428 OHAPTEE XXXIV. 



hyde solation, and after twenty-four hours transfers to pure 

 3'5 per cent, bichromate for at least 2 days (retina), or 3 to 

 6 (central organs) . He finds that by this means precipitates 

 are almost entirely avoided. This I also find, but I seem to 

 get a too abundant impregnation of capillaries. 



Gerota {Intern. Monatuschr. Anat., xiii, 1896, p. 108) first 

 hardens (brain) for a week or two in 5 to 10 per cent, formol 

 solution, then puts small pieces for three to five days into 

 4 per cent, bichromate, then into the silver. 



Similarly Bolton {Lancet, 1898, p. 218; Journ. Roy.Mic. 

 8uc., 1898," p. 244). 



ScHEBiBEE {Anat. Anz., xiv, 1898, p. 275) obtained good 

 results (on appendages of Crustacea which were impervious 

 to the osmic mixture) with mixtures of five parts 2'5 per 

 cent, bichromate to one of 4 per cent, formaldehyde, or one 

 part 2'5 per cent, bichromate to two of 5 per cent, foi-malde- 

 hyde, the specimens remaining for one day in the first, for 

 two days in the second. 



Similarly Duboscq {Arch. %. Exper., 1899, p. 483), warm- 

 ins' the mixture to 40 C. 



VAN Gehuchten {in litt.), and other observers, have not 

 obtained good results with formaldehyde. 



Kallius {Encycl., p. 564) finds these mixtures good for 

 brain, but not so much so for other organs. 



818. Acetic Aldehyde. — Vassalb and Donaqgio (Moiiitore Zool., 

 Ital., vi, 1895, p. 82) harden pieces of at most 1 cm. in thickness for 

 fifteen to twenty days in a mixture of five parts of aldehyde with 100 of 

 3 to 4 per cent, bichromate, changing the fluid after a few days, as soon 

 as it has become dark. The rest as Golgi. 



819. Modifications of the Silver Impregation. — Ramon y 

 Cajal {Eev. trim. Hist., No. 2, 1888, note) found the addition of a very 

 little formic acid to the silver bath facilitated reduction. According to 

 VAN Gehuchten {La Cellule, vii, 1891, p. 83), 1 drop of the acid should 

 be added to 100 c.c. of the silver. But the practice is now generally 

 abandoned. 



Berkeley (Johns Hopkins Hasp. Rep., vi, 1897, p. 1 ; Journ. Boy. 

 Mic. Soc, 1898, p. 242) impregnates, after hardening in the osmio- 

 bichromate, in a freshly prepared solution of two drops of 10 per cent, 

 phosphomolybic acid to 60 c.c. of 1 per cent, silver nitrate, which in 

 winter should be kept at a temperature of about 26'-' C. 



Hill (op. cit, § 813) takes instead of silver nitrate a f per cent, solu- 

 tion of silver nitrite, with O'l per cent, of formic acid added. 



