PROTOPLASMIC STRUCTURE 67 



activity, and automatic regulation of composition and 

 properties ? 



The experimental studies and observations of the last 

 twenty years have led more and more to the conclusion 

 that the general or fundamental structure of protoplasm 

 corresponds more closely to that of an emulsion than to 

 that of any other simple non-living physical system. 

 The most general facts of its chemical composition are in 

 agreement with this conclusion. Water-insoluble con- 

 stituents (lipoids) occur in association with colloidal 

 constituents which have water-combining powers (pro- 

 teins). The whole resulting complex is during life 

 immiscible with water, and typically is bounded from 

 the external watery medium forming its immediate 

 environment by a layer or surface-film having semi- 

 permeable properties. The semi-permeability and the 

 water-immiscibility of the surface layer appear to be 

 interdependent properties; they suggest the existence of 

 a continuous external layer of water-insoluble material 

 of fatty or similar nature.^ The unit of organic struc- 

 ture, the cell, would thus appear to be a system with 

 an aqueous internal phase limited externally by a thin 

 water-insoluble phase or boundary layer. The aqueous 

 internal phase forms one component of a system, the 

 cell protoplasm, which is structurally and chemically 

 highly complex, and emulsion-like in its general physical 

 constitution. 



It is evident that the general properties of emulsions 

 do not in themselves explain the properties of living 

 matter. What seems highly probable, however, is that 

 the original structural foundation upon which the proper- 



^ Cf. Quincke, Ann. Physik., XXXV (1888), 580; cf. pages 629-30. 



