INTRODUCTION TO VITAL COLOURING 275 



Beauchamps.^^ It is a curious fact that while many others were 

 playing unscientifically at non-vital dyeing, these men were 

 making such a profound study of the living cell that their papers, 

 published from 1886 to 191 1, can be studied with profit at the 

 present day. Ehrlich regarded his work on vital dyes as the basis of 

 his later pharmacological researches. He was struck by the strange 

 specificity of vital dyes, and looked for chemical agents that 

 would be equally specific in attaching themselves to harmful 

 parasites. ^-^ In introducing methylene blue as a vital dye for nerve- 

 axons,^^^ he indicated that he had been trying to find how poison- 

 ous substances might distribute themselves differentially among the 

 tissues of the body, and used a dye in his experiments because its 

 colour would announce its distribution. 



Certain non-living parts of organisms may be coloured equally 

 well and in the same way whether the animal be alive or dead, and 

 without any necessity for the dye to enter living cells. The jelly of 

 colonial Radiolaria, the tubes of various Protozoa, the intercellular 

 matter of many Metazoa, and the peritrophic membrane of insects 

 provide examples of what de Beauchamps ^^ called 'pseudovital' 

 colouring. This is a process that does not require special considera- 

 tion here, because there is no important difference from the dyeing 

 of any other kind of non-living matter. 



The use of vital coloration has become less frequent since the 

 introduction of phase-contrast and interference microscopy, be- 

 cause these methods enable one to study the living cell in as natural 

 a state as possible. Extremely valuable though they are, phase- 

 contrast and interference depend wholly on differences in refrac- 

 tive index between an object and its surroundings. They cannot, 

 therefore, give us any direct information about chemical com- 

 position. The main virtue of vital dyeing is that it calls attention to 

 the heterogeneity of cell-inclusions. Some of them colour with one 

 dye, some with another. These are indications of chemical 

 diversity, and vital dyes sometimes give useful pointers towards 

 composition. Further, an inclusion that is of the same or nearly 

 the same refractive index as the ground cytoplasm and therefore 

 invisible or nearly so by phase-contrast or interference, may take a 

 vital dye strongly.*^* Beyond this, as we shall see, the vital 

 activity of the cell in responding to the presence of the dye may 

 give us information of particular interest. The examination of the 

 untreated cell by the newer optical methods should go hand-in- 

 hand with studies made by direct microscopy with vital coloration. 



