THE HISTOGENESIS OF THE NERVOUS TISSUES 



301 



the nervous system. The differentiation of these tissues has been studied by 

 Hardesty in pig embryos (Amer. Jour. Anat., vol. 3, 1904). 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 sharp outlines and form a compact, 



Marginal layer 



Mantle layer 

 A 



terminal 

 cell 



Marginal layer Ependymal layer 

 Mesoderm Marginal layer 



Ependymal layer 



Germinal 

 cell 



Internal limiting membrane 



Ependymal layer 



Germinal 

 cell 



membrane 



Internal 



membrane 



Mesoderm Marginal layer * Mantle layer Ependymal layer 



Fig. 305. — Three stages in the early development of the neural tube showing the origin ot the syn- 

 cytial framework (after Hardesty). X 690. A, From rabbit before the closure of neural tube; B, 

 from 5 mm. pig after closure of tube; C, from a 7 mm. pig embryo; D, from a 10 mm. pig embryo. 

 *, Boundary between nuclear layer and marginal layer. 



cellular syncytium which is bounded, on its outer and inner surfaces, by an exter- 

 nal and internal limiting membrane (B). In a 10 mm. embryo the cellular strands 

 of the syncytium are arranged radially and nearly parallel (D). The nuclei are 

 now so grouped that there may be distinguished three layers: (1) an inner epen- 

 dymal zone with cells abutting on the internal limiting membrane, their processes 



