1 2 URODELA 



CHAP. 



(tail-vertebrae), or entirely by the pair of chief dorsal arcualia. 

 There is consequently no neuro-central suture. Moreover, the 

 central region of each vertebra is strongly pinched in laterally, 

 widening towards the ends. Another feature of the vertebral 

 column of tiie Urodela is the possession of a considerable amount 

 of intervertebral cartilage, by which the successive vertebrae are 

 held together. This cartilage does not ossify, and it either remains 

 continuous, serving in its entirety and owing to its flexibility as 

 a joint, or it becomes more or less imperfectly separated into a cup 

 and ball portion, the cup belonging to the posterior end of the 

 vertebra. Such joints are called opisthocoelous, and occur in the 

 Desmognatliinae and Salamandrinae. In the adult the cup and 

 ball fre(|uently calcify, and the chorda dorsalis or notochord is 

 completely destroyed. Those vertebrae between which the inter- 

 vertebral cartilage remains unbroken, are called amphicoelous, 

 since in them, most obviously in macerated or dried skeletons, 

 the vertebrae appear hollowed out at either end. In such 

 amphicoelous vertebrae a consideral)le amount of the chorda 

 always remains, running in an unbroken string through the 

 whole length of the vertebral column. Towards adult life 

 the chorda becomes constricted, and is ultimately squeezed out 

 or destroyed, in the middle of the vertebra, by the invasion of 

 cartilage from the chief arcualia. This intravertebrally situated 

 cartilage has been described erroneously as chordal cartilage. 



The development of the vertebrae proceeds as follows. First 

 appear a pair of basidorsalia and a pair of basiventralia (Fig. 1, i, 

 R.I), B.V), blocks of cartilage, imbedded in and resting upon the 

 thin sheath of the chorda dorsalis. Next appears a pair of inter- 

 dorsal blocks, immediately behind the basidorsals ; and somewhat 

 later appears a pair of interventral blocks. These four pairs of 

 curtilages or " arcualia " each meet, above or below the chorda, and 

 I'oriii semi-rings, wliich again by extending upwards or downwards 

 fuse into complete rings, in such a way that the interdorsal and 

 interventral elements form the intervertebral mass spoken of above. 

 The l)asidorsals fuse with the basiventrals, and ibrm the body of 

 the vertebra, the fusion being effected chiefly by the calcification 

 and ossification of the lateral connecting portion of tlie skcleto- 

 genous layer. The basidorsalia form the neural arches with 

 their unpaired short spinous or neural, and the paired anterior 

 and ])0s1('rini' zygapopliysial processes. Concerning tlie })asi- 



