i6o 
Walter Stiles 
by him. Robertson is of opinion that the maintenance of the in¬ 
tegrity of the cell surface can be adequately accounted for by supposing 
that the cell is surrounded by a thin concentrated film of protein. 
Such films are thought to exist very generally, and perhaps universally, 
at the bounding surfaces of droplets of protein solutions (cf. also 
Ramsden, 1903). Robertson thinks it possible that a discontinuous 
layer of lipoid substance may underlie the outer film of protein. 
Lepeschkin (1910 a, 1910 b, 19n a, 19 n b), while agreeing that 
lipoid substances may be present in the plasma-membrane, adduces 
experimental evidence in favour of the importance of proteins therein. 
The protoplasm of plant cells coagulates at definite temperatures, the 
protoplasm of Spirogyra sp. at about 50-5° C., that of the epidermal 
cells of Tradescantia sp. at about 70*7° C. This behaviour is similar to 
that of proteins (cf., for example, Schryver, 1913). The coagulation is 
visible in plasmolysed cells under the microscope owing to the change 
from a shining homogeneous to a granular appearance. At the same 
time there is a sudden increase in permeability made obvious by the 
sudden shrinkage of the protoplast. Coagulation of the protoplast by 
organic substances can also be brought about in the same way as that 
of non-living proteins such as egg-albumin. 
Lepeschkin consequently comes to the conclusion that the plasma- 
membrane contains both protein and lipoid substances in important 
quantities. He dismisses, however, the view of Nathansohn (1904 a) 
that the membrane consists of a mosaic of lipoid substances and 
protein responsible respectively for the intake of lipoid-soluble and 
water-soluble substances, since the entry of these two sorts of sub¬ 
stances is not independent one of the other. Thus, the entry into the 
cell of salts and dyes soluble in water is diminished in the presence 
of narcotics soluble in fats. The value of this argument is, however, 
doubtful. 
Lepeschkin’s own view is that the plasma-membrane is largely 
composed of lipoid substances loosely combined with protein. 
It will be observed that both Czapek and Lepeschkin worked 
with vacuolate cells, and there is no evidence to decide whether they 
are dealing with the whole thickness of the protoplasm, or only a 
thin limiting layer of it. Lepeschkin recognised this quite clearly, 
simply using the term plasma-membrane to designate that part of 
the protoplasm which is the seat of the phenomena he investigated. 
This plasma-membrane is the surface layer and an unknown thick¬ 
ness of cytoplasm within it, even possibly the whole thickness. 
For an account in English of the work of Czapek and Lepeschkin, 
