206 CHAPTER XV. 



then stains for three to five minutes in the anilin-water gentian solution, 

 treats with the iodine solution for one to three hours, and finally differentiates 

 with alcohol. 



274. Thionin. The hydrochloride of thioniii, or violet of 

 Lauth, is a colour chemically nearly allied to methylen blue. 

 It may be obtained from Griibler & Hollborn. I have 

 classed it here as a regressive stain, but its action is so 

 selective from the first that it may almost be considered to 

 be a progressive stain. If you stain for only a short time (a 

 few minutes) in a concentrated aqueous solution,, hardly any- 

 thing but the chromatiii will be found to be stained. If the 

 staining be prolonged, plasmatic elements will begin to take 

 tip the colour. After a short stain no special differentiation 

 is required ; all that is necessary is to rinse with water, 

 dehydrate, and mount. After a strong stain you differentiate 

 with alcohol in the usual way, with this advantage, that the 

 stain is so highly resistent to alcohol that there is no risk 

 whatever of overshooting the mark ; the stain will not be 

 more extracted in an hour than that of gentian or dahlia is 

 in a minute, so that the process may be controlled under the 

 microscope if desired. For this reason I think this stain 

 may be useful to beginners, but I myself prefer gentian. It 

 is a very powerful stain. 



Thionin is a specific stain for mucin, q. v. Some observers have found 

 the stain to fade. WOLFF (Zeit.f. wiss. Mik., xv, 3, 1899, p. 312) says that, 

 to avoid this, preparations should be mounted in a little solid colophonium 

 or balsam melted over a flame. 



275. Other Regressive Stains.- -The foregoing, I think, may 

 -uiHice for most practical purposes, but the following may be 

 mentioned. 



Dahlia (FI.EMMING, Arcli. f. mik. Anat., xix, 1881, p. 

 -;l7).--The stain is paler in the nuclei than with gentian or 

 -alYanin. The cytoplasmic granulations of certain cells aiv 

 sharply stained. 



Dahlia is also a useful nuclear stain for fresh tissues (v. EHKLICH, Arch, 

 f. mile. Anat., xiii, 1876, p. 203). For these the aqueous solution must be 

 aL-idulated with (7'5 per cent.) acetic acid ; or you may stain in a neutral 

 solution, and wash out with acidulated water. 



Victoria Blue (Victoriablau) (LDSTGARTEN, Mcd. Jahrh. /,-. 

 Ges. <L Aerzte zu Wien, 1886, pp. 285291). 



