458 VERTEBRAL COLUMN OF THE AMNIOTA. 



opistbocoelous. A longitudinal section through the vertebral column 

 at this stage is diagrammatically shewn in fig. 320 0. 



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 

 vertebrae 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 t(jgether. 



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

 mode of development to the neural arches. 



The imconstricted portion of the notochord, which persists in each 

 vertebra, becomes in part converted into cartilage. 



Anura.. In the Anura the [)rocess of formfition of the vertebral 

 column is essentially the same as that in the Salaniandriflse. Two tyi)es may 

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

 and mainly differs fi'om that in Salamandra in (1) the earlier 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 })arts of the notochord; 

 (3) the complete division of these intervertebral thickenings into anterior 

 and ))osterior portions, which unite with and foi-m the articular surfaces of 

 two contiguous vertebra?. The vertebrae are moreover procoelous instead 

 of lieing O|iistliocoelous. 



The unconstricted vertebral sections of the notochord always persist till 

 the ossification of the vertebrfe has taken ])lace. In some forms they 

 rt^main through hfe (Rana), while in other cases they eventually either 

 wholly or partially disai)pear. 



The second type of vertebral development is found in Bombinator, 

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

 verteV)ra takes place almo.st entirely on the dorsal side of the not((chord ; 

 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 unconstricted parts of the notochord appear however 

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

 the vertebral column in thej<e genera has been distinguished as epicliordal 

 in contradistinction to the more nomal or perichordal type. 



Amniota. In the Amniota 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 roimd the notochord, with a neural ridge 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. 



B/eptilia. lu Reptilia (Gegenbaur, No. 416) a cartilaginous tube 

 is formed round tiie not(jchord, which is continuous with the carti- 



