38 



VERTEBRATE SKELETON 



phyllospondylous vertebrae (fig. 42, A) neural and haemal parts have 

 joined, but the haemals do not meet beneath the notochord, the result 

 being an imperfect centrum with no evident pleurocentra. In both 

 these types the bone apparently formed a thin layer on the outside 

 of cartilage which has left no trace. Stegocephals had long, well- 

 developed ribs, frequently bicipital and bearing uncinate processes, 

 like those of birds, which strengthened the skeletal basket. 



Urodela have the cartilage elements laid down early, both cranial 

 and caudal parts occurring, the former soon disappearing as discrete 

 parts (pleurocentra or intercalaria). The haemals are small in the 



Fig. 43. — Two stages in development of vertebrae of Amblystoma. cc, cartilage in 

 centre of vertebra; ei, elastica interna; i, incisure cutting ic, intercentral cartilage; n, 

 notochord; v, vertebra (bone) black. 



trunk. Ossification begins early around the cartilages and noto- 

 chordal sheath, much as in Teleosts, resulting in an elongate biconical 

 ring of bone (fig. 43). Between each two of these rings {i.e. inter- 

 vertebrally) the cartilage increases, extends inwards, constricting 

 and eventually obliterating the notochord here. From these points 

 cartilage gradually grows forwards and backwards, between bone and 

 sheath, to the middle of the centrum where the notochord itself may 

 chondrify. This intervertebral cartilage persists unmodified in 

 Perennibranchs where no true joint forms between the amphicoelous 

 vertebrae. In most Salamandrina the intervertebral cartilage is 



