50 Biological Stains 



cess will be greatly influenced by the chemical nature of the differ- 

 ent parts of the cell. Substances of an acid character would ad- 

 sorb basic dyes more readily, and retain them in the adsorbed 

 state more firmly than they would acid dyes, and substances of a 

 basic character would adsorb acid more readily than basic dyes. 

 Amphoteric substances are usually acid or basic in their prevailing 

 character. Acid substances such as cell nuclei exert a decided 

 chemical affinity for basic dyes and accordingly adsorb them 

 readily. They have, however, inferior degrees of affinity for acid 

 dyes and in order to obtain staining with such dyes it is only 

 necessary to convert them into a favorable physical form for ad- 

 sorption. Basic substances, on the other hand, such as cytoplasm, 

 although ordinarily stained only by acid dyes, may be stained by 

 basic dyes if the latter are employed in a favorable physical form. 

 Favorable conditions for these unusual types of staining are ob- 

 tained merely by altering the reaction of the staining solutions. 

 Thus one does not have to assume chemical combination between 

 dye and tissue in order to account for the different type of staining 

 obtained in solutions of varying pH value. 



It is extremely easy to theorize in regard to the mechanism of 

 staining but very difficult to propose any conclusive argument. 

 Dye chemists have obtained actual quantitative data as to with- 

 drawal of dyes from solution by the fibers of which textiles are 

 made up, which point very strongly to the process being physical 

 rather than chemical. These conclusions are not directly appli- 

 cable to biological staining for two reasons : In the first place fixed 

 tissues are decidedly different from the animal or vegetable fibers 

 studied by the dye chemists and quantitative data seem to be 

 lacking as to how such tissue removes any dye from solution. 

 In the second place, staining effects are in most instances brought 

 about by the action of decolorizing agents after the staining has 

 taken place, and this introduces a factor which dye chemists have 

 not had to take into account in explaining the affinity of textiles 

 for dyes. 



Evidence is still lacking, in fact, to prove or to disprove any 

 of the physical theories on the one hand or the chemical theory 

 on the other. The difference, perhaps, is not one of immense 

 importance. All of the theories teach about the same practical 

 lessons as to the type of dye to select for any particular purpose 

 and as to the influence of salt content or H-ion concentration of 

 staining solution or tissues upon the rate or intensity of staining. 

 The user of biological stains should know enough about the vari- 

 ous theories of their action to understand the influence of ionic 

 concentration upon basic dyes on the one hand and acid dyes on 

 the other; but realizing that the same phenomena can be explained 

 by more than one theory, he can safely leave to the physical chem- 

 ists all discussion of the mechanism involved. 



