158 ' NERVOUS TISSUES 



sheath being supplied probably by joint influence of the new axis cylinder 

 and the neurolemma cells. 



Mitochondria of granular and rod forms are abundant in the neuro- 

 blasts. Meves, Duesberg and others have claimed that these differentiate 

 into neurofibrils. The recent work of Cowdry (Amer. Jour. Anat., 15, 4, 

 1914) on chick embryos proves the untenability of this view. Cowdry 

 shows that the neurofibrils arise as a differentiation of the ground substance 

 of the neuroblast ; and that mitochondria persist in undiminished numbers 

 throughout the period of neurofibril-development. Moreover, it is now 

 known that mitochondria are present also in adult neurons. They are ap- 

 parently fundamental cytoplasmic constituents of a metabolically active 

 cell. 



Spinal ganglion cells of certain adult mammals (cat and rabbit) have 

 been kept alive in tissue cultures for as long as twenty days (Minea, Anat. 

 Anz., 46, 20, 1914). The cells remain apparently normal, augment their 

 amount of chromophilic substance, produce new neurofibrils, develop short 

 processes with end-plates (neuropodia) and become fenestrated, but do not 

 proliferate. 



