THE DIRECT ATTACHMENT OF DYES TO TISSUES 203 



Unfixed proteins are generally neither strongly basiphil nor 

 strongly acidophil. Chemical fixation generally increases their 

 colourability by dyes. This is well shown by fibrin films. ^''^'^'^ 

 Fixation by heat has the same effect. ^^^ Certain fixatives favour the 

 action of basic dyes, others that of acid dyes; others again allow 

 easy coloration by both. 



Formaldehyde favours basic dyes more than any other fixative 

 does. Mercuric chloride has the same tendency, but in a less 

 marked form. These facts are related to what is known of the re- 

 actions of these two fixatives with proteins, for in both cases there 

 is blocking of -NHg groups (pp. 53 and 59). Kelley ^^^ con- 

 siders that mercuric chloride favours acid dyes; he thinks the main 

 effect of the salt is to block carboxyl groups, while amino-groups 

 are left free for linkage wdth acid dyes. This is contrary to what 

 is known of the chemistry of fixation by mercuric chloride, and 

 also to the observable facts of dyeing. 



From some of the remarks in the literature one might imagine 

 that osmium tetroxide almost abolishes the capacity of tissues to be 

 dyed. Even Seki,*^^ so reliable as a general rule, gives this impres- 

 sion. Actually this fixative leaves cytoplasm readily colourable by 

 basic dyes (after bleaching), though scarcely at all by acid ones. 

 Anyone can prove this for himself by carrying out the experiment 

 described in the Appendix (p. 325). Basic fuchsine gives particu- 

 larly intense coloration of the convoluted tubules of the mammalian 

 kidney. These facts cannot be correlated with what is known of the 

 reaction of osmium tetroxide with proteins (p. 62). From the 

 effects of dyes one would suppose that amino-groups were almost 

 eliminated, but carboxyl groups left free for linkage wdth basic 

 dyes. It must be mentioned that basic dye-lakes (p. 207) act less 

 strongly than basic dyes on tissues fixed by osmium tetroxide. 



Ethanol is intermediate between such fixatives as formaldehyde, 

 mercuric chloride, and osmium tetroxide, which make cytoplasm 

 basiphil, and the ones to be mentioned below, which have the 

 reverse effect. It allows fairly easy dyeing of proteins and cyto- 

 plasm by both basic and acid dyes. This must be correlated with 

 the fact that it is a non-additive fixative, which would not be ex- 

 pected to have much effect on the proportions of the acidic and 

 basic groups available for linkage with dyes. 



In sections of tissues fixed with acetic acid, the cytoplasm shows 

 a general resemblance in its reaction to dyes to that fixed by ethanol, 

 for both basic and acid dyes are taken up fairly readily. 



