248 CHAPTER XVIII. 



GEEOTA (Arch. f. Anat. u. Phys., Phys. AUh., 1897, 

 p. 428; Zeit. f. iviss. Mik., xv, 3, 1899, p. 348) greatly 

 recommends reduction in a hydroquinone developing solu- 

 tion, followed by fixation in hyposulphite of soda, just as in 

 photography. 



342. After-blackening. LEGEOS (Journ. de VAnat., 1868, 

 p. 275) washes his preparations after reduction in hyposulphite 

 of soda, which he says prevents after-blackening. Accord- 

 ing to DUVAL (Precis, p. 230) they should be Avashed for a 

 few seconds only in 2 per cent, solution, and then in dis- 

 tilled water. 



343. Silver Impregnation of Marine Animals. On account 

 of the considerable quantity of chlorides that bathe the tissues 

 of marine animals, these cannot be treated directly with 

 nitrate of silver. 



HEETWIG (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 treat- 

 ing for six minutes with 1 per cent, solution of silver 

 nitrate. 



HAKMEE (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. This 

 method gave good results Avith Loxosoma and Pedicellinu, 

 with Medusae, Hydroids, Sagitta and Appendicularia. 



HAEMEE thinks that for some 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. 



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

 includes the important bichromate-and-silver method of 

 GOLGI, see Part II. 



345. Double -staining Silver-stained Tissues. The nuclei of 

 tissues impregnated with silver may be stained with the 

 usual reagents, provided that solutions containing free am- 



