288 DYEING 



the cytoplasm, caused by the ready penetration of the cell- 

 membrane by the leucobase. 



The fact that the leucobase colours the living cell or parts of it 

 shows that the cell is able to restore the dye by oxidation. As 

 Ehrlich showed, the cell is also able to reduce the dye to its leuco- 

 base. The dehydrogenases of the cell take hydrogen from their 

 substrates and use the dye as acceptor, unless something is done to 

 keep the dye oxidized. Supervital preparations are usually exposed 

 to atmospheric oxygen while they are being vitally coloured, so as 

 to prevent this reduction. One really needs anaerobic conditions 

 at the start, to help penetration, and then abundant oxygen to 

 convert all leucobase into dye. A convenient technique has not 

 been w^orked out for common use, though Harris and Peters -^^ 

 have achieved this end in a rather elaborate way. 



The tendency of vital dyes to be reduced to their leucobases 

 varies considerably. Thionine is particularly easily reduced, 

 neutral red with difficulty. That is to say, thionine readily gives up 

 its ionic form in the presence of reducers, by accepting an electron, 

 while neutral red does not. Vital dyes arrange themselves in this 

 order of oxidation-potentials, beginning with the most easily 

 reduced (that is to say, the strongest oxidizer): thionine, brilliant 

 cresyl blue, methylene blue, Janus green, neutral red. 



It is rather surprising that neutral red should penetrate cells so 

 particularly easily, in view of its strong tendency to retain the 

 ionic form. Its resistance to anaerobic conditions are well seen in 

 supervital work, for the colour is well retained even under a cover- 

 slip sealed at the edges. 



Despite the small value or uselessness of acid dyes in general 

 vital dyeing, it is desirable to record shortly what is known about 

 the capacity of some of them to enter living cells. A few acid dyes, 

 such as orange G, that occur in aqueous solution in the form of 

 particles not exceeding 0-64 m/x in radius, are able to enter certain 

 cells that are impervious to those acid dyes that are dispersed in 

 larger particles. ^^^ The indophenols as a group are also able to 

 enter living cells. It has been pointed out ^- that these are acid dyes 

 that scarcely ionize, and it seems that entry is denied, as a rule, not 

 to acid dyes as such, but to acid dye-ions. The indophenols are not 

 further considered in this book, since they have only limited 

 applications in microtechnique. The ability of certain lipid-soluble 

 acid dyes to enter cells has already been mentioned (p. 285). 



Mdllendorff ^^^ called attention to one of the rare exceptions to 



