Theory of Staining 47 



action, moreover, is strongly indicated by the fact that in dilute 

 solutions the tissues take up relatively larger quantities of the dye 

 than in concentrated solution. Accordingly the observation that 

 not all the dye is withdrawn from the solution is a poor argu- 

 ment against chemical action. 



Furthermore, if adsorption is assumed to take place only at a 

 specific site (i.e. at the COOH 017 NH3 group), the physical and 

 chemical concepts merge into one and cannot be distinguished 

 from each other. It has been shown, for instance, that cells in 

 which the COOH groups are tied up in methyl esters do not stain 

 well with basic dyes. This observation is hard to explain on a 

 purely physical basis; to explain it bj- adsorption requires the as- 

 sumption that the phenomenon occurs only at a specific chemical 

 site. Such an explanation comes quite close to assuming chemical 

 action. 



In brief, the chemical theory of staining is that the tissues have 

 certain definite chemical affinities which are satisfied by the chem- 

 ical affinities of the dyes; therefore, when the tissue is put in a 

 solution of the dye the latter combines with those portions of the 

 tissue or of the individual cells which have the proper chemical 

 nature. This theory, it will be seen, is especially well adapted to 

 explain the differential staining which takes place when we find 

 a certain stain acting only on the nuclei. It must be remembered, 

 however, that it is merely a theory. As a matter of fact, the 

 probabilities are that staining is both a chemical and a physical 

 phenomenon. 



The chemical theory of staining depended largely upon the 

 question of the acid or the basic character of the dye molecule. It 

 will be recalled that all ordinary dyes are encountered either as 

 sodium or potassium salts of dye acids or as dye salts of colorless 

 acids, the former being the acid dyes and the latter the basic dyes; 

 while certain compound stains are neither acid nor basic dyes, in- 

 asmuch as the property of color exists in both the anion and the 

 kation. 



The fundamental principle involved in this theory is that 

 certain parts of animal or plant cells are acid in character and 

 hence have an affinity for the basic dyes. The nuclei of the cells, 

 or especially the chromatin within the nuclei, are assumed to be 

 acid in character (due largely to their constituent nucleic acid), 

 and there is no question but that they have a strong affinity for 

 basic dyes; while the cytoplasm has an affinity for acid dyes and is 

 assumed to be basic in character. (See Stearn and Steam, 1929- 

 30.) 



This theory assumes that the acids and bases which go to make 

 up body tissue are ordinarily amphoteric, capable of acting as 

 bases in acid solutions and as acids in basic solutions, the H-ion 

 concentration at which any such compound changes from an acid 



