86 Elementary Zoology. 



the second, or axis. After this come seven vertebrae with 

 well-developed transverse processes, and an eighth vertebra 

 with particularly strong transverse processes, which support the 

 pelvic bones. The anterior set may be termed " dorsals," and 

 the vertebra which supports the pelvis " sacral." In order to 

 satisfactorily compare the vertebrse of the frog with those of 

 the other vertebrates, it will be necessary to enter into the 

 development of the vertebral column. 



Unlike the skull, the entire vertebral column is formed out 

 of cartilage. The cartilages, when they first appear, appear 

 round the notochord, the development of which structure, the 

 precursor of the vertebral column, is dealt with on another page. 

 There are typically four pairs of cartilaginous elements aiTanged 

 in sets, of which, however, some may be suppressed. They have 

 been called by the following names : basidorsalia, basiventralia, 

 interdorsalia, interventralia. In the common frog the inter- 

 ventralia are suppressed, and the entire vertebra is formed 

 by the coalesced basidorsalia, interdorsalia, and basiventralia. 

 In the bird's vertebral column the separate vertebrse are 

 formed of other elements. The centrum of the vertebra is 

 formed of the interventralia, the basidorsalia form part of the 

 neural arch, and, finally, the basiventralia are converted into 

 the intercentra, where these latter exist. The same statements 

 hold for the vertebral column of the mammal. In both these 

 latter groups the atlas is peculiar, in that its centrum remains 

 free from the interventralia, and becomes attached to the 

 centrum or the following vertebra, the axis. It is clear, 

 therefore, that a vertebra of the frog does not exactly corre- 

 spond to a vertebra of either a bird or a mammal. Moreover, 

 apart from their being formed out of partly different elements, 

 and therefore not exactly corresponding, there is not even 

 a rough correspondence between the atlas of the frog, on the 

 one hand, and of the bird and mammal on the other. It 

 has been pointed out that the frog has only ten pairs of 

 cranial nerves ; the fowl and the mammal have twelve. This 

 naturally leads to the view, supported by other data, that the 

 skull of the higher vertebrates is more extensive than that of 

 the frog. There are, in fact, rudiments of, apparently, at least 



