136 CHAPTER XT. 



in other cases a similar effect has been shown to be due to the presence 

 of impurities in the dyes ; so with iodine green and methyl green, which 

 mostly contain an admixture of methyl violet. 



It has been held by some that nietachromasy is an optical pheno- 

 menon, but that is clearly only true of certain cases. 



207. The Preparation of Tissues for Staining. It is generally 

 found that precise stains can only be obtained with carefully 

 fixed (i. e. hardened) tissues. Dead, but not artificially 

 hardened tissues stain indeed, but not generally in a precise 

 manner. Living tissue elements in general do not stain at 

 all, but resist the action of colouring reagents till they are 

 killed by them (see, however, next section). 



It appears probable, as was first pointed out, I believe, by 

 MAYER (Mitth. Zool. Stat. Neapel, ii, 1880, p. 17), that most 

 of the histological stains obtained with fixed tissues are 

 brought about in one of two ways. Either they result from 

 the combination of the colouring agent with certain organic 

 or inorganic salts, phosphates, for instance, that existed in 

 the tissue elements during life and were thrown down in situ 

 by the fixing or hardening agent employed, as seems to 

 happen when such a fixing agent as alcohol is employed. 

 Or they result from the combination of the colouring agent 

 with certain compounds that did not pre-exist in the tissues, 

 but were formed by the combination of the constituents of 

 the tissues with the chemical elements brought to them by 

 the fixing agent, as seems to happen when such a fixing 

 agent as chromic acid is employed the compounds in 

 question being probably chiefly metal al humiliates. It 

 follows that correct fixation and careful washing out are 

 essential to the production of good stains. 



Objects which have been passed through alcohol generally 

 stain better than those which have only been in watery 

 fluids. 



But long preservation of tissues in alcohol is generally 

 unfavourable to staining. 



208. Staining " intra vitam." -Some few substances (which 

 are almost always basic dyes) possess the property of staining 



-or rather, tingeing living cells without greatly impairing 

 ibcir vitality. Such are in very dilute solutions cyanin 

 (or quinolem), methylen blue, Bismarck brown, anilin black, 



