Permeability 29 
that conclusions relating to permeability changes based on the as¬ 
sumption that the electrical conductivity is a quantitative measure 
of permeability to ions must be accepted with very great caution. 
This concludes our review of the methods that have been devised 
for the study of absorption of dissolved substances and the per¬ 
meability of the cell to such substances. These methods are very 
varied and many of them decidedly ingenious. From this survey it 
is obvious that reliable data are often readily obtainable with regard 
to qualitative relations in permeability, and the more important 
qualitative results have been recorded in the course of this chapter. 
But when we consider quantitative relations in absorption and per¬ 
meability the case is very different. Although many results have been 
recorded professing to give quantitative data with regard to intake 
of dissolved substances and the permeability of cells to such sub¬ 
stances, yet in most cases the presence of disturbing factors renders 
the results open to criticism. The reason for this is to be found in the 
complexity of the system involved, combined with the very general 
assumption that the 1 conditions are simple. Thus, to take only one 
case, that of the plasmolytic method, it is assumed in this method 
that the substance which enters the cell passes through a proto¬ 
plasmic membrane and then accumulates in the cell sap as such, and 
that there are no disturbing factors to complicate this simple arrange¬ 
ment. Exosmosis, which certainly takes place, is neglected. The 
possibility of combination of the diffusing substance with a cell con¬ 
stituent or its adsorption with such a cell constituent, which we have 
seen may certainly take place in certain cases, is also neglected. 
In the next chapter, where the quantitative relations of the cell 
to dissolved substances are considered, it will therefore be necessary 
to subject the data dealt with to some scrutiny, in order to have as 
clear an idea of what the results obtained really mean, as the difficulty 
and complexity of the subject make possible. 
(To be continued) 
