AND OP THE CAUDAL END OF THE SPINAL CORD. 187 



face of the skin. In the first embryo the tail remnant is surrounded by a minute 

 fvuTow, while in the 33 mm. specimen it appears as a rounded eminence; and finally, 

 at the 39 mm. stage, the remnant of the tail is represented by a small papilla. 

 These remnants contain groups of cells from the primitive neural canal. The 

 apex of the caudal conical eminence, the caudal tubercle, according to linger and 

 Brugsch, in which the cell-strand of the neural canal enters, is a part of the lost- 

 vertebrse tail, or so-called non- vertebra ted tail. The various stages in the reduc- 

 tion of the tail as shown on the skin surface do not present the same appearance in 

 every embryo; but on section evidences of its reduction and disappearance are 

 invariably found dorsal to the caudal end of the vertebral column — that is, dorsal 

 to the coccygeal tubercle and in the median line of the embryo. I have seen no 

 case in which the remnant of the tail is situated just at the caudal end nearly in 

 line with the extended axis of the vertebral column — namely, at the top of the 

 coccj'geal tubercle. At this stage the caudal ligament is well developed and con- 

 sists of bands of connective-tissue. The curve of the vertebral column is quite 

 jM-ominent at the thirtieth, thirty-first, and thirty-second vertebrae. This flexion 

 of the caudal portion of the column begins at about the 25 mm. stage, although 

 sometimes it does not appear until the 33 mm. stage. These embryos show a small 

 tail at the caudo-dorsal end. In the 30 mm. embryo (fig. 43) and the 39 mm. 

 embryo, where the tail is disajjpearing from the surface of the skin, this curving 

 of the vertebral column becomes more marked than in the younger specimens. 



Concerning the disappearance of the tail in the human embryo, I am of the 

 ojjinion that, while the lost-vertebne portion of the tail disappears from the skin 

 surface, a few vertebrse of the tail fuse with the one above, u.sually the thirty-fourth, 

 a part of which disappears by dedifferentiation; and that the caudal jjortion of the 

 column, which consists of the thirty-first, thirty-second, thirty-tliird, and thirty- 

 foiu'th vertebrae, is bent to the ventral side, sinking into the embryonic tissue 

 between the rectum and the coccygeal tubercle. After the tail entirely disappears 

 there appears outside of the ventral region of the tail root a blunt conical eminence 

 known as the coccygeal tubercle, or eminentia coccygeal^. This tubercle is a tempo- 

 rary swelling formed by bulging of the caudal end of the vertebral column and the 

 addition of embryonic tissue contained in the lost- vertebrae tail at an earlier stage. 

 The tubercle disappears at some time between the 33 and 52 mm. stage, while the 

 caudal end of the vertebral column, the so-called internal tail (after Braun), sinks 

 deeper into the soft tissues which surround and envelop it (figs. 44 and 45). 



In describing the embryonic tail in mammals, Braun divides it into internal 

 tail {die innere Schwanz) and external tail (aussere Schwanz). This theoretical 

 arrangement may be the better one. In the human embryo, at least in my speci- 

 mens, it can be clearly demonstrated. In an embryo of 21 mm. one can recognize 

 the external tail, which may be divided into two portions — vertebrated and non- 

 vertebrated. In the 27, 30, and 39 mm. specimens the thirty-first, thirty-second, 

 thirty-third, and thirty-fourth vertebrae belong to those of the internal tail. I 

 agree with other authors that the human embryo has a true tail at a certain stage 

 of its development and that the second coccygeal vertebrae and those caudal to it 

 in the adult are the true tail vertebrae in the philogenetic sense. 



