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 generally well ossified biconcave discs. In some forms however, like Euchirosaurus, the centra are imperfectly ossified, and consist of bony rings traversed by a wide notochordal 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.
In the tail region of other forms each vertebra consists of an anterior centrum bearing the neural arch, and a posterior intercentrum[61] bearing chevron bones. Vertebrae of this type are called embolomerous. Haemal arches similar to the neural arches are often found as in Urodela. The transverse processes are sometimes well developed and are divided into tubercular and capitular portions.
In Gymnophiona the vertebrae are biconcave and are very numerous, they sometimes number about two hundred and thirty. Only quite the last few are ribless and so can be regarded as post-thoracic vertebrae. The first vertebra has nothing of the nature of an odontoid process.
In Anura the number of vertebrae is very greatly reduced, only nine and the urostyle being present. Of these, eight are presacral and one sacral. The urostyle is post-sacral and corresponds to three or more modified vertebrae. The first vertebra is without transverse processes, the remaining presacral vertebrae have the transverse processes fairly large, while the sacral vertebra has them very large, forming in some genera widely expanded plates. The urostyle is a long cylindrical rod which articulates with the sacrum generally by two facets. Ankylosed to its anterior end are the remains of two neural arches.
In Anura remains of the notochord are found in the centre of each vertebra, i.e. vertebrally, while in the Urodela they only occur intervertebrally.
The vertebrae in Anura are, as a rule, procoelous. The eighth vertebra is however generally amphicoelous, while the ninth commonly has one convexity in front, and two behind.