THE SKELETON IN AMPHIBIA. VERTEBRAL COLUMN. 171 



to a certain extent vertebrally, while intervertebrally there is 

 a development of cartilage. The connection between the bony 

 vertebrae is effected mainly by the expanded notochord. 



2. In the next stage, as seen in Gyrinophilus porphyriti- 

 cus, the growth of intervertebral cartilage has caused the 

 almost complete obliteration of the notochord intervertebrally, 

 and its entire disappearance vertebrally, i.e. in the centre of 

 each vertebra. The intervertebral cartilage now forms the main 

 connection between successive vertebrae, and sometimes cases 

 are found whose condition approaches that of definite articula- 

 tions. Readily recognisable remains of the notochord are still 

 found at each end of the intervertebral constriction. 



3. In the third stage differentiation and absorption of the 

 intervertebral cartilage has given rise to definitely articulating 

 opisthocoelous vertebrae. These are found in most adult 

 Salamandrina. 



The transverse processes of the earlier trunk vertebrae 

 are divided into two parts, a dorsal part which meets the 

 tubercular process of the rib and is derived from the neural 

 arch, and a ventral part which meets the capitular process of 

 the rib, and is derived from the ventral or haemal arch. In 

 the caudal vertebrae and often also in the posterior trunk 

 vertebrae the two processes are fused. 



Siren and Proteus, although they possess minute posterior 

 limbs, have no sacral vertebrae, while Cryptobranchus lateralis 

 has two. The caudal vertebrae, except the first, have haemal 

 arches very similar to the neural arches. 



In Labyrinthodontia the centra of the vertebrae are gene- 

 rally well ossified biconcave discs. In some forms however, like 

 Euchirosaurus, the centra are imperfectly ossified, and consist 

 of bony rings traversed by a wide notochord al canal. Each 

 ring is formed of four pieces, a large well-ossified neural arch, 

 a basal piece, and a pair of lateral pieces. Vertebrae of this 

 type are called rachitomous. 



