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 
Fic. 38. Fic. 39. 
Fic. 38.—Two caudal vertebre of alligator. c¢, centrum; ha, hemapophysis; hs, 
hemal spine; na, neurapophysis; ns, neural spine; poz, prz, post- and prezygapophyses; 
t, transverse process. The arrow passes through the neural arch. 
Fic. 39.—Diagrams of (A and B) fish vertebre and (C) vertebra from higher groups. 
b, basal stumps; ¢, capitular head of rib; ct, centrum; d, diapophysis; /7, fish rib; ha, hemal 
arch; ma, neural arch; ~, parapophysis; 7, rib; ¢, 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 
