PROTOZOAL DISEASES 271 



stained by a given dye, the cells of some other organ 

 in the same animal reacted differently to the dye. 

 In other words some dyes showed what is known 

 as a selective affinity for certain cells. These dyes 

 were used in the study of dead tissues. As time 

 went on it was found that certain aniline dyes, in 

 high dilution, exerted but a slight or no observably 

 injurious effect on living cells, whilst a certain 

 amount of the colouring matter was absorbed into 

 the living tissue. Such staining of living cells be- 

 came known as intra-vital staining and it served 

 many a useful purpose in the laboratory. Ehrlich, 

 continuing his studies on immunity, was struck by 

 this phenomenon and its possible bearing upon the 

 action of drugs in the body. Dilute methylene 

 blue for instance had been found to produce intra- 

 vital staining in nerve tissue. Ehrlich argued that 

 this power to take up certain substances like dyes 

 afforded an indication that the cells of the body 

 might take up drugs in a similar manner. The 

 drug might augment the natural resisting power 

 of the body when it reacted to the stimulus of the 

 invading parasite or the drug might show a selec- 

 tive and more directly injurious action upon the 

 microorganism itself. He began by studying blood 

 parasites which cause trypanosomiasis, a group 

 name for diseases in vertebrate animals of which 

 sleeping sickness in man may be taken as an 

 example. He discovered an aniline dye to which 

 he gave the name of trypan-red because he found 



