any doubt on the subject, that the os coccygis of man is a real analogue of the animal's tail-root, while it also makes clear to us how the same has reached its special form. It is further confirmed by the fact that the inversion in which the coccyx takes part is not observed in the embryonal life of man nor in the earliest infancy, but first appears when the child begins to carry its body erect. The tail-like prolongation of the human vertebral column is evidently a rudimentary formation—an inheritance from the animal condition which, perhaps, persists simply because the inturned vertebra of the os coccygis has adapted itself to a new function, instead of becoming useless.
There is found in the human embryo, in the first stage of its embryonal life, just as in other vertebrates, a considerable and conformable tail-structure, which it is not hard to interpret according to biogenetical principles. The length of this taillet, in proportion to that of the rest of the body, is at first considerable. In embryos that have completed their third week the tail is, perhaps, about twice as long as the lower limbs. It is one of the pruderies that still live to vex us that some anatomists. Prof. His, 
Fig. 2.—Lower Part of an Embryo 15·5 mm long with Tail. From Ecker. of Leipsic, for example, object to calling this appendage a tail. But Prof. Ecker unequivocally upholds this designation, and in the Archiv für Anatomie und Physiologie (1880, No. 6, p. 442) formulates the following principles in elucidation of the matter:
1. The name "tail" can only be applied to the part of the hinder end of the body projecting over the cloacum.
2. In embryos of the second class—that is, those which are from eight to fifteen millimetres long—the "tail" overtopping the cloacum appears as a free pointed projection upward and forward.
3. This tail consists of a vertebra-containing and a vertebra-free section, the latter of which contains only a chorda and a marrow-tube.
4. Only the latter section suffers a reduction, by the chorda dorsalis being mostly converted into a knot, while the rest disappears.
5. The vertebra-containing section persists for a longer time than the so-called coccygeal lump. The latter disappears gradually under the surface, chiefly in consequence of the gradually stronger curvature of the os sacrum and os coccygis, and partly of the more prominent development of the pelvic band and its musculature.
We should also distinguish two processes in the gradual disappearance of the embryonal tail of man: an atrophy of the tail--
