceding ovum, magnified. Fig. 189—Human ovum of twenty to twenty-two days. (From _Allen Thomson._) Opened. The chorion forms a spacious vesicle, to the inner wall of which the small fœtus (to the right above) is attached by a short umbilical cord. Fig. 190—Human fœtus 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 für Anthropologie,_ Band XV, p. 129). Fig.191. Human embryo of sixteen to eighteen days. Fig. 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 (Fig. 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.