MODIFICATIONS OF GOLGl's METHOD. 377 



three days (embryos of fowl) in the osmium-bichromate mix- 

 ture, the preparations are put for thirty-six hours into nitrate 

 of silver solution (0*5 to O75 per cent.). They are then 

 brought back into the same osmium-bichromate mixture, or 

 into a weaker one containing only 2 parts of osmic acid solu- 

 tion to 20 of the bichromate. After treatment with this they 

 are washed quickly with distilled water, and put for a seconcf 

 time into the silver solution for thirty-six to forty-eight hours. 

 It is important to hit off the proper duration of the first 

 impregnation in the bichromate. If it has been too long 

 (four days) or too short (one day), the second impregnation 

 will not succeed. In this case a third impregnation must be 

 resorted to, the objects being again treated with the weak 

 osmium-bichromate mixture, and afterwards again with the 

 silver solution. 



For retina the process is given as follows : Put the retina 

 (batrachians and reptiles) with the sclerotic for twenty-four 

 hours into the weak osmium-bichromate mixture. Then put 

 it into 1 per cent, silver solution for twenty-four hours, then 

 back into the osmium mixture for twenty-four hours, then for 

 twenty-four hours again into the silver-bath. 



716. OYARZUN (Arch.f. mik. Anat., xxxv, 1890, p. 380) used 

 the first-given process of Ramon y Cajal in his researches on 

 the fore-brain of Amphibia. He gives the times as twenty- 

 four hours in the osmium mixture, and the same in the silver- 

 bath. This is for frogs. For Triton and Salamandra twenty 

 hours is enough. Thirty to forty hours is too much. 



717. VAN G-EHUCHTEN (La Cellule, vii, 1891, p. 81 ; viii, 2, 

 p. 225; see Journ. Roy. Mic. Soc., 1892, p. 153, and Zeit. f. 

 wiss. Mik., ix, 2, 1892, p. 237), who has done some very im- 

 portant work on nerve-centres with Grolgi's process, also 

 follows the first-given modification of Ramon y Cajal, with 

 this exception, that he employs formic acid to assist the action 

 of the silver-bath (as suggested by Ramon y Cajal in an early 

 paper). He treats the tissues (spinal cord and cerebellum in 

 small pieces) with the osmium mixture for two to two and a 

 half days at the ordinary temperature (or for less time at a 

 temperature of 35 to 40 C.) . They are then brought into 

 the silver-bath, to which is added a little formic acid (1 to 2 

 c.c. to a litre, or 1 drop to 100 c.c. will do; care must be taken 



