METHYLEN BLUE. 235 



fibrils " are sharply stained of a violet-blue, showing no 

 granular precipitate, and the " interfibrillar " and " peri- 

 fibrillar " substance, as well as nuclei, are either colourless 

 or very lightly stained. The usual methods, on the other 

 hand, give an " inverse " reaction, the " primitive fibrils " 

 remaining colourless, whilst the interfibrillar substance and 

 protoplasm of the nerve-fibres are impregnated with a finely 

 granular greenish -black or violet precipitate, nnd the nuclei 

 are usually stained. 



328. Preservation of the Preparations. There are consider- 

 able difficulties in the way of obtaining permanent prepara- 

 tions, as the stain is so very unstable that, as above explained, 

 it begins to discharge after a short time, even in the living 

 and not yet totally impregnated tissue. It may, however, 

 be fixed, and more or less permanent preparations be made 

 by one or other of the following methods : 



DOGIEL (Arch.f. m-ili'. Anat., xxxiii, 4, 1889, pp. 440 et *eq.} 9 

 following AKNSTEIN (AnQt.Au.zeig., 1887, p. 551), brings the 

 preparations, in order to fix the colour, into saturated aqueous 

 solution of picrate of ammonia, in which they are allowed to 

 remain for half an hour or more, and are then removed, 

 washed in fresh picrate of ammonia solution, and studied in 

 dilute glycerine, or mounted permanently in glycerine satu- 

 rated with picrate of ammonia. More recently (Zeit. f. ic-is*. 

 Af/fc., viii, 1, 1891, p. 15) he has recommended an increased 

 duration of the picrate of ammonia bath up to eighteen or 

 twenty-four hours, and mounting, without washing out, in 

 chemically pure glycerin, free from acid. There is a defect 

 in this process, namely, that picrate of ammonia has a very 

 injurious action, of a macerating nature, on some tissues. 

 This may, however, be avoided by adding to the fixing-bath 

 I to 2 per cent, of a 1 per cent, osmic acid solution. (If it- 

 be desired to harden the tissues so that sections ma}' be cut, 

 the proportion of osmium solution should be increased four- 

 fold.) 



S. MAYER (Zeit. f. wiss. Mik., vi, 4, 1889, p. 422) preferred 

 a mixture of equal parts of glycerin and saturated picrate 

 of ammonia solution, which served to fix the colour and 

 mount the preparations in. This was also in principle the 

 method followed by RETZIUS (Intern. Monatsschr. Anat. u. 



