The Spinal Cord 295 



Hum about the spinal canal go to form the skeletal frame- work and 

 the nerve cells grow out into these deeper parts from the col- 

 umnar epithelium which lines the cavity. These nerve cells, 

 or neuroblasts, are at first spheroidal in form and show upon 

 their surface one to several prolongations, which are later to 

 constitute the polar elongations, or axis cylinders, of the nerves. 

 The spinal nerves develop first by the formation of the spinal 

 ganglia, which appear in the neural folds at a very early period, 

 and from the inner sides of which there grow out nerve fibers 

 into the spinal cord, to constitute the superior, or dorsal, roots of 

 the spinal nerves. They become the centripetal, or sensory, 

 roots of the nerves. The ventral, or motor, nerves arise very 

 early as small outgrowths from the lower part of the sides of the 

 spinal cord, in the position they occupy during adult life. These 

 ventral roots grow outward to meet the dorsal roots just beyond 

 the ganglia and fuse with them, after which they again divide 

 into dorsal and ventral twigs, which are mixed nerves. 



