142 METALLIC STAINS (IMPREGNATION METHODS). 



has discovered that many marine animals will live for some 

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

 potash in distilled water. By washing them in this way they 

 may be freed from the greater part of their chlorides, and 

 may then be treated with silver nitrate in the usual way. This 

 method gave good results with Loxosoma and Pedicellina, with 

 Medusae, Hydroids, Sagitta and Appendicularia. 



VOSMAER has been able by this means to demonstrate the 

 epithelium of Chondrosia and Thenea, which Sollas was unable 

 to see ; and MEYER has obtained good results with Annelids 

 and ova of Teleostea. Few animals resist the action of nitrate 

 of potash so well as Loxosoma and Pedicellina, but die in the 

 solution in a few minutes. Their tissues, however, suffer but 

 little change, and give good impregnations. Harmer thinks 

 that for these animals other solutions having the same density 

 as sea- water might be substituted for the nitrate of potash, 

 and recommends a 4'5 per cent, solution of sulphate of soda. 



See also DEKHUYSEN'S method, ante, 213. 



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

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

 see Part II. 



219. 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, as this would dissolve out the silver. These stains 

 will only succeed, however, with successful negative impreg- 

 nations, 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. 



Gold. 



220. The Characters of Gold Impregnations. Gold chloride 

 differs from nitrate of silver in that it generally gives positive 

 ( 210) impregnations only. It only gives negative images, 

 so far as I know, when caused to act on tissues that have first 

 received a negative impregnation with silver, the gold sub- 

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



