ON THE THEORY OF THE VERTEBRATE SKULL 6ol 



Rathke, ' Schildkroten,' p. 66. — In birds, the centra commence as bony rings 

 which closely encircle the chorda, and lie internal to the general cartilaginous 

 mass of the vertebra. The bony substance extends inwards and constricts the 

 notochord, outwards it permeates the vertebrse. 



" In the caudal vertebrae, and perhaps in all the cervical vertebras, 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, independ- 

 ently 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 substance 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 I in. to i in. 3 lines in length, the notochord ran straight 

 through the already existing rudiments of the intervertebral ligament 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 {Bogenschenkeln) of the vertebras, 

 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 vertebra;, 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 vertebral 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 chondrification which takes place 

 in the immediate neighbourhood of the notochord and in the neural and haemal 



' The relations of the ganglion to the rudiment of the rib and neural arch and segment 

 <if the dorsal muscles in the mouse's embryo are the same as in that of the bird. 



