THE PERMEABILITY OF PROTOPLASM 135 



THE CONSTITUTION OF THE PROTOPLASMIC SURFACE. 



It has been already pointed out that Overton supposed 

 the surface of protoplasm to be covered over with a layer 

 of lipoid substances, such as lecithin or cholesterin. This 

 hypothesis was based upon the behaviour of such bodies, 

 which, in accordance with the well-known thermodynamic 

 deductions of Willard Gibbs, tend to accumulate on the 

 surface. 



The view was subsequently advanced by Czapek that 

 the true structure of the surface was that of a fatty emul- 

 sion in a colloidal solution of protein. In answer to 

 Lepeschkin's criticism (1913), Czapek (1914) has recently 

 emphasized the fact that he does not regard the fat as 

 being in globules of any considerable size, for such a 

 mixture would have a surface tension approximating to 

 that of pure water. An emulsion, however, the droplets 

 of which are for the most part less than a micron in size 

 can have a low surface tension. Particles which are 

 smaller than 6 \*.p the limit of visibility with the ultra- 

 microscope are termed amicronic. Czapek accordingly 

 assigns to the protoplasmic surface the structure of an 

 amicronic emulsion. 



With regard to Czapek's measurements of the surface 

 tension of living cells, Vernon (1913) has raised the objec- 

 tion that, out of twenty-nine solutions of substances which 

 lower surface tension, twenty-two have no toxic action till 

 the surface tension is about 0-68 (that of water to air being 

 taken as unity), as shown by Czapek. The remaining 

 seven, however, are injurious even when their surface 

 tensions are only from 0-82 to 0-99. In reply the argument 

 has been brought forward that such exceptions may readily 

 be accounted for as due to a chemical poisonous action, 

 and that, when a single case can be instanced in which a 



