CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 417 



4. Mitochondria are present in the early stages in the differ- 

 entiation of the nerve cells of the chick (embryos somites to 

 32) and there is ample evidence that they occur throughout cy- 

 tomorphosis. The neurofibrils, on the other hand, are only pres- 

 ent in later stages (embryos of 15 somites on) in evident adap- 

 tation to functional demands. Mitochondria may therefore be 

 regarded as cytoplasmic elements of a generalized nature, not 

 participating in so specialized a cell function as the develop- 

 ment of neurofibrils; while the neurofibrils are to be looked upon 

 as indicative of the differentiation of the cells in which they are 

 found (p. 403). 



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