Permeability 157 
present in the cell are various fatty substances which lower the 
surface tension of water considerably. These fatty substances include 
the true fats which are glyceryl esters of higher fatty acids, the 
lipines (Leathes, 1910), which are complex compounds of fatty acids 
with nitrogen-containing groups, and phospholipines containing 
phosphorus in addition, of which the best known is lecithin. Although 
Quincke (1879, 1888, 1894) had earlier conceived the protoplast as 
surrounded with a film of oil, undoubtedly the popularity of the 
theory of a lipoid plasma-membrane dates from the researches of 
Overton (1895, 1896, 1897, 1899 a, 1899 b, 1901) who showed in a 
series of investigations with a number of different cells and a large 
number of substances, including aniline dyes, that those substances 
soluble in lipoids easily enter the cell, while those which are insoluble 
in lipoids do not readily penetrate living cells. Meyer’s theory of 
narcosis (1899) rests on a similar foundation. 
In this connection Overton laid particular stress on the import¬ 
ance of lecithin and also cholesterol, which is not a true lipoid sub¬ 
stance at all, but a complex alcohol (Windaus and Stein, 1904 a, b). 
Czapek (1910 a, 1910 b, 1911 b, 1914) on the other hand, rather 
emphasised the importance of the simpler neutral fats. He came to 
his conclusions as the result of a series of experiments on the exosmosis 
of tannin from plant cells, mainly those of the sub-epidermal mesophyll 
of the leaf of Echeveria, brought about by treatment with various 
toxic substances. Czapek came to the conclusion that, with few 
exceptions, for exosmosis to occur the concentration of the dissolved 
substance must be great enough to lower the surface tension of the 
solution (against air) to o-68, that of pure water being taken as unity. 
He further found that a strong emulsion of neutral fat has a surface 
tension against air of o-68, and that this is a minimal value; however 
strong the emulsion the surface tension is not lowered below this. 
Czapek was thus led to think of the surface layer of the cell as a 
fatty emulsion, which might indeed contain other substances, but 
of which the principal constituents are neutral fats. On surrounding 
the cell with a solution of lower surface tension than o-68, a breaking 
down of the organisation of the semi-permeable membrane is supposed 
to result with consequent exosmosis of the contents. 
Some substances, however, were found to produce exosmosis 
when presented to the tissue in solutions of considerably higher 
surface tension than o-68. To such substances Czapek attributes a 
specific toxic action, the substances in question apparently acting 
chemically on the limiting membrane. 
