PROPERTIES OF PROTOPLASMIC MEMBRANES 135 



physico-chemical or non- vital factors can, however, 

 be shown in some cases to have definite relations to 

 certain characteristic physiological effects; e.g., in the 

 action of salts and lipoid-solvent compounds on Hving 

 cells. These are the substances through whose action 

 the physiological properties and activities of cells may 

 most readily be altered in a reversible manner; and this 

 action is referable in many cases to a direct alteration 

 of the plasma membranes. 



PERMEABILITY AND STRUCTURAL CHARACTER 



OF MEMBRANES 



Semi-permeability requires a certain closeness of 

 physical texture; i.e., density of structural material; 

 apparently also the structure-forming substances must 

 be water-insoluble. In all semi-permeable artificial 

 membranes water-insolubility is essential; the most 

 perfect examples are the precipitation-membranes of the 

 ferrocyanides and other insoluble salts of heavy metals. 

 That the chief compounds composing the plasma 

 membranes are also water-insoluble is shown by the 

 characteristic insolubihty of Hving cells in their normal 

 aqueous media. 



The relation between structural density and permea- 

 bility in artificial membranes is well shown in the ''ultra- 

 filters" devised by Bechhold for the^ separation of various 

 colloids from their suspension-media.^ These filters 

 consist of disks of gelatine hardened in formalin. Bech- 

 hold found that the higher the colloid-content of these 

 partitions, the more numerous were the colloidal sub- 



^ Bechhold, Colloids in Biology and Medicine, EngUsh translation, 

 or third Gennan edition, 1920. 



