n VERTEBRAL COLUMN 21 



Only two vertebrae, generally the tenth and eleventh of the 

 whole column, are clearly visible, each being composed of a pair 

 of dorsal and a pair of ventral cartilaginous blocks. The sacral 

 vertebra articulates with the coccyx by one or two convexities, 

 but in the Aglossa, in some Pelobatidae, and a few others, the 

 coccyx is fused with the sacral vertebra. Beyond the first 

 and second component vertebrae of the embryonic coccyx, the 

 cartilage is continued in the shape of two dorsal, and one ventral, 

 bands, which soon fuse with each other. Dorsally this cartilage 

 surrounds the spinal cord ; the latter degenerates towards the 

 end of the tadpole-stage, leaving, however, the empty spinal 

 canal. The chorda, completely surrounded by cartilage, persists 

 into the post -larval stage, but is destroyed long before the 

 creature attains maturity. Ultimately the whole coccyx ossifies. 



The tail proper, namely that portion which is absorbed 

 during the metamorphosis, remains throughout its existence in 

 an apparently primitive condition. The chorda dorsalis and the 

 spinal cord extend through its whole length, surrounded by 

 continuous connective tissue without any cartilage ; in fact it 

 represents a piece of typical vertebral column before the appear- 

 ance of cartilage. The reduction of this swimming organ begins 

 at the hind end. 



The vertebral column of the adult. The first vertebra 

 (we will call it the atlas since it carries the skull) is not, as in 

 the Urodela, provided with an odontoid process. It articulates 

 by two cups with the condyles of the occiput. In some Anura it 

 co-ossifies, rather incompletely, with the second vertebra, regularly 

 in the fossil Palaeobatrachus, often in Ceratophrys, Breviceps, 

 and occasionally in Pelobates, Bufo, Sana, and Xenopus. 

 This is, however, no justification for looking upon the first 

 vertebra as a complex of two vertebrae, although the atlas is 

 frequently very thick and broad, and even carries, in the 

 Aglossa, considerable lateral wings or diapophyses. Those of the 

 trunk-vertebrae are often very long, acting thereby as substitutes 

 for ribs which are absent, except on the second, third, and fourtli 

 vertebrae of the Discoglossidae, and on the second and third of 

 the Aglossa. In the adult Aglossa these ribs fuse with the 

 processes which carry them. 



The diapophyses of the sacral vertebra carry no ribs, the 

 ilia being attached to them directly. They are either cylindrical 



