1101] From this it may be seen that the number of vertebrae or 

 their equivalent is fairly if not quite constant in embryos 

 between eight and sixteen millimeters in length. We have, 

 then, seven vertebrae in the embryonic tail at its highest 

 period of development. The stages studied by His and by 

 Kosenberg were either too young or too far advanced to 

 show the maximum number of vertebrae. That the reduction 

 takes place by fusion, as is maintained by Fol, is confirmed 

 by the study of the embryos described above. In the older 

 embryo (16 mm.), in which an exceptionally large number 

 of segments was present, partial fusion between several of 

 the adjacent vertebrae had taken place. In still older embryos, 

 as seen in the table, the number of segments is inconstant; 

 most probably this is due to the varying extent to which 

 fusion hasr taken place, though it is possible that it may be 

 due in part to a difference in the original number. As Stein- 

 bach'*' shows, the usual number of segments is thirty-four, 

 i. e. five coccygeal, although the number may be less or, in 

 rare instances, even increased by one. 



The spinal ganglia of the caudal region, as Keibel has 

 shown, also sufl^er reduction. There are never quite so many 

 ganglia developed as vertebrae, and the last ones are always 

 more or less rudimentary; but there are always more formed 

 than persist in the adult. For instance, in an embryo of 10 

 mm. Phisalix described thirty-six ganglia; in an embryo of 

 11.5 mm. Keibel found thirty-four; in the embryo of 14 mm. 

 described above there were thirty-three, and in the embryo 

 of 16 mm. thirty-two, while in the adult there are but thirty- 

 one. The segmental arteries of the distal caudal segments 

 also become obliterated as development proceeds. 



We conclude, then, with Keibel that, while as far as out- 

 ward form is concerned the embryonic tail disappears largely 

 as a result of the growth of the extremities and the gluteal 

 region, a certain amount of regressive change takes place in 

 the caudal appendage itself. This is manifest not only in the 

 absorption of the caudal filament, as supposed by Ecker and 



36 E. Steinbach : Die zahl der Caudalwirbel beim Menschen. Inaug. 

 Diss., Berlin, 1889. 



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