THE MODE OF ACTION OF VITAL DYES 285 



the process of penetration is not under the direct control of the 

 experimenter. 



The ability of dyes to enter living cells was first studied 

 systematically by Overton. ^^^ He was engaged on the researches 

 that proved the existence of a cell-membrane having different 

 properties from the ground cytoplasm. He found that basic dyes 

 entered cells readily, but sulphonated acid dyes slowly or not at 

 all. He noticed that basic dyes were soluble in melted cholesterol 

 and in lecithin dissolved in warm benzene, and also that lecithin 

 and other phospholipids suspended in water take up basic dyes 

 strongly. Most acid dyes, on the contrary, are insoluble in these 

 solvents, and scarcely or not at all taken up by suspended phospho- 

 lipid. Overton concluded that the penetration of basic dyes into 

 cells was made possible by the presence of lipid in the cell- 

 membrane. He was confirmed in this view by the facts that methyl 

 orange and tropaeolin, though acid dyes, have some capacity to 

 enter cells, and differ from most acid dyes in being somewhat 

 soluble in lecithin solution and capable of absorption by suspended 

 lecithin. 



Fischel ^^^ made an interesting generalization about the chemis- 

 try of vital dyes. He pointed out that in these dyes the hydrogens 

 of the amino-groups were commonly (though not always) substi- 

 tuted by methyl or ethyl. Pararosaniline and rosanilin, for instance, 

 are not usable vitally, but the related methyl violet (with methyl 

 groups substituting some of the amino-hydrogens) and especially 

 dahlia (with ethyl groups) are useful vital dyes. Fischel' s generaliza- 

 tion applies to seven of the eight especially valuable vital dyes 

 listed on p. 278. The substitution by methyl and ethyl would tend 

 towards solubility in lipids. Not all basic dyes are lipid-soluble. 

 As Seki *^^ showed, it is especially those that are soluble in solu- 

 tions of lecithin that are able to enter living cells. 



The capacity of dyes to pass through a layer of a liquid lipid or 

 of a lipid-solvent such as chloroform may be studied by the use 

 of quite simple apparatus. ^^^ A horizontal glass tube is turned 

 vertically upwards at each end and a vertical tube is connected 

 with it at its middle point. Enough lipid or lipid-solvent is put in 

 the apparatus to fill the horizontal tube and rise some distance in 

 the vertical ones. A solution intended to represent a cellular fluid 

 may now be added to one of the end-tubes. In the original experi- 

 ments actual sap taken from the very large vacuole of Valonia 

 (Siphonocladiales) was used. The aqueous solution of a dye is put 



