Permeability 159 
of which against air is less than that of protoplasm against air should 
bring about cytolysis, while finally Czapek himself quotes examples 
of fats, aqueous emulsions of which lower surface tension considerably 
below the critical value of o-68. 
An argument frequently advanced against the theory of a lipoid 
limiting membrane is that water, which enters most cells so' readily, 
could not so pass through a continuous film of fatty substance. This 
argument, however, can scarcely be maintained against Czapek’s 
view that the fat does not form a continuous layer, but is in the 
form of an emulsion. Also lecithin, on which Overton laid stress, 
when shaken up with water, forms an emulsoid system consisting of 
a less aqueous disperse phase in a more aqueous dispersion medium, 
and it is presumably in this form that the lecithin would be held in 
the surface layer of the protoplasm. Unfortunately for Overton's 
theory, lecithin in this form is not a solvent for lipoid soluble sub¬ 
stances only (Nathansohn, 1904 a). Membranes of lecithin, and also 
of cholesterol, were found to be completely impermeable to all dyes 
examined, while when the lecithin membrane was saturated with 
water, dyes both soluble and insoluble in lipoids penetrated the 
membrane, not merely the lipoid-soluble ones (Ruhland, 1909 a). 
For a further discussion of the properties of lecithin and cholesterol 
in regard to cell permeability reference may be made to Bayliss 
(1915). Ruhland (1912, 1913 a, 1913 b, 1914) moreover, now holds 
that the intake or non-intake of dyes and other substances by 
plant cells depends rather on the size of the molecules or molecular 
aggregates of the dyes, and not on their solubility or otherwise 
in lipoid substances. 
Pfeffer (1900) regarded the plasma-membrane as largely com¬ 
posed of protein, as indicated by the “fixation” of the limiting layer 
on treatment of the cell with various reagents such as dilute acids, 
mercuric chloride and iodine, and by microchemical tests applied to 
the membrane after fixation. The action of dilute salt solutions in 
rendering the cell more and more permeable (Pfeffer, 1877, 1890; de 
Vries, 1885) could also be explained as due to coagulation of protein 
in the membrane. Pfeffer realised that other substances might also 
be present in the limiting layer. 
Robertson (1908) pointed out that Overton’s theory is invalid 
for a variety of reasons, the most cogent of which is the fact that the 
relation between solubility in lipoid substances and power to enter 
the cell does not hold generally. Robertson found no correspondence 
between penetrability and power to enter cells in the dyes examined 
