218 CHAPTER XVII. 



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. 



Haemee {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 fi-ee am- 

 monia be avoided. These stains will only succeed, however, 

 with successful negative impregnations, as nuclei that have 

 been impregnated will not take the second stain. 



Impregnation with silver may be followed by impregna- 

 tion 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 pro- 

 duced. See hereon Geeota, Ioc. cit., § 355. 



358. Impregnation of Nerve Tissue, — For this suhject, which 

 includes the important hichrcviate-and-silver method of Golgj, 

 and the neurojiliril methods of Bielschowskt and Eamon y 

 Cajal, see Fart II. These give impertant results, not only 

 ivith Nervous tissue, hut iciih various forms of Ccmiective 

 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) impregnations only. It generally gives negative 

 images onlj' with such tissues as have first received a 

 negative impregnation with silver, the gold substituting 

 itself for the silver. In order to obtain these images you 



