TAIL-LIKE FORMATIONS IN MEN. 351 



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 in- 

 fancy, 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 in- 

 turned 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 ac- 

 cording 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, per- 

 haps, 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, 

 of Leipsic, for example, object to calling this 

 appendage a tail. But Prof. Ecker unequiv- 

 ocally upholds this designation, and in the 

 Archiv fiir Anatomie und Physiologie (1880, 

 No. 6, p. 442) formulates the following prin- 

 ciples in elucidation of the matter : 



1. The name " tail " can only be applied to j.^^_ o._Lower Pakt of 

 the part of the hinder end of the body project- an Embryo 15-5 mi. 

 ing over the cloacum. ^°^^' ^ 7"" "^^^^ 



^ . From Ecker. 



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 dis- 

 appears. 



5. The vertebra-containing section persists for a longer time 

 than the so-called coccygeal lump. The latter disaxjpears grad- 

 ually 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 dis- 

 appearance of the embryonal tail of man : an atrophy of the tail- 

 voL. XL. — 27 



