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. 



ff 



FIG. 138. Structure of the wall of the neural tube. Trans- 

 verse section through the region of the twenty-first somite 

 of a 29 s embryo. Drawn with Zeiss 2 mm. oil-immersion. 

 c. 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; karyokinetic 

 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. 



