Strong, Review of the Golgi Method. 103 



' ' {a) Hardening of pieces in a solution of potassium bi- 

 chromate. 



' ' {b) Immersion of the hardened pieces in a solution of silver 

 nitrate. 



' ' {a) Hardening in bichromate. Although there are no es- 

 pecial rules for the hardening other than those which must usu- 

 ally be followed to obtain a good uniform hardening, yet it is 

 this part of the process which requires the most care. This is 

 the more so because the time necessary to harden the pieces to 

 the degree required for the action of the second reagent varies 

 very considerably according to different circumstances and espe- 

 cially according to the temperature. 



" For the first immersion of the pieces, I use either a sim- 

 ple two per cent, solution of potassium bichromate or the usual 

 formula of Miiller. (The reagents should be pure.) There 

 must be an abundant quantity of fluid in proportion to the quan- 

 tity of pieces to be hardened. 



"The part of the brain or spinal cord to be treated is cut 

 into tolerably small pieces (about i to i ^^ ccm.). It is important 

 that the pieces be fresh ; the fresher the pieces, the better the 

 results. It is well to use, preferably, the brains of animals just 

 killed, yet satisfactory results can also be obtained 24 to 48 hours 

 after death. It is hardly necessary to say that the pieces must 

 be cut regularly and in definite directions (according to the part 

 to be studied) so as to permit orientation as to the part and the 

 location of the elements in the future study. 



" That the hardening may proceed with some rapidity and 

 be uniform, it is well to successively increase the concentration 

 of the fluid, raising the quantity of bichromate from 2^ to 2^, 

 3, 4 and 5%. 



"Whether the fluid is increased in strength in hardening 

 the pieces, or remains the same strength, it is always necessary 

 to change it from time to time to avoid the formation of moulds 

 which, as is well known, develop abundantly in bichromate so- 

 lution when the pieces are to some extent neglected. For the 

 same reason it is advantageous to place in the vessels with the 

 pieces a small quantity of some substance which will prevent 



