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. 



XlJ 





ti-^j^lal- 



Fig. 138. — Section of the neural tube, 29 s embryo. 



c. C, Central canal, cp. C, Epithelial cells, g. C, Ger- 

 minal cells. 1. m. ex., External limiting membrane. 1. m. in., 

 Internal limiting membrane. Ms'ch., Mesenchyme, m. v., 

 Marginal velum. 



The germinal cells are rounded cells situated next the central 

 canal between the inner ends of the epithelial cells; karyokinetic 

 figures are very common in them. AccorcUng to His the germina) 

 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. 



