SKELETON 



53 



of the arches penetrate the external elastic layer and migrate into 

 the sheath, the tissue thus formed being later converted into cartilage 



(fig. 44)- 



As is the case with the arches, there may be a duplicity of centra 

 in the vertebrates, a condition (diplospondyly) well known, for in- 

 stance, in the tail of Amia. In most vertebrates, however, the two 

 half centra fuse to form a single (true) centrum. This fusion may 

 take place in two ways. In some regions of a few fishes the cranial 

 and caudal half centra of the same (myotomic) somite unite so that 

 the resulting centrum coincides in position with the myotome. But 

 in the great majority of vertebrates the caudal half centrum of one 

 somite fuses with the cranial half of the succeeding somite to form 

 the definitive centrum of the adult. In this way there is brought 

 about an alternation of myotomes (and the resulting muscles) and 

 vertebrae, a condition more advantageous for the interaction of skele- 



FiG. 47. — Stegocephalan vertebrae, after Zittel and Woodward. A, phyllospondy- 

 lous; B, rachitomous of Chelydrosaurus; C,Callopterus; D,emho[omeTOus of Eurycormus, 

 hs, hypocentrum arcuale; hp, hypocentrum pleurale; np, neurapophysis; «j, neural spine; 

 h, hypocentrum; p, pleurocentrum. 



ton and muscles, for where vertebrae and muscles are coextensive, 

 flexure of the body to right or left cannot be so well effected. 



In the higher vertebrates all traces of this double origin of the 

 vertebrae are lost in the adult. But in many of the ichthyopsida 

 different conditions occur. In the tail of Amia referred to above and 

 in several extinct ganoids and stegocephals there are apparently two 

 centra and but a single arch for each muscle segment (fig. 47, D). 

 Of these the incomplete element is usually called the intercentrum. 

 In other stegocephals more complex conditions occur (fig. 47, B). 

 Here each vertebra consists of several parts to which the names in 

 the legend of the cut have been applied, and this type of vertebra has 

 been called rhachitomous or temnospondylous. No certainty has 

 been reached as to the homologies between these and other vertebrae, 

 but it appears probable that the parts labelled hs and «/> represent 

 the bases of the haemal and neural portions of the caudal half-scle- 



