taken place. As to the fact that a tissue which has a strong affinity 

 for some particular dye never withdraws that dye completely from 

 a very dilute solution, those who favor the chemical theory point 

 out instances where chemical reactions are known to take place 

 and yet to stop before either component is exhausted; and they 

 further claim that chemical action is strongly indicated by the fact 

 that in dilute solutions the tissues take up relatively larger quan- 

 tities of the dye than in concentrated solutions. 



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; tnerefore, 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, as when we find 

 a certain stain acting only on tde nuclei or even exclusively upon 

 certain structures within the nuclei. The chemical theory is not 

 yet firmly established, and the probabilities are that staining is 

 both a chemical and physical phenomenon; but it is coming to be 

 more widely accepted than the physical theory. Hence it deserves 

 a more detailed discussion. 



The chemical theory of staining is dependent 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. 



Briefly stated, the fundamental chemical theory of staining 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 by this 

 theory 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 

 dves and is assumed to be basic in character. 



Now this is by no means the whole of the chemical theory of 

 staining. The theory is so complex and has so many ramifications 

 and special applications that it takes an intensive study of the 

 subject to comprehend it fully. It is well knowni for instance, that 

 certain basic dyes have stronger affinities for certain parts of the 

 nuclei than for others, and that of the various cytoplasmic struc- 

 tures outside of the nucleus some are more readily stained by 

 certain acid dyes and some by others. Such special applications 

 as these, of course, are not explained on the theory of acid or basic 

 character alone. It is possible, for example, to use the Flemming 

 triple stain, which employs the acid dye orange G and the two basic 



102 



