224 bulletin: museum of comparative zoology. 



2. Silver nitrate Avas treated with a 20% solution of formalin, at 

 room temperature. There was no evidence of chemical change. 

 When heat was applied there was a deposit of metallic silver. 



3. Ammoniacal silver oxide solution, when treated with pyrogallic 

 acid at room temperature, yielded a heavy precipitate of finely divided 

 silver. 



4. Ammoniacal silver oxide solution, when treated with a solution 

 of formaldehyde, immediately yielded a copious, heavy, somewhat 

 spongy, precipitate of metallic silver, gray to black in color. 



On the basis of these tests then, it appears, that there are good 

 chemical reasons for expecting Bielschowsky's fluid to serve better 

 than silver nitrate solution as a means of impregnating tissues with 

 metallic silver, since it is much more readily reduced to the metallic 

 state than is silver nitrate, whether the reducing agent be formaldehyde, 

 or the developer used by photographers, and this at ordinary room 

 temperature. The undesirability of leaving tissues for several days 

 in the warm, non-preservative fluids required by Ramon y Cajal's 

 method is obvious. While all of the reagents used in developing 

 photographic plates are strong reducing agents, formaldehyde, is 

 sufficiently active for the purpose, and possesses the advantage that 

 while it is acting as a reducing agent it is also acting as a preservative. 



The merit of any method, however, is determined by the results 

 which it yields rather than by theoretical considerations. The Ramon 

 y Cajal process has many warm advocates. I tried it without success 

 in this investigation, but have since secured excellent impregnation of 

 nerve terminations by its use. 



My experience has been that success in impregnating the nerve 

 terminals of the ear requires the use of a stronger solution of silver 

 oxide and a longer treatment than is necessary for central organs. 

 In order to arrive at once at a knowledge of the optimum conditions 

 as to concentration and duration of treatment, I ran through a multiple 

 series of ear sacs, varying the concentration of the Bielschowsky's 

 fluid and the duration of treatment by regular intervals. The most 

 satisfactory preparations proved to be those made from material which 

 had remained for 30 to 45 minutes in a fluid prepared by adding to 

 20 cc. of 2% silver nitrate solution 5 drops of a 40% solution of sodium 

 hydroxide, and dissolving the precipitate of silver oxide thus formed 

 in the smallest possible amount of ammonium hydroxide. 



Serial sections have been cut, in the usual way. The most advan- 

 tageous thickness has been found to be 10 mikra. Sections of that 

 thickness are sufficiently transparent, and permit one to study a larger 



