Walter Stiles 
146 
surrounding the protoplasm. This is regarded by Seifriz as the 
plasma-membrane after it has become changed by “ fixing” or coagu¬ 
lation. The occurrence of this “degenerate” membrane is cited by 
Seifriz as further evidence for the existence of the plasma-membrane. 
There is similar evidence for the existence of an internal plasma- 
membrane surrounding the vacuole, and also one separating the 
protoplasm and nucleus (the nuclear membrane). 
It is to be noted particularly that the plasma-membrane is capable 
of undergoing reversible change from sol to gel, of readily adjusting 
itself to changes of shape and area, and if destroyed is in most cases 
readily and immediately re-formed at the surface of the cytoplasm, 
from which it is not capable of isolation until the cytoplasm has 
undergone degeneration. It is obviously to be regarded as part of 
the protoplasm. 
3. Evidence derived from a Consideration of the 
Water Relations of the Cell 
The simplest case that can be considered in this connection is that 
of a cell unprovided with a cell wall and in which vacuoles are not 
present. Such cells, common in animals, are comparatively unusual 
in plants. Animal cells are often surrounded by a solution of a 
number of salts, and although the concentration of this solution 
varies in different cases, the salts present in it are very generally 
those of sea water, and the relative proportion of the salts may be the 
same as in that medium. If cells, for example, red blood corpuscles, 
or pieces of tissue such as muscle or sheep’s eyes, are placed in dis¬ 
tilled or tap water, the cells absorb water, and this absorption may 
continue until the swelling is such that the cell or tissue is ruptured. 
The cells are thus behaving as if they consisted of a solution possessing 
an osmotic pressure and surrounded by a semi-permeable membrane. 
If this be the case, as the osmotic pressure exerted by the colloids of 
the cell must be very small, while the movement of water into the 
cell indicates a very considerable osmotic pressure, the existence of 
the latter must be due to crystalloids. 
This is the usually accepted explanation of the swelling of cells 
in distilled water or in weak solutions, and of the corresponding 
shrinkage of cells in solutions of high osmotic pressure. It involves 
the assumption of a semi-permeable membrane enclosing the general 
body of the protoplasm. As Loeb (1906) has truly pointed out, it is 
not necessary that the membrane should be impermeable to salts for 
the absorption of water from a solution of such salts to take place, 
