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 

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

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

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

 and a peripheral process from each neuroblast (Fig. 139) ; the 

 former corresponds to the axone and the latter to the dendrites 

 of the medullary neuroblast. The central axone enters the dorsal 

 zone of the neural tube, and the peripheral process 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 



