CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 407 



c. Staining reactions. No variations or change could be 

 detected in the results of fixation or in the staining reactions of 

 mitochondria coincident with the development of neurofibrils. 

 Their degree of solubility in acetic acid is apparently constant, 

 and throughout the series of preparations (which of course is 

 much more complete than it was possible to represent on the plates) 

 they appear a dark blue black color, which is the same in figure 

 15 a, at a stage when no neurofibrils are differentiated, as it is 

 in figure 20 a after a large nunber of neurofibrils have been 

 formed. Moreover, the staining reactions of mitochondria in the 

 neural tube of this series of preparations, running parallel as the}' 

 do to the differentiation of neurofibrils, do not differ, in smy ap- 

 preciable wa}^ from those of mitochondria in other neighboring 

 tissues of the same embryos. They are Hkewise similar to those 

 of mitochondria occurring in the neural tube before the differen- 

 tiation of any neurofibrils. The only variation in the staining 

 reactions of the neural tube in this series seems to be in the case 

 of the background and to be dependent upon the length of dif- 

 ferentiation in iron alum and the duration of the subsequent 

 washing in tap water. 



It is very difficult for me to determine whether a change in 

 the reaction of neurofibrils to silver nitrate during the course of 

 development actually exists in my preparations. The varia- 

 tions in color which I have observed may be due to a host of 

 possible factors among which the duration and temperature of 

 impregnation, the presence or absence of light, and so forth, may 

 be mentioned. I have absolutely no reliable evidence, therefore, 

 of any change in the chemical composition of neurofibrils from 

 the time that they are first laid down to the most highly special- 

 ized embryos in my series. 



d. Relative amount of mitochondria and neurofibrils.^ As I ha\'e 

 alreadj^ stated, the figures on plates 3 and 4 constitute the essential 

 features of a comparison of the results yielded by Meves' iron 

 hematoxylin method and by Cajal's silver nitrate technique 



' The relatively smaller size of the nuclei in figures 15 a to 20 a is indicative 

 of the fact, already mentioned (p. 399), that the shrinkage is greater in the mito- 

 chondrial preparations than it is in the neurofibrillar ones. 



