156 
Walter Stiles 
Summary of the Evidence for the Existence of 
limiting Semi-permeable Plasma-membranes 
Physico-chemical considerations indicate that the surface layer 
of the protoplasm must differ physically and chemically from the 
rest of the latter, the actual composition of the limiting layer de¬ 
pending not only on the composition of the protoplasm, but also on 
that of the medium with which the protoplasm is in contact. Any 
change in such a medium will bring about changes in the limiting 
layer. Ultramicroscopic, as well as microscopic, observations, in¬ 
cluding those made with the aid of microdissection, show that in 
some cells there may be a comparatively wide border (the hyaloplasm 
or ectoplasm) differentiated from the internal protoplasm, but that 
there may also be a much more delicate limiting layer which is 
regarded as the true plasma-membrane. These lines of investigation 
do not give definite information on the question whether the per¬ 
meability properties of the limiting layer differ considerably from 
those of the internal protoplasm, except that as the surface layer 
should be more concentrated and viscous, it should offer greater op¬ 
position to the passage of dissolved substances (compare Chapter IV). 
The relations of the cell to water and dissolved substances are 
generally regarded as yielding evidence of the existence of a semi- 
permeable membrane bounding the protoplast, but an analysis of 
the evidence indicates that it would be premature to draw such a 
conclusion from existing data. The low electrical conductivity of 
tissues affords some evidence of the presence of a semi-permeable 
membrane. In this connection, unfortunately, experimental observa¬ 
tions are very scanty, while with plants observations on non-vacuolate 
cells appear to be non-existent. 
Conclusions appear to have been drawn incorrectly in the past 
owing to the distinction between vacuolate and non-vacuolate cells 
not having been drawn with sufficient definiteness. In the case of 
the vacuolate cell there appears to be every reason to believe that 
the protoplasm acts as a membrane, to some extent semi-permeable, 
separating the vacuole from the external medium. 
Composition of the Limiting Layer 
This is a question that has given rise to very considerable discussion. 
We have noticed that dissolved substances which lower the 
surface tension of the solvent tend to accumulate in the surface layer. 
For this reason the various constituents of the protoplasm may be 
expected to be present in the plasma-membrane. Among substances 
