so 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



coelous) . They may bear a ball at one end and a corresponding 

 socket at the other; the socket may be in front (procoelous) or be- 

 hind (opisthocoelous). In the birds a saddle-shaped articulation 



Fig. 43. — Diagrammatic sagittal sections of (A) amphicoelous; (S), procoelus; (C), 

 opisthocoelous; and (£>), amphiplatyan vertebrae; the head supposed to be at the left. 



occurs (fig. 55), while in mammals the ends of the centra are usually 

 flat (amphiplatyan), though in the necks of ungulates the vertebrae 

 are jopisthocoelous. 



Fig. 44. — Section of developing vertebra of 45 mm. Amblystoma. c, cartilage of 

 intercentrum; cs^, outer chorda sheath; cs^, inner chorda sheath; dm, dura mater; e, 

 epithelioid layer of notochord (elastica interna); end, endorhachis, torn from wall of 

 vertebral canal; »/>,_neurapophysis (ossified); ns, neural spine of preceding vertebra; 

 nt, notochord; sc, spinal cord; sd, sub-dural space. 



Comparative anatomy, embryology and paleontology all agree in 

 showing that the vertebral formation began with the arches and ex- 

 tended from these to the centra. In what must be regarded as the 



