476 



THE NEKVOUS SYSTEM 



The development of the posterior nerve root ganglia forms a 

 striking example of the history of the neuroblast, as above de- 

 scribed. The manner in which these cells are derived from cells 

 lying in the wall of the neural canal can be readily appreciated by 

 examining successively Figures 374 to 376. After their develop- 

 ment has been completed each of these neurones comprise a nerve 

 cell situated in the posterior nerve root ganglion of a spinal nerve, 

 from which one branch of its T-shaped process passes through the 

 spinal nerve toward the periphery, while the other branch, through 

 the posterior root, enters the spinal cord to terminate in its cen- 

 tral grey matter either at or near the level at which it enters, or 

 possibly at a much higher and 

 more remote level. 



FIG. 374. DIAGRAM OF A TRANSECTION 

 OF THE SPINAL CORD OF AN EARLY 

 EMBRYO, SHOWING THE MIGRATION 

 OF NEUROBLASTS TOWARD THE MAR- 

 GINAL VEIL AND THE DORSAL NERVE 



ROOT. 



a, neural canal ; 6, dorsal root. 

 (After His.) 



FIG. 375. TRANSECTION OF THE SPINAL CORD 

 OF A HUMAN EMBRYO OF FOUR WEEKS. 



The central canal is immediately sur- 

 rounded by ependyma cells. The peripheral 

 nerve cells are shown on the left of the figure. 

 The nerve roots are already pushing out- 

 ward from the primitive cord, d, dorsal ; t>, 

 ventral nerve roots. (After His.) 



Myelinization. Having attained its full area of distribution, 

 the last change in the neurone to mark the completion of its 

 development is the appearance of its myelin or medullary sheath. 

 The period at which this sheath is obtained varies with the differ- 

 ent tracts of fibres in the nervous system, and seems to be syn- 



