CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 41.1 



this coloration of the neurofibrils by mitochondrial dyes is marked 

 in adult cells (Cowdry '12 b, figs. 1, 2, 5, 7 and 13) should be taken 

 into consideration before regarding it to be indicative of transi- 

 tions between mitochondria and the primitive neurofibrils. 



Let us now consider the statement that the primitive neuro- 

 fibrils may be stained by both the mitochondrial and the neuro- 

 fibrillar methods (i.e., the second phase). The completeness of 

 the demonstration of mitochondria by the iron hematoxylin 

 method depends upon the presence in the fixative of chromic acid, 

 osmic acid and acetic acid, in suitable amounts, and on the mor- 

 danting action of iron alum; while their complete absence in the 

 neurofibrillar preparations is due to the unmodified action of sil- 

 ver nitrate. The neurofibrils seem to have a specific affinity for 

 silver nitrate, upon which all silver impregnation methods de- 

 pend. So that it is extremely unlikely, especially in the absence 

 of direct evidence, that so widely divergent methods stain the 

 same thing, namely, the primitive neurofibrils, for if the primi- 

 tive neurofibrils may be stained by both methods, they must of 

 necessity partake of the microchemical properties of both mito- 

 chondria and neurofibrils which are, to some extent, mutually 

 incompatible. 



Finally, the neurofibrils are said to enter on a third phase in 

 their history characterized by the loss of their affinities for mito- 

 chondrial dyes. I have.nevertheless failed to find an}- conclusive 

 evidence that the neurofibrils change their chemical composition 

 after their first formation. My failure may be due to the un- 

 standardized condition of the neurofibrillar methods of tech- 

 nique which still prevails. In any case the burden of suppljdng 

 the evidence rests with those who make the statement. 



If neurofibrils are formed by a chemical transformation of 

 mitochondrial substance into neurofibrillar material, one would 

 expect to find variations in the effects of fixation and in the 

 staining properties of mitochondria during the formation of 

 neurofibrils. I have shown that the exact converse obtains. Both 

 the solubilit}' of mitochondria in acetic acid and their staining 

 reactions in the cells of the neural tube in which neurofibrils are 

 being formed are remarkably uniform and constant. Alore- 



