451 



" In the caudal vertebrae, and perhaps in all the cervical vertebrae, the 

 bony substance of the rings extends gradually to the surface of their centra. 

 In the dorsal vertebrae, on the other hand, there is formed at the fifteenth 

 day, independently of these rings, a broad though thin bony plate on the 

 upper, and a second on the lower face of the centrum, with which the sub- 

 stance of the ring, as it extends, coalesces. The notochord at the eighteenth 

 day may be seen traversing the intervertebral articular cavities like a 

 thread." 



Development of the Spinal Column of Mammalia. 



Rathke, ' Schildkroten,' pp. 66, 67. "In the pig and sheep the 

 bony substance is deposited immediately around the notochord, in such 

 a manner that, at first, as in birds, it forms a narrow and thin ring, from 

 which it passes partly towards the surface, partly towards the ends of the 

 separate centra, and after a time reaches the surface, but not the ends. To 

 complete the centra, there arise in the latter two special disks of bone for 

 each vertebra, which afterwards apply themselves to the previously 

 ossified middle part, and wholly coalesce with it. 



" In pig-embryos of 1 in. to 1 in. 3 lines in length, the notochord ran 

 straight through the already existing rudiments of the intervertebral liga- 

 ment like a delicate filament " (p. 77). 



Bischoff, in his various works, shows that the earliest changes in the 

 vertebral column of mammals are the same as in birds. 



" In like manner as in the Mammalia and birds, the ribs in the Chelonia 

 grow out as simple rays from the neural arches (Bogenschenkelii) of the 

 vertebrae, quite close to their bodies. Very close to the places where they 

 have arisen, however, the ribs of birds and mammals send, as a rule, a 

 process downwards and inwards, which increases more or less in length 

 and thickness, enlarges somewhat at its free end, and becomes closely 

 applied thereby to one, or two, bodies of vertebrae, by the intermediation 

 of the articular capsule which now becomes formed. This process is the 

 neck and head of the rib." Rathke, Schildkroten, p. 97. 



" The cervical transverse processes of birds and Mammalia attain their 

 forked form in quite a different manner from the ribs, namely, by the 

 coalescence at one end, of what are properly two transverse processes 

 which have grown out of the vertebra, while, on the other hand, a rib has 

 become forked, because, though originally a perfectly simple ray, it has 

 sent out a secondary process from one of its ends." 



It results from the observations which have just been detailed, that 

 with certain real or apparent exceptions which have been duly noted, 

 there is a very great uniformity in the mode of development of the ver- 

 tebral column in all Vertebrata. 



The primary processes up to, and inclusive of, segmentation or division 

 into somatomes, appear to be the same in all ; and there is every reason to 

 believe that the somatomes become differentiated in the same general 

 way *. There seems to be no difference, save in degree, in the chondrifi- 



* The relations of the ganglion to the rudiment of the rib and neural arch and 

 segment of the dorsal muscles in the mouse's embryo are the same as in that of 

 the bird. 





