THE NERVOUS SYSTEM 



the dorsal surface of the neural tube, the neural crest soon separates from it 

 and comes to lie in the angle between it and the myotomes. In this position 

 the neural crest gives rise to a series of sensory ganglia. From neuroblasts 

 located in these ganglia arise the sensory fibers of the cerebrospinal nerves. 



Marginal layer Mantle layer Ependymal layer 



' I 



.Germinal 

 cell 



Marginal layer Ependymal layer 

 Mesoderm Marginal layer 



\Germinal 

 cell 



S^p 



Internal limiting membrane 



Ependymal layer 



9 



^Germinal 



1 >' J cell 



External limiting membrane Mantle layer Internal limiting membrane 



External limiting membrane 



ST 1 



Germinal cell Internal limiting membrane 



Mesoderm Marginal layer 



Mantle layer 



Ependymal layer 



Fig. 19. Early stages in the differentiation of the neural tube: A, From a rabbit embryo 

 before closure of neural tube; B, from a 5 mm. pig embryo after closure of tube; C, from a 7 mm. 

 pig embryo; D, from a 10 mm. pig embryo. *, Boundary between nuclear and marginal layers. 

 (Hardesty, Prentiss-Arey.) 



This last statement requires some qualification. The fibers of the olfactory nerve 

 arise from cells in the olfactory mucous membrane. The fibers of the mesencephalic root 

 of the trigeminal nerve, which in all probability are sensory, arise from cells located within 

 the mesencephalon. The optic nerve is also an exception, but this is morphologically a 

 fiber tract of the brain and not a true nerve. An ingenious theory, advanced by Schulte and 

 Tilney (1915), attempts to bring this mesencephalic root and the optic nerve into more ob- 



