842 



DEVELOPMENT. 



[CH. LVIII. 



Between the inner ends of the spongioblasts (fig. 660, S) 

 numerous rounded cells called germinal cells (G) next appear. 

 These rapidly divide, and so form neuroblasts (N). The neuro- 

 blasts are pear-shaped ; each has a large oval nucleus, and its 

 tapering stalk is directed towards the outer surface of the cord ; 



Fig. 660. Inner ends of tpongioblasts (8), -with germinal cells (G) between them. N N, 

 neuroblasts which have resulted from, the division of a germinal cell ; M, rayelo- 

 spongium formed by the branching outer ends of the spongioblasts. (After His.) 



the process ultimately pierces the basement membrane (fig. 66 1). 

 These are the primitive nerve cells ; their processes are the axis 

 cylinder processes which grow out as nerve fibres. The nerve 



Fig. 661. Three neuroblasts, each with a nerve fibre process, growing out beyond the 

 basement membrane of the embryonic spinal cord. (After His.) 



fibres are first without sheaths ; the formation of the sheaths 

 comes later (see pp. 105, 609). 



The neuroblasts collect into groups, one of which, especially 

 large, is at the situation of the future anterior horn ; the pro- 

 cesses of the primitive nerve-cells pass out of the cord as the 

 beginnings of the anterior roots (fig. 662). The somewhat oblique 

 coursing of these fibres before they leave the cord forms the 

 beginning of the anterior white column. The posterior white 

 columns simultaneously begin to appear on each side of the 

 narrow dorsal part of the canal. They are formed by the pos- 

 terior roots entering the cord. 



As the cornua of grey matter grow out from the central mass, 



