CHAPTER l6 



The Mode of Action oj Vital Dyes 



Three requirements must be satisfied if a dye is to be used in 

 general vital work. It must be able (i) to enter cells of various 

 kinds; (2) to diffuse through the protoplasm without killing the 

 cells; and (3) either to colour certain pre-existent cell-inclusions 

 distinctively, or to colour the whole of the cytoplasm of particular 

 cells so strongly that the cells and their processes stand out from 

 the surrounding intercellular material and from other cells. In 

 short, the three requirements are penetration, harmlessness, and 

 some degree of specificity. 



The dyes that satisfy the three requirements are all basic, but 

 by no means all basic dyes satisfy them. A glance at the list of 

 particularly useful general vital dyes on p. 278 will show that they 

 do not all belong to any particular chemical group. Indeed, nearly 

 all the groups mentioned in the chapter on the classification of 

 dyes (p. 169) provide examples that can be used vitally. Heiden- 

 hain ^^^ claimed that most vital dyes, and the best, were oxazines, 

 thiazines, and azines, and that when, as in safranine, one of the 

 central nitrogen atoms became quinquevalent (or as we should say 

 today, positively charged), vital dyeing was no longer possible. There 

 is some truth in these statements, but in fact it is not possible to 

 generalize about the chemical composition of vital dyes in such 

 simple terms. The special characters of vital dyes will here be 

 considered under the separate headings of penetration, harmless- 

 ness, and specificity. 



Penetration 



A few kinds of cells are so large that dyes may be injected into 

 them, but the danger of mechanical damage usually outweighs 

 the advantage of certain and controlled entry, and anyhow the 

 method is not applicable to most cells. In ordinary vital work one 

 arranges that the cell shall be bathed in a solution of the dye, and 



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