PERMEABILITY OF SURFACE LAYER OF CF.LLS. 



in nature, early expressed by Quincke and others, has thus been 

 confirmed by a study of the permeability, since fat-solvents will 

 readily enter and traverse a layer consisting largely of fatty sub- 

 stances. Overton has shown that the permeability of cells is in 

 general such as would be expected if the plasma-membrane con- 

 sisted largely of lecithin and cholesterin, the chief members of the 

 so-called lipoid group of substances, which appear to be constant 

 constituents of protoplasm. He found that those dyes which 

 readily enter cells (infra vitam dyes) are very generally soluble in 

 lipoid mixtures or in organic solvents containing lipoids-- even if 

 insoluble in triolein and other simple glycerides-- while the 

 group of non-vital dyes, particularly the sulphonic acid derivatives, 

 are insoluble in such solvents. 1 A relation between lipoid-solu- 

 bility and ability to enter cells thus exists for dyes, according to 

 Overton. The rule that such solubility implies ready penetration 

 of a dye into a cell and vice versa appears, however, not to be with- 

 out exceptions ; 2 the matter is complicated by the readiness with 

 which the normal permeability may be modified by the very 

 presence of the substance whose penetration is in question. 

 There are, however, other facts which indicate that the distinctive 

 vital permeability of cells is intimately related to the presence of 

 these substances. Chief among these are the peculiar relations of 

 lecithin and cholesterin to haemolysis, a change which seems to de- 

 pend primarily on alteration of the permeability of the surface layer 

 of the erythrocytes ; the action of various hasmolytic substances 

 (saponin, agaricin, solanin, cobra poison, tetanolysin) is furthered or 

 checked in a characteristic manner by the addition of lipoids to the 

 blood serum. Again, the observations of Pascucci 2 in Hofmeister's 

 laboratory on the permeability of artificial membranes impregnated 

 with lipoids, and on the composition of the stroma of blood cor- 

 puscles, afford strong support to the view that lipoids play an es- 

 sential part in the formation of the plasma-membrane. Naturally 

 the latter is not to be regarded as a simple layer of lipoids ; it 

 appears rather to be a surface film of highly complex composi- 



1 Overton, " Jahrbiicher fur wissenschaftliche Botanik," 34, 1900, p. 669. 



2 Cf. the critique of Brailsford Robertson, Journal of Biological Chemistry, 1908, 

 vol. 4, p. i. Robertson nevertheless regards the surface film as partly composed of 

 lipoids, although he ascribes more importance to the surface film of modified protein. 



3 Pascucci, Hofmeister 's Beitrdge, 1905, 6, pp. 543, 552. 



