328 Prof. J. S. Macdonald. < [Ape 
the dye in different parts of the nerve-fibre, I proceeded to study more 
minutely the manner in which the dye is actually thrown down from its © 
solution in water by the addition of varied quantities of potassium salts— 
examining the precipitates under the same powers of the microscope as had 
been used in the study of the nerve-fibre. Precipitation I found to be largely 
a matter of time. Using a 0:05-per-cent. solution of the dye, I found that the 
addition of even 1 per cent. of potassium chloride precipitates a considerable 
portion of the dye in 24 hours. Immediate precipitates such as I had 
obtained in the nerve were only to be obtained by the addition of much 
larger quantities of the salt, beginning at about 5 per cent. Such precipitates, 
however, no matter how rapidly formed, betrayed a crystalline structure 
when examined with the microscope. The rapidity of precipitation is greatly 
increased by the addition of dextrose to the solutions of the dye, but here 
again the precipitate is crystalline. It seemed possible that the granular 
character of the precipitate within the nerve might be the result of the 
viscosity of the solution present there, and this made me try precipitation 
experiments in strong solutions of gelatine. Even here, however, I was not 
successful, although there were certain suggestive alterations in the grouping 
-of minute crystals to be observed. On the other hand, I was able to obtain 
spherical granular precipitates of the dye in a manner which only seemed to 
confuse the issue. Thus, bile-salts precipitate the dye in just the form 
required. So also do solutions of Witte’s peptone. The addition of blood- 
serum to solutions of the dye resulted in the precipitation of yellow granules 
exactly resembling the granules seen in the nerve-fibre, when the amount of 
serum added was such as to ensure a precipitation of globulins. Iwas, there- 
fore, placed in a new perplexity as to the meaning of the granules observed in 
nerve-fibre, when, fortunately for my conclusions, Macallum’s paper was 
published. The drawings accompanying this paper at once convinced me of 
the fact that the differential distribution of the hypothetical salt solution 
deduced from the neutral-red staining did indeed represent an actual 
differential distribution of potassium salts. The drawings given by Macallum 
correspond exactly with the appearance seen by me, with the exception 
of some slight changes produced by the acid contained in his reagent upon 
the myelin sheaths. Axis-cylinder for axis-cylinder, the appearances 
represented by Macallum and those seen by me are the same. It is 
unnecessary to say that his method is far superior to mine, that will be 
gathered at once from the ambiguities related above. Neutral-red has, how- 
ever, this advantage over the definite precipitant, that the effects produced 
can be watched much more readily in the course of their development. 
I have obtained Macallum’s reagent from the source from which he obtained 
