STRUCTURE OF FOUR-DAY CHICKS 



119 



the cord become greatly thickened in contrast with the dorsal 

 and ventral walls which remain thin. In this process the lumen 

 (central canal) becomes compressed laterally until it appears in 

 cross section as little more than a vertical slit. The thin dorsal 

 wall of the tube is known as the roof plate; the thin ventral 

 wall as the floor plate; and the thickened side walls as the 

 lateral plates. 



The Spinal Nerve Roots.— During the fourth day the estab- 

 lishment of the spinal nerve roots has begun. The growth of 

 nerve fibers from the neuroblasts can only be traced with the aid 

 of special methods of staining. The more general steps in the 



spinal cord 



neuron of 

 ventral root 

 (motor) 



mesenchyme 



sympathetic 

 ganglion cells 



Pig. 44. — Drawing to show the structure and relations of a spinal ganglion 

 id the roots of a spinal nerve. The left half of the drawing represents struc- 

 ^tures as they appear after treatment by the usual nuclear staining method. The 

 right half of the section shows schematically the nerve cells and the fibers grow- 

 ing out from them as they may be demonstrated by the Golgi method. {Nerve 

 cells and fibers after Ramon y Cajdl.) 



levelopment of the roots of the spinal nerves can, however, be 



followed in sections prepared by the ordinary methods. 



In the adult each spinal nerve is connected with the cord by 



TO roots, a dorsal root which is sensory in function and a ven- 



:al root, which is motor in function. Lateral to the cord the 



'"dorsal and ventral roots unite. The spinal ganglion (dorsal 



root ganglion) is located on the dorsal root between the spinal 



cord and the point where dorsal and ventral roots unite. Distal 



to the union of dorsal and ventral roots is a branch, the ramus 



