54 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



rotome, while those with the letters hp and pi are the corresponding 

 cranial elements of the succeeding body somite. The relations of 

 the parts in the phyllospondylous vertebra (fig. 47, A) are even 

 more uncertain, since here the bone (alone preserved) was apparently, 

 a coating over the cartilage. 



Fig. 48. — Trunk vertebrae of Rhynchobatus, after Dum6ril. h, haemal process; 

 t, intercalary plate; », neural process; r, rib; s, spinous process. 



In the formation of the arches derivatives of both caudal and 

 cranial half sclerotomes may persist (fiig. 48). The two may be dis- 



FiG. 49. — Earlier and later stages of development of a vertebra of Amblystoma. cc 

 cartilage in centre of vertebra; «, elastica interna; i, incisute cutting through ic, inter- 

 central cartilage; «, notochord; ns, notochordal sheath; v, vertebra (bone) black. 



tinguished by their relation to the nerve roots. Thus the ventral 

 root usually penetrates the caudal plate (in the figure fused with the 

 centrum), while the dorsal root passes between cranial and caudal 



