240 THE DEVELOPMENT OF THE CHICK 
cannot be regarded as proved. Similarly intermediate stages 
between epithelial and true gla cells are apparently shown in 
Fig. 148. 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. 148. — 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 
