3 oo 



HISTOGENESIS 



entiated the nervous tissues, with the single exception of the nerve cells 

 and fibers of the olfactory epithelium. 



Differentiation of the Neural Tube. The cells of the neural tube 

 differentiate into two products. There are formed: (i) nerve cells and 

 fibers, in which irritability and conductivity have become the predominant 



Ependymal layer 



Marginal layer Ependymal layer 

 Mesoderm Marginal layer 



Internal limiting membrane 



Ependymal layer 



External limiting membrane 

 External limiting membrane 



Internal limiting membrane 



Germinal cell 

 1 



Internal limiting membrane 



Mesoderm Marginal layer * Mantle layer Ependymal layer 



FIG. 305. Three stages in the differentiation of the neural tube (after Hardesty). X 690. 

 A, From a rabbit embryo before the closure of the neural tube; B, from a 5 mm. pig embryo 

 after closure; C, from a 7 mm. pig embryo; D, from a 10 mm. pig embryo. *, Boundary be- 

 tween mantle and marginal layers. 



functions; (2) neuroglia cells and fibers, which constitute the supporting, 

 or skeletal tissue, peculiar to the nervous system. The differentiation 

 of these tissues has been studied by Hardesty (1904) in pig embryos. 

 The wall of the neural tube, consisting at first of a single layer of columnar 

 cells, becomes many-layered, and, finally, three zones are differentiated 

 (Fig. 305 A-D). As the wall becomes many-layered the cells lose their 



