44 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



specialized portion of a continuous axis, but it is not possible to carry 

 the comparison into details. The idea of Oken that the skull is a com- 

 plex of three or four vertebrae has long been overthrown. The skull 

 differs markedly from the vertebral column in the presence of numerous 

 membrane bones. 



VERTEBRAL COLUMN. 



The notochord (p. 12) is the foundation around which the verte- 

 bras and the posterior part of the skull are developed. It is a cylin- 



dm 



FIG. 37. Section of developing vertebra of 45 mm. Amblystoma. c, cartilage of inter 

 centrum; cs l , outer chorda sheath; cs 2 , inner chorda sheath; dm, dura mater; e, epithelioid 

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

 np, neurapophysis (ossified); ns, neural spine of preceding vertebra; nt, notochord; sc, 

 spinal cord sd, subdural space. 



drical rod of entodermal origin, without segmentation, 1 extending from 

 the infundibulum (see brain) to the posterior end of the body. Its cells 

 become vacuolated and at length most of the protoplasm, together' 

 with the nuclei, migrate to the surface of the cord, where they appear 

 like an epithelium, which, together with its basal membrane, is called 

 the internal elastic membrane (elastica interna, fig. 37, e}. 



1 Segmental undulations occur in the notochords of some mammals, but their significance 

 is not dear. 



