Permeability 221 
Ruhland thus comes to the conclusion that the entrance or non¬ 
entrance of a substance into the cell is related entirely to the mag¬ 
nitude of its molecules or molecular aggregates, the plasma-membrane 
acting exactly as an ultra-filter. 
Ruhland’s theory has been subjected to criticism by Hober and 
Nast (1913). It will be recalled that Ruhland found a few highly 
colloidal basic dyes that would not penetrate the cells examined. 
Hober and Nast, on the other hand, found that all these dyes will 
stain living animal cells with the exception of gallamin blue, and this 
dye, according to Hober and Nast, is an acid and not a basic dye. 
These dyes are not very soluble and are easily precipitated by 
electrolytes, and Hober and Nast consider it possible that the dyes 
were precipitated before they reached the plasma-membrane, possibly 
by the electrolytes in the cell walls. In any case the ultra-filter theory 
is not supported by the work of Hober and his collaborators on animal 
cells. 
The ultrafiltration theory has been criticised by Collander (1921) 
from another point of view. The last-named writer points out that 
the cells which take up the dye when Kuster’s method is used are 
those in the neighbourhood of the vascular bundles, and Collander’s 
work on the intake of sulphonic acid dyes has shown that these cells 
may have a quite abnormally high capacity for absorbing acid dyes. 
If the theory were sound all cells should absorb acid dyes of a high 
degree of dispersion, but this has been shown by Collander not to 
be the case, while the earlier experiments of Ruhland (1908 a, b) 
himself indicate the same thing. It is clear that the ultrafiltration 
theory cannot be accepted as a complete and general theory of cell 
permeability. 
The Lipoid Theory 
The lipoid theory of cell permeability propounded by Overton 
(1895, 1896, 1899, 1900, 1901) is a solution theory of permeability, 
assuming as it does that the penetrating capacity of different sub¬ 
stances runs parallel with their solubility in lipoid substances of 
which the plasma-membrane is supposed to be composed. 
Overton founded his theory on experiments with a wide range 
of tissues and a large number of different substances including many 
dyes and other organic compounds of different groups. 
As already mentioned, most acid dyes are insoluble in lipoid 
substances, but Ruhland states that some sulphonic acid dyes are 
soluble in lipoids and do not stain living cells, and three acid dyes, 
cloth red 3(L4, true red A and wool violet S, were found by Hober 
