PROPERTIES OF PROTOPLASMIC MEMBRANES 145 



dyes in solutions of lecithin and cholesterol,^ Overton 

 referred the characteristic permeability of the plasma 

 membrane specifically to the presence of lipoids. This 

 evidence is in itself scarcely conclusive, especially since 

 other factors are now known to be of importance in the 

 penetration of dyes into cells, and many exceptions have 

 been found to the rule that lipoid-solubility connotes 

 ready penetration. The state of colloidal subdivision 

 has been shown to be an important factor,^ and the 

 negative electrification of the cell-surface must also 

 play a part by favoring the adsorption and penetration 

 of the positive particles of the basic dyes, which Overton 

 found to penetrate most readily. But other evidence 

 of various kinds supports Overton's conclusion in the 

 main;^ and there can be little doubt that the lipoids 

 form essential constituents of plasma membranes, even 

 although other compounds, especially proteins, may be 

 of equal importance; e.g., as furnishing a structural 

 support to the lipoids. The influence of lipoid-solvents 

 on the permeability and activity of cells, the character- 

 istic cytolytic action of these compounds, the influence 

 of salt-solutions on oil-water emulsions, the peculiar 

 relations of cholesterol to the mechanical properties of 

 the plasma membranes, the properties of artificial lipoid- 

 impregnated membranes, and many other facts all 

 indicate the important role of lipoids in determining the 

 properties of plasma membranes. It is not to be con- 



^ Overton, Arch. ges. Physiol. y XCII (1902), 115. 



* Ruhland, loc. cit. 



3 Especially the results with strong and weak acids and bases, above 

 cited. Cf. the discussion in chap, viii, pp. 428 £f ., of Hober's Physikalische 

 Chemie der Zelle. 



