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chemically from the phases which they separate. These layers are no 
doubt largely composed of loosely bound adsorption compounds, but 
it is always possible that the meeting of substances adsorbed from 
the one and the other of the two phases in contact may result in the 
formation of more stable chemical compounds which are not present 
in the interior of either of the two adjoining phases. In such a case 
the plasma-membrane would be regarded as produced in much the 
same way as the precipitation membranes discussed in Chapter V. 
The formation of a rigid film at the surface of separation of a protein 
solution and some other phase (Ramsden, 1903), presumably by the 
protein adsorbed at the surface undergoing some irreversible change, 
is of significance in this connection. 
In any case it is clear that from purely physical and physico¬ 
chemical considerations there is good reason to suppose that where 
the protoplast is in contact with the cell wall and the vacuole there 
are thin layers which differ in physical and chemical properties from 
the general mass of the protoplasm. It is possible and, indeed, to be 
expected, that among other properties in which these layers differ 
from the internal cytoplasm, is their permeability to different sub¬ 
stances. 
2. Evidence from Cytological Observations 
It is convenient to group under this second heading all the 
evidence bearing on the existence of plasma-membranes which is 
based on actual observations made on the protoplasm of the cell. 
Such observations are of three kinds: (1) ordinary microscopic obser¬ 
vations, (2) ultramicroscopic observations, and (3) those made by 
the aid of microdissection. 
(1) Microscopic observations. Incidental reference has already 
been made to the clear hyaloplasmic layer of Myxomycetes and of 
the cells of other organisms. The hyaloplasm is presumably more 
viscous than the internal mass of the cytoplasm as it appears free 
from moving granules which would thus seem to be repelled from 
entering it. It is to be noted that the existence of the hyaloplasm 
appears to be due to surface forces, for it remains constant in thick¬ 
ness when the cell increases or diminishes in volume owing to absorp¬ 
tion or loss of water (Bayliss, 1915). 
Sometimes in cells that have been killed a very line membrane 
can be recognised limiting the now dead protoplasm. As Bayliss 
(1915) points out, this “is no proof that the membrane was in existence 
in the living cell in the same state as that seen.’’ 
