SELECTIVE PERMEABILITY 337 



the properties of the cell, since according to that conception, the 

 entire cytoplasm is a potential plasma-membrane and, conse- 

 quently, as soon as one hypothetical membrane is ruptured 

 another one is immediately formed in its place. 



Many attempts have been made to account for the selective 

 permeability of the living cell toward dissolved substances. 

 The first explanation that suggested itself was that the plasma- 

 membrane acted as a sieve allowing molecules of smaller size to 

 pass and barring those of larger size, but this hypothesis had to 

 be abandoned inasmuch as by the means of studying permea- 

 bility which are at our disposal it was found that there is no 

 relation between the size of the molecule and its ability to enter 

 the cell. It was thus found that the plasma-membrane is per- 

 meable to the large molecules of alkaloids while it is impermeable 

 to the comparatively small molecules of the amino acids. The 

 case was found to be the same with reference to the artificial 

 ferrocyanide membrane which is, for instance, permeable to 

 sodium salycilate, while it is impermeable to potassium sulphate. ^^ 

 The other hypotheses which have been offered in explanation 

 of the diosmotic properties of the cell are based on the solubility 

 in the plasma-membrane. The plasma-membrane, according 

 to this view, represents a continuous layer of a substance which 

 has definite solvent properties. Only those substances which 

 are soluble in the plasma-membrane are capable of entering the 

 cell. The different hypotheses differ with reference to the chemi- 

 cal nature of the substance or substances constituting the plasma- 

 membrane which conditions its solvent properties. 



Quinke^*^ came to the conclusion, on the basis of his studies 

 of the periodic expansion of fluid surfaces and of the motion 

 phenomena caused by it, that the cell content is enveloped by a 

 thin oil membrane and that thin oil lamella with solid and fluid 

 protein impregnate the entire mass of protoplasm. The alka- 

 line proteins react with the fatty acids contained in the oil; 

 and protein soaps are periodically formed which dissolve and 

 expand on the border between the oil and the surrounding fluid. 



15 Hober, 1. c, p. 167. 



i« Quinke, G., Annalen d. Physik und Chemie N. F. 63: 622. 1894. 



