THE NERVOUS SYSTEM 



237 



the dorsal root and of the sympathetic system. The first kind 

 only is considered here, and they are usually called the gan- 

 glionic neuroblasts s.s., because they alone remain in the spinal 

 ganglia. Like the medullary neuroblasts these neuroblasts form 

 outgrowths that become axis cylinder processes; but they differ 

 from the latter in that each ganglionic neuroblast forms two 

 axones, one from each end of the spindle-shaped cells, which are 

 arranged with their long axis parallel to the long axis of the 

 ganglion (Fig. 139). Thus we may distinguish a central process 

 and a peripheral process from each neuroblast, the former grow- 

 ing towards and the latter away from the neural tube (Fig. 139). 

 In other words each ganglionic neuroblast is bipolar, as contrasted 

 w^ith the unipolar medullary neuroblasts. The central axone 

 enters the dorsal zone of the neural tube, and the peripheral one 

 grows out into the surrounding mesenchyme. 



Fig. 141. — Transverse section through the spinal 

 cord of a 9-day chick, prepared by the method 

 of Golgi. (After Ramon y Cajal.) 

 Col., Collaterals, d. R., Dorsal root. G., Gray 



matter. Gn., Ganglion. Nbl. 4, Neuroblast of the 



ventral horn (motor), v. R., Ventral root. W., 



White matter. 



In the course of the later development the cell-body moves 

 to one side so that the central and peripheral branches appear 

 nearly continuous (Fig. 141). Farther shifting of the cell-body 

 produces the characteristic form of the ganglionic nerve-cell with 

 rounded body provided with stem from which the central and 

 peripheral branches pass off in opposite directions. The central 

 process enters the marginal velum near its dorsal boundary and 



