SKELETON. 



45 



Next, mesenchymatous cells, derived from the sclerotomes, form 

 a notochordal sheath, bounded externally by an elastica externa. 

 The mode of formation and the history of the sheath vary in different 

 groups, for accounts of which reference must be made to special 

 papers. Other skeletogenous tissue extends outward from the sheath 

 toward the periphery, as described on a previous page (p. 38, fig. 30) 

 from which the ribs of all vertebrates are developed, the cyclostomes 

 passing but little beyond this membranous condition in the trunk 

 region. 



With the appearance of cartilage segmentation is introduced into 

 the skeleton. As cartilage is firm and comparatively unyielding, in 



C 



na 



FIG. 38. FIG. 39. 



FIG. 38. Two caudal vertebrae of alligator, c, centrum; ha, haemapophysis; hs, 

 haemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; 

 t, transverse process. The arrow passes through the neural arch. 



FIG. 39. Diagrams of (A and B) fish vertebrae and (C) vertebra from higher groups. 

 b, basal stumps; c, capitular head of rib; ct, centrum; d, diapophysis;/r, fish rib; ha, haemal 

 arch; na, neural arch; p, parapophysis; r, rib; t, tubercular head. 



order that the trunk may bend, the cartilage becomes divided into 

 separate blocks, which, in order that they may be moved by the muscles 

 connected with them, must alternate with the myotomes. Hence the 

 metamerism of the vertebral column is the result of that of the muscular 

 system. 



A typical vertebra, whether of cartilage or bone, consists of several 

 parts, the names of which are necessary for the understanding of the 

 following account. Surrounding the notochord is the body or centrum, 

 developed from the notochordal sheath or from tissue surrounding it. 

 A neural arch, enclosing the spinal cord, extends dorsally from the 

 centrum. It consists of a plate on either side (neurapophysis), the 



