418 CHAPTER XXXII. 



1896, p. 1). He thinks the stain depends on the formation of a " reduced 

 salt (subsalt) of silver," not of a silver chromate, and that the reduction 

 takes place not in the nervous fibrils, but in the liquid or semi-liquid 

 " neuroplasm " with which they are bathed. He finds the impregnated 

 material will stand imbedding in celloidin for many days. For impregna- 

 tion he recommends instead of silver nitrate a f per cent, solution of silver 

 nitrite with O'l per cent, of formic acid added. Other details loc. cit. 



AZOULAY (Comptes Rend. Soc. Biol. [10], i, 1894, p. 839) has followed 

 the process under the microscope, and holds that it is due to a crystallisa- 

 tion of chromate of silver in the tissues. 



Modifications concerning the Impregnation of the Tissues. 



748. RAMON Y CAJAL, who has done a great deal of im- 

 portant work by GOLGI'S method, has always used the rapid 

 process. For the times and strengths used by him in his 

 researches on the cerebral cortex of mammals see his paper 

 in La Cellule, vii, 1891, p. 125, or Zeit. f. iviss. Mik., ix, 2, 

 1892, p. 239; also Journ. Hoy. Mic. Soc., 1892, p. 154. He 

 found it useful to adopt SEHRWALD'S gelatin process ( 757) 

 for avoidance of peripheral precipitates. He prefers not to 

 adopt GKEP PIN'S treatment with hydrobromic acid, nor 

 OBREGIA'S treatment with gold chloride, finding that, although 

 they serve to render the preparations permanent, they obscure 

 the finer relations of fibres. 



For embryos of the fowl he employs the same process ; 

 see his paper in Anat. Anz., v, 1890, 85, or Zeit. f. wiss. J/YA-., 

 vii, 2, 1890, p. 235. 



749. RAMON Y CAJAI/S Double-Impregnation Process. In a 

 paper on the structure and relations of the sympathetic- gan- 

 glia (which I have not seen, and quote from Z<"if. /'. /r/.v.s-. 

 Mik., loc. cit.) RAMON Y CAJAL describes a process of " intensi- 

 fied " or " double impregnation." After hardening for three 

 days (embryos of fowl) in the osmium-bichromate mixture, 

 the preparations are put for thirty- six hours into nitrate of 

 silver solution (0'5 to 0'75 per cent.). They are then brought 

 back into the same osmium-bichromate mixture, or into a 

 weaker one containing only two parts of osmic acid solution 

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

 washed quickly with distilled water, and put for a second 



