20 

 ness of the animal. Half an hour after- the firal injection, 

 the brain was removed, cut into thin slices, and then exposed 

 to the air until the tint had become a brighter blue. In the 

 conversion of the unstable methylen-blue stain of the fresh 

 tissues into the insoluble form, I have not been successful 

 with the method recommended by &ethe i'd'z). The use of the 

 picrate of ammonia as a preliminary fixer has seemed to actual- 

 ly impair the clearness of the final preparation. I obtained 

 the best results with the solution given by Weyer ('96): 



Distilled water ----- lOO c.c. 



Affimonium molybdate - - - - - 10 granns 



Hydrochloric acid - - - - - 10 drops 



Heat the first two ingredients together, then add the acid. 



The pieces of brain were placed in this mixture, cooled 

 with ice, for four hours. They were then washed with iced 

 water for two hours. Dehydration with cooled alcohols, and 

 imbedding in paraffiri were hastened as much as practicable; 

 in fact, it is well to have the tissues in paraffin on the 

 same day when the injection was begun. The preparations were 

 used chiefly for the study of the architectural relations between 

 the neurones, and so the sections were cut quite thick. 



The results given by this method are characterised by 

 exceptioi.al clearness, due, in large measure, to the selective 

 coloration of certain neurones, only. The attainiient of the 

 desired end is far from constant, however. After experience 

 had shown the rule, care was always taken to apply this tech- 

 nique only to those animals which had been in an active condi- 



