THE NEURON 



95 



Axon 

 Collateral 



cells of this ])late, a neural gi'oove is formed and deepens until the 



thickened folds fuse dorsally to form a neural tube which lies l)elovv 



and sei)arate from the ectoderm. The cells of this neural tube ^i\'e 



rise to the major i)art of the ner^•e tissue. The anterior portion of 



the tube forms the brain anrl the 



posterior portion the s])inal cord. 



Between the nein*al tube and the 



ectoderm a longitudinal band of 



cells a])])ears on each side and 



forms the neural crest. From 



these cells are derived the s])inal 



and cranial ganglia and indirectly 



the sym])athetic ganglia. The 



cells of the neural tube undergo a 



series of divisions and finally 



differentiate into the nerve cells 



or neurons and the neuroglia 



cells which form the supporting 



tissue of the nervous system. 



THE NEURON. 



A study of nerve tissue involves 

 an understanding of the neuron, 

 or nerve cell. A description of 

 it includes a consideration of 

 the cyton, which consists of the 

 nucleus and surrounding portion 

 of cytoplasm, and the cytoplas- 

 mic processes which extend out 

 from the cyton. (Fig. 51.) 



The Cyton.— The size of this 

 portion of the neuron \'aries from 

 a few to several himdred microns 

 in diameter. The limiting sur- 

 face is a denser cytoplasm but 

 not a distinct cell membrane, as 

 found in other types of cells. 

 There is considerable variation in shape, some cell bodies are 

 spherical, others oval, pyriform, fusiform, or even stellate. Such 

 variations are largely due to the nmnber and location of the processes 

 which extend out from the cvtons. 



Telodendrion 



Fig. 51. — Diagram of a neuron. 



