252 CHAPTER XVIII. 



or protoplasmic stain, sometimes an extra- cellular impregna- 

 tion similar to that of nitrate of silver. And the preparations 

 thus obtained are anything but permanent. 



351. LOWIT'S Method (Sitzgsber. Akad. Wien, Bd. Ixxi, 

 Abth. 3, 1875, p. 1). The principle of this process is that, 

 in order to facilitate the penetration of the gold and its sub- 

 sequent reduction in the tissues, the tissues are made to 

 swell up by treatment with formic acid before being brought 

 into the gold-bath, and formic acid is employed to assist the 

 reduction after impregnation. 



The following directions, which may serve as a type, are 

 ta.ken from FISCHER'S paper on the corpuscles of Meissner 

 (Arch.f. mik. Anat.,xii, 1875, p. 366). 



Small pieces of fresh skin are put into dilute formic acid 

 (one volume of water to one of the acid of 1*12 sp. gr.),aiid 

 remain there until the epidermis peels off. They then are 

 put for fifteen minutes into gold chloride solution (1 per 

 cent, to 1 per cent.), then for twenty-four hours into dilute 

 formic acid (1 part of the acid to 1 3 of water), and then 

 for twenty-four hours into undiluted formic acid. (Both of 

 these stages are gone through in the dark.) Thin sections 

 are then made and mounted in dammar or glycerin. Suc- 

 cessful preparations show the nerves alone stained, but it is 

 not possible always to control the results. 



352. RANviuii's Formic Acid Method (Quart, Journ. Hie. Sci. 

 [N. S.J, Ixxx, 1880, p. 456). The method of LOWIT has been 

 modified by many workers by omitting the final treatment 

 with undiluted formic acid, and also in some other details. 

 RAN vi KB proceeds as follows : Reflecting that the action of 

 the one third formic acid in which LOWIT placed his tissues 

 must be hurtful to the final ramifications of the nerves, he 

 combines the formic acid with a fixing agent designed to 

 antagonise its altering action, and takes for this purpose the 

 chloride of gold itself. The tissues are placed in a mixture 

 of chloride of gold and formic acid (four parts of 1 per cent, 

 gold chloride to one part of formic acid) which has been 

 boiled and allowed to cool (RANVIER'S Trait/', p. 826). They 

 remain in this until thoroughly impregnated (muscle twenty 

 minutes, epidermis two to four hours) ; the reduction of the 



