Permeability 19 
urea (de Vries, 1889 a, b). Later Overton (1895) tested the plasmo- 
lytic action of a large number of substances, and with many was 
unable to bring about plasmolysis at all, notably with the alcohols 
of the fatty series, various narcotics (ether, chloral hydrate) and other 
organic substances. 
“Permeability Coefficients By determinations of the concentra¬ 
tion of penetrating substances required to produce plasmolysis and 
the concentrations of a non-penetrating substance required to pro¬ 
duce plasmolysis of the same cells, Lepeschkin (1908 a) and Trondle 
(1909, 1910) claim to be able to obtain values termed “permeability 
factors” by the former and “permeability coefficients” by the latter, 
which are measures of the permeability of the cell. Thus let it be 
supposed that the osmotic pressure of a non-penetrating substance 
that just brings about plasmolysis is P 0 while the osmotic pressure 
of the penetrating substance required to effect plasmolysis is P. The 
osmotic pressure of the penetrating substance isotonic with the solu¬ 
tion of non-penetrating substance is also P 0 . As P is greater than P 0 , 
it follows that the isotonic coefficient of the penetrating substance 
determined by plasmolysis will be less than that obtained by direct 
measurements of the osmotic pressure with a perfectly semi-per¬ 
meable membrane, or that calculated from determinations of the 
osmotic pressure from other physico-chemical data. 
If i is the true isotonic coefficient calculated from physico¬ 
chemical data and i f is the isotonic coefficient obtained by the plasmo- 
lytic method with the particular cells and substance under investiga¬ 
tion, the permeability factor or permeability coefficient is given by 
i f 
^ = 1 ~ 1 
and this permeability factor or coefficient is held to be proportional 
to the permeability of the cells employed to the particular substance. 
It is difficult to understand how such considerations can lead to 
results having any definite quantitative significance. We have ob¬ 
served that the reason why a higher osmotic concentration of a sub¬ 
stance that penetrates is required to effect plasmolysis than of one 
which does not, is because some of the dissolved substance immedi¬ 
ately enters the cell and so increases the osmotic concentration of 
the cell sap. The difference between the osmotic concentration of a 
penetrating and a non-penetrating substance required to bring about 
incipient plasmolysis is therefore a measure of the amount of sub¬ 
stance which has entered the cell between immersion in the solution 
of the penetrating substance and the moment of observation. The 
