3io 



HISTOGENESIS 



nected with it are differentiated the nervous tissues, the single exception being 

 the nerve cells and fibers of the olfactory epithelium. 



The Differentiation of the Neural Tube. — The cells of the neural tube dif- 

 ferentiate along two fines: There are formed: (i) nerve cells and fibers, in which 

 irritability and conductivity have become the predominant functions; (2) neu- 

 roglia cells and fibers which form the supporting or skeletal tissue peculiar to 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 



Neural groove Neural groove 



Neural plate 



Neural plate 



Ectoderm 



Neural groove. 



Ectoderm, 



~Neural tube 



^Neural cavity 



c J> 



Fig. 297. — Four sections showing the development of the neural tube in human embryos: A, of an 

 early embryo (Keibel); B, through head of a 2 mm. embryo (Graf Spee); C, neural tube of 2 mm. em- 

 bryo (Mall); D, neural tube of 2.69 mm. embryo (Kollmann). 



layered and finally three zones are differentiated (Fig. 298 A-D.) When the wall 

 becomes many layered the cells lose their sharp outlines and form a compact 

 cellular syncytium (Fig. 298 B) . On its outer and inner surfaces there is differ- 

 entiated from the cytoplasm an external and internal limiting membrane. In a 

 10 mm. embryo the cellular strands of the syncytium are radially arranged and 

 directed nearly parallel to each other (Fig. 298 D) . The nuclei are now so grouped 

 that there may be distinguished three layers: (1) an inner ependymal zone with 

 cells abutting on the internal limiting membrane, their processes extending 

 peripherally; (2) a middle mantle or nuclear zone, and (3) an outer or marginal 



