354 TH E EVOLUTION OF MAN. 



the coalescence of the skeleton-plates of a pair of primitive 

 vertebrae, and which encloses within itself a part of the 

 chorda, consists originally of a somewhat soft cell-mass, which 

 afterwards passes into a second firmer, cartilaginous state, 

 and finally into a third, permanent, bony state. These three 

 different conditions are generally distinguishable in the 

 greater part of the skeleton of the higher Vertebrates ; at 



Pig. 113 — Third human neck -vertebra. 

 Fig. 114. — Sixth human chest -vertebra. 

 Fig. 115. — Second human lumbar-vertebra. 



first, most parts of the skeleton are quite tender, soft, and 

 membranous; then, in the course of development, they 

 become cartilaginous, and finally they ossify. 



All the bony vertebrae which afterwards compose the 

 backbone, or vertebral column, arise, as we have already 

 observed, entirely from the inner portion of the primitive 

 vertebrae, from the skeleton-plate. The outer portion, on 

 the other hand, which we have called the " muscle-plate ' 

 (Fig. 112, Tnp), produces the great mass of the dorsal 

 muscles (the dorsal " side muscles of the trunk "), as well as 

 the leather skin, which covers the flesh of the back. This 

 muscle-plate is in direct communication with that portion 

 of the side-plates which develops into the ventral skin and 

 the ventral muscles. 



