458 VERTEBRAL COLUMN OF THE AMXIOTA. 



opisthocoelous. A longitudinal section through the vertebral column 

 at this stage is diagrammatically shewn in fig. 320 C. 



To the centre of each of these vertebrae the neural arches, the 

 origin of which was described above, become in the meantime firmly 

 attached ; and grow obliquely upwards and backwards, so as to meet 

 and unite above the spinal cord. The transverse processes of the 

 vertebra would seem (Fick) to be developed independently of the 

 arches, though they very soon fuse with them. According to Gotte 

 the transverse processes are double in the trunk, there being two 

 pairs, one vertically above the other for each vertebra. The pair on 

 each side eventually fuse together. 



In the tail luemal arches are formed, which are similar in their 

 mode of development to the neural arches. 



The unconstricted portion of the notochord, which persists in each 

 vertebra, becomes in part converted into cartilage. 



Anura. In the Anura the process of formation of the vertebral 

 column is essentially the same as that in the Salamandridge. Two types may 

 however be observed. One of these occurs in the majority of the Anura, 

 and mainly differs from that in Salamandra in (1) the eailier fusion of the 

 arches with the cellular sheath of the notochord; (2) the more rapid growth 

 of the intervertebral thickenings of the cellular sheath, which results in the 

 early and complete obliteration of the intervertebral parts of the notochord; 

 ('.}) the complete division of these Intel-vertebral thickenings into anterior 

 and posterior portions, which unite with and form the articular surfaces of 

 t\vo contiguous vertebrae. The vertebrae are moreover proecelous instead 

 of being O|>isthoccelous. 



The unconstrictecl vertebral sections of the notochord always persist till 

 the ossification of the vertebrae has taken place. In some forms they 

 remain through life (Rana), while in other cases they eventually either 

 wholly or partially disappear. 



The second type of vertebral development is found in Bombinator, 

 Pseudis, Pipa, and Pelobates. In these genera the formation of the 

 vertebra takes place almost entirely on the dorsal side of the notochord ; 

 so that the latter forms a band on the ventral side of the vertebral column. 

 In other respects the history of the vertebral column is the same in the two 

 cases ; the vertebral unconstrictecl parts of the notochord appear however 

 to become in part converted into cartilage. The type ot formation of 

 the vertebral column in these genera has been distinguished as epichordal 

 in contradistinction to the more no-mal or perichordal type. 



Amniota. In the Amiiiota all trace of a distinction between a 

 cellular notochord sheath and an arch tissue is lost, and the two are 

 developed together as a continuous whole forming an unsegmented 

 tube round the notochord, with a neural ridiie which does not at first 

 nearly invest the neural cord. This tube becomes differentiated, in 

 the manner already described for other types, into (1) vertebral 

 regions with true arches, and (2) intervertebral regions. 



Eeptilia. In Replilia (Gegenbaur, No. 416) a cartilaginous tube 

 is formed round the notochord, which is continuous with the carti- 



