EMULSION-STRUCTURE OF PROTOPLASM 289 



' alcohol-water layer can be seen to swell up rapidly and burst, and the 

 fine membranes which surrounded them can then be seen falling 

 down through the alcohol-water. If we now add several volumes of 

 alcohol and shake up the liquid, the chloroform droplets all disappear 

 and what we now have is a clear, homogeneous solution, in which 

 innumerable minute membranes can be clearly seen floating. 



The phenomena of Relative Semipermeability may also be illustrated 

 by these droplets. Substances which are soluble in water and also in 

 chloroform penetrate the membranes, and if, like Alcohol, Ether or 

 Ethyl Acetate they chance to be more soluble in chloroform than in 

 water, the chloroform in the droplets may take up so much of the sub- 

 stance that they swell to the extent of rupturing their enveloping 

 membranes. If, however, the substances in which the droplets are 

 immersed are sufficiently insoluble in water they fail to penetrate the 

 membranes and then the droplets may be " plasmolyzed," that is, 

 the chloroform may be extracted from them leaving the enveloping 

 membranes shrunken and empty. This occurs when the droplets are 

 suspended in Toluol, Xylol or Carbon Bisulphide. 



Fat emulsions which contain protein tend to form films at surfaces 

 with which they come in contact, consisting of a more concentrated 

 emulsion, both in respect to fat and in respect to protein, than that 

 which constitutes the body of the liquid. This is very well illustrated 

 by the film which forms on the surface of Milk when it is heated. The 

 heating of the milk renders the Calcium Caseinate which it contains 

 somewhat less soluble, and the concentrated layer of calcium caseinate 

 and fat particles which forms at the surface becomes, at temperatures 

 above 45, sufficiently viscous to assume the consistency of a semi-solid 

 film, which, owing to its high viscosity, does not readily pass back into 

 solution upon cooling. A pure solution of calcium caseinate becomes 

 markedly opalescent on heating to 45 but does not form a sufficiently 

 viscous film at its surface to be mechanically separable from the 

 underlying liquid. 



A living cell consists essentially of a more or less finely emulsified 

 suspension of fat-like substances in a semi-gelatinous solution of pro- 

 tein. The film which forms at the surface of warm milk may be 

 regarded as an extreme illustration of the type of surface-layer which 

 we may therefore expect to exist at the periphery of living cells, namely 

 an emulsion of fat and protein, more concentrated and, therefore, 

 more viscous than the emulsion which constitutes the underlying 

 protoplasm. 



The emulsion-structure of the superficial layer in cells enables us to 

 account for a very widespread property of living cells which would 

 otherwise be almost inexplicable, namely the property of One-sided 

 Permeability. This phenomenon is very well illustrated by the follow- 

 ing experiment of Overton's: If tadpoles are immersed in a five or 

 six per cent, solution of cane-sugar or a 0.6 per cent, solution of sodium 

 chloride they are unaffected either in size or in any other notable 

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