354 THE EVOLUTION OP 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 



FIG. 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, mp), 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. 



