240 



THE DEVELOPMENT OF THE CHICK 



cannot be regarded as proved. Similarl}^ intermediate stages 

 between epithelial and tnie glia cells are apparently shown in 

 Fig. 143. However, there can be but little doubt that the prin- 

 cipal source of the neuroblasts of the mantle layer is the germinal 

 cells, that migrate outwards between the bodies of the epithelial 

 cells. The germinal cells continue in active division up to at 

 least the eleventh day, and their activity seems sufficient to 

 provide for all the cellular elements of the mantle layer, whereas 

 the epithelial cells apparently do not divide at all. Moreover, 

 mitoses are not infrequent in some cells of the mantle layer itself. 



Fig. 143. — Transverse section of the cord of a 

 nine-day chick to show neuroglia and ependymal 

 cells ; prepared by the method of Golgi. (After 

 Ramon y Cajal.) 

 D., Dorsal. Ep., Ependymal cells. N'gl., Neu- 

 roglia cells, v., Ventral. 



The form of the gray matter in the cord in successive 

 stages is shown in Figs. 144, 145, and 146, representing sections 

 of the cord at five, eight, and twelve days. It will be seen that 

 the gray matter gains very rapidly in importance between the 

 fifth and the eighth days. 



Attention should be directed to a group of neuroblasts situated at 

 the external margin of the white matter just above the ventral roots. 

 This is known as Hoffmann's nucleus; it extends the entire length of the 

 cord (Fig. 146, twelve days). 



The white matter of the cord gains in importance at an equal 

 rate (Figs. 144, 145, 146). Its production is due to ascending 



