234 THE DEVELOPMENT OF THE CHICK 
cells the nuclei are at different levels; thus in a section several 
layers of nuclei appear. These cells are not closely packed 
together, except at their outer ends, but are more or less separated 
by intercellular spaces that form a communicating system of 
narrow channels. 
PS Fe) 
~ 
ins ch. 
(rs ows! 
I'ig. 138. —Structure of the wall of the neural tube. Trans- 
verse section through the region of the twenty-first somite 
of a29sembryo. Drawn with Zeiss 2 mm. oil-immersion. 
e.C., Central canal. ep. C., Epithelial cells. g. C., Ger- 
minal cells. Ms’ch., Mesenchyme. 
The germinal cells are rounded cells situated next the central 
canal between the inner ends of the epithelial cells; karyokinetie 
figures are very common in them. According to His the germinal 
cells are the parent cells of the neuroblasts alone; it is probable, 
however, that they are not so limited in function, and that they 
represent primitive cells from which proceed other epithelial 
cells and embryonic neuroglia cells as well as neuroblasts. 
