S1LVEU IMPREGNATION OF MARINE ANIMALS. 141 



of the reduced silver being dissolved ; hence the mixture must 

 be renewed from time to time. When all the silver has dis- 

 solved, a darker colour is permanently assumed. The nerve- 

 cells are left in this mixture for five to seven days. 



215. After-Blackening. LEGROS (Journ. de I'Anat., 1868, 

 p. 275) washes his preparations after reduction in hyposulphite 

 of soda, which prevents after-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. 



216. The Hoggans' Histological Rings are vulcanite rings 

 made in pairs, in which one ring just fits into the other, so as 

 to clip and stretch pieces of membrane between them. They 

 will be found described and figured in Journ. Roy. Mic. Soc., 

 ii, 1879, p. 357, and in ROBIN'S Journ. de VAnat., 1879, p. 54. 

 They may be obtained, in sets of various sizes (that of seven 

 eighths of an inch being the most convenient for 3x1 slides), 

 of Burge and Warren, 42, Kirby Street, Hatton Garden, 

 London, E.G., price ten shillings the dozen pairs. 



This useful little apparatus has lately been reinvented by 

 Eternod (Zeit. f. wiss. Mik., iv, 1, 1887, p. 39), and is made 

 according to his designs by Demaurex, Bandagiste, Fusterie, 

 Geneva (Switzerland). 



The Hoggans' histological rings were described by me in 

 the first edition of this work, p. 375, and in the Traite d. 

 Meth. techn., LEE et HENNEGUY, p. 138 ; so that I am in no 

 way responsible for the waste of time involved in this reinven- 

 tion on the part of the inventor, and the editors of the Zeit. 

 f. wiss. Mik. and other journals. 



217. 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 ni- 

 trate of silver. 



HERTWIG (Jen. Zeit., xiv, 1880, p. 324) recommends fixing 

 them with a weak solution of osmic acid, then washing with 

 distilled water until the wash-water gives no more than an in- 

 significant 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, pp. 44 to 56) 



