Permeability 51 
to demonstrate an outer layer differing in consistency from the 
rest of the cell. Price from ultra-microscopic observations states 
that the presence of an outer layer of the protoplasm differentiated 
from the interior seems quite definite. In any case it must be 
emphasized that the surface of the cell constitutes a boundary 
between two immiscible phases and as such the properties of the 
surface will differ greatly from those of the main bulk of the proto¬ 
plasm. Indeed, the surface layers of the protoplasm are generally 
regarded as differing so much from the rest of the protoplasm as 
to have different permeability properties. The evidence for and 
against this view will be more suitably dealt with in a later chapter 
when the cell membranes are considered in more detail. 
The chemical composition of protoplasm is rendered difficult of 
determination as the methods of chemical analysis in themselves 
necessarily change the living matter into something essentially non¬ 
living and therefore different. Chemical analysis has however pro¬ 
vided some information regarding the components of the living 
substance. Water often comprises about 80 or 90 per cent, of the 
total weight of protoplasm. The classical analysis of Reinke and 
Rodewald (1880) of the plasmodium of the myxomycete Fuligo 
varians showed that about 60 per cent, of the dry weight of the 
protoplasm consisted of proteins, about 20 per cent, of carbo¬ 
hydrates and fatty substances, while the remainder consisted of 
amino-acids and other organic acids, various organic bases and 
inorganic salts. In different samples of protoplasm the relative 
quantities of the substances of which it is composed vary; pro¬ 
teins may form as little as 40 per cent, of the total dry matter. It 
has been a matter of controversy whether cytoplasm is a single 
definite substance peculiar to living matter, or whether it is com¬ 
posed of a large number of relatively simpler substances. In the 
elaboration of the former view the molecule or “biogen” is held 
to consist of a stable central nucleus to which are attached side- 
chains capable of undergoing various chemical transformations such 
as oxidation and reduction, and to which the activities of the cell 
are due (Verworn, 1903). The opinion that protoplasm comprises 
a large number of different substances is more generally held to¬ 
day. Thus Czapek (1911) remarks: “But we have to concede that 
the chemical nature of protoplasm is not founded upon the pecu¬ 
liarities of one particular substance which is characteristic of living 
protoplasm. There are, we are certain of it, a great number of 
constituents of protoplasm which form the substratum of cell-life.” 
