202 METALLIC STAINS (IMPREGNATION METHODS). 



blackening. According to DUVAL (Precis, p. 230) they should be washed 

 for a few seconds only in 2 per cent, solution and then in distilled 

 water. 



GEROTA (Arch. Anat. Phys., Phys. Abth., 1897, p. 428) reduces in a 

 hydroquinone developing solution, followed by fixation in hyposulphite 

 of soda, just as in photography. 



356. Impregnation of Marine Animals. On account of the 

 chlorides that bathe the tissues of marine animals, these cannot be 

 treated directly with nitrate of silver. 



HERTWIG (Jen. Zeit., xiv, 1880, p. 322) recommends fixing them 

 with a weak solution of osmic acid, then washing with distilled water 

 until the wash-water gives no more than an insignificant precipitate 

 with silver nitrate, and then treating for six minutes with 1 per cent, 

 solution of silver nitrate. 



HARMER (Mitth. Zool. Stat. Neapel, v, 1884, p. 445) washes them 

 for some time (half an hour) in a 5 per cent, solution of nitrate of 

 potash in distilled water ; they may then be treated with silver 

 nitrate in the usual way. For some animals he recommends a 

 4-5 per cent, solution of sulphate of soda. 



357. Double-staining Silver-stained Tissues. The nuclei of tissues 

 impregnated with silver may be stained with the usual reagents, 

 provided that solutions containing free ammonia be avoided. These 

 stains will only succeed, however, with successful negative impregna- 

 tions, as nuclei that have been impregnated will not take the second 

 stain. 



Impregnation with silver may be followed by impregnation with 

 gold. In this case the gold generally substitutes itself for the silver 

 in the tissues, and though the results are sharp and precise, the effect 

 of a double stain is not produced. See hereon GEROTA, loc. cit., 

 355. 



358. Impregnation of Nerve Tissue. For this subject, which 

 includes the important bichromate-and-silver method of GOLGI, and the 

 neurofibril methods of BIELSCHOWSKY and RAMON Y CAJAL, see 

 Part II. These give important results, not only with Nervous tissue, 

 but with various forms of Connective tissue, mitochondrial formations, 

 etc. 



Gold. 



359. The Characters of Gold Impregnations. Gold chloride differs 

 from nitrate of silver in that it generally gives positive ( 348) im- 

 pregnations only. It generally gives negative images only with 

 such tissues as have first received a negative impregnation with 



