SELECTIVE PERMEABILITY 341 



certain substances and their solubility in the lipoids. This 

 parallelism may be a mere coincidence. Thus it was found by 

 Traube that the lipoid-soluble substances lower the surface 

 tension of a solution and enter the cell in accordance with his 

 theory of osmosis in which the principal part is played by the 

 tendency of a phase of lower surface tension to move into a 

 phase of higher surface tension.-^ The parallelism itself does 

 not seem to be perfect, according to Ruhland^^ who found for 

 instance, that malachite green and thionin which are only spar- 

 ingly soluble in lipoids and methylene green which is absolutely 

 insoluble in them enter the cell very easily while rhodamin 

 which is very soluble in lipoids does not seem to penetrate the 

 cell at all. There is besides some intrinsic contradiction in 

 Overton's hypothesis as it stands. Nathanson" has shown 

 that the compounds of cholesterin which were substituted by 

 Overton in his theory for cholesterin itself in order to account 

 for the permeability of the cell to water, become on swelling 

 permeable to lipoid-insoluble substances also. 



Nathanson^e has offered a modification of Overton's lipoid 

 hypothesis. According to him, the lipoids of the plasma- 

 membrane are represented by substances like cholesterin which 

 are entirely impermeable to water. The plasma-membrane, 

 however, does not represent a continuous envelope of lipoids, 

 as it is supposed by Overton, but a mosaic of protoplasm and 

 cholesterin, the latter filling in the interstices in the former. 

 The cholesterin in the plasma-membrane conditions the per- 

 meability of the cell to such substances as dyes, narcotics, etc.; 

 only those which are soluble in it enter the cell. The passage 

 into the cell of such substances is entirely physical in nature 

 and continues till equal concentration is reached in the outside 

 medium and within the cell. The protoplasmic parts of the 

 membrane condition the permeability of the cell to water to 

 certain neutral salts and to other lipoid-insoluble substances 



^^ Harvey, I.e. 

 2* L.c. 



" Nathanson, A., Uber die Regulation der Aufnahme Anorg. Salze, etc. 

 Jahrb. f. Wiss. Bot. 39: 608-644. 1903-04. 

 "L.c. 



