1 2 URODELA 



(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 vertel>ral 

 column of the Urodela is the possession of a considerable amount 

 of intervertel3ral 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 l^ecomes more or less imperfectly separated into a cup 

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

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

 Desmognathinae and Salamandrinae. In the adult the cup and 

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

 completely destroyed. Those vertebrae between which the inter- 

 verteliral 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 verteljrae a consideralile amount of the chorda 

 always remains, running in an unbroken string through the 

 whole length of the vertebral column. Towards adult life 

 the chorda liecomes constricted, and is ultimately squeezed out 

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

 cartilage from the chief arcualia. This intravertelirally situated 

 cartilage has been described erroneously as chordal cartilage. 



The development of the vertel)rae proceeds as follows. First 

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

 B.I), B.A^), Ijlocks of cartilage, imbedded in and resting upon the 

 tliin 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 

 cartilages or " arcualia " each meet, above or below the cliorda, and 

 form semi-rings, whicli again by extending upwards or downwards 

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

 interventral elements form tlie intervertebral mass spoken of above. 

 The liasidorsals fuse with the basiventrals, and form tlie body of 

 the vertebra, tlie fusion being effected chiefly by the calcification 

 and ossification of the lateral connecting portion of tlie skeleto- 

 genous layer. The basidorsalia form the neural arches with 

 their unpaired short spinous or neural, and tlie paired anterior 

 and posterior zygapophysial processes. Concerning the basi- 



