vV 



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CHAPTER VIII 

 THE NERVOUS SYSTEM 



I. The Neuroblasts 



The account given in Chapters V and VI outlines the origin 

 of the larger divisions of the central nervous system and ganglia. 

 The subsequent growth and differentiation is due to multiplica- 

 tion of cells, aggregation of embryonic nerve-cells, or neuro- 

 blasts, in particular regions or centers, the formation and growth 

 of nerve-fibers which combine to form nerves and tracts, and 

 the origin and differentiation of nerve-sheaths, and the support- 

 ing cells, neuroglia, of the central system. The most important 

 factors are the origin of the neuroblasts and of nerve-fibers in 

 connection with them; these fibers form the various nerve-tracts 

 and commissures within the central nervous system and the 

 system of peripheral nerves. The origin of neuroblasts and the 

 development of fibers is the clue to differentiation in all parts 

 of the nervous system. 



Neuroblasts are found in two primary locations in the embryo; 

 (1) in the neural tube, and (2) in the series of ganglia derived 

 from the neural crest; these are known as medullary and gang- 

 lionic neuroblasts respectively.^ 



The Medullary Neuroblasts. In the neural tube of the chick, 

 up to about the third day, there are present only two kinds of 

 ceils, the epithelial cells and the germinal cells (Fig. 138). 



The epithelial cells constitute the main bulk of the walls, 

 and extend from the central canal to the exterior; their inner 

 ends unite to form an internal limiting membrane lining the 

 central canal, and their outer ends to form an external limiting 

 membrane. Each cell in the lateral walls of the tube is much 

 elongated and usually shows three enlargements, viz., at each 

 end and in the region of the nucleus, the cell being somewhat 

 constricted between the nucleus and each end. In different 



1 Neuroblasts arise also in the olfactory epithelium. (See Chap. IX.) 



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