556 VERTEBRAL COLUMN OF THE AMNIOTA. 



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

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

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

 and mainly differs from that in Salamandra in (i) 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 parts of the notochord ; 

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

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

 two contiguous vertebrae. The vertebrae are moreover proccelous instead 

 of being opisthocoelous. 



The unconstricted 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 unconstricted parts of the notochord appear however 

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

 the vertebral column in these genera has been distinguished as epichordal 

 in contradistinction to the more normal 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 round 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 (i) vertebral regions with true arches, and (2) inter- 

 vertebral regions. 



Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous 

 tube is formed round the notochord, which is continuous with 

 the cartilaginous neural arches. The latter are placed in the 

 vertebral regions, and in these regions ossification very early sets 

 in, while the notochord remains relatively unconstricted. In the 

 intervertebral regions the cartilage becomes thickened, as in 

 Amphibia, and gradually constricts the notochord. The carti- 

 lage in each of the intervertebral regions soon becomes divided 

 into two parts which form the articular faces of two contiguous 

 vertebrae. 



