Permeability 151 
and so it is very natural to account for this differential intake of 
substances into the cell by the presence of a superficial layer of the 
protoplasm which acts in the same way as an artificially prepared 
precipitation membrane. Other phenomena, such as the presence in 
the protoplasm of dissolved substances, including free electrolytes 
(Hober, 1912 h , 1913), which do not diffuse out into the external 
medium, and the deplasmolysis of plasmolysed cells in glycerol and 
various salt solutions, are also readily explained on the same assump ¬ 
tion. Also artificial membranes of collodion impregnated with calcium 
phosphate (Meigs, 1913) or an ethereal extract of muscle (Philippson, 
1913) have been prepared which behave very similarly to certain 
cells in regard to the passage of salts. 
We have already seen when considering the evidence for the 
presence of a plasma-membrane yielded by the facts of the water 
relations of the cell, that there are those who deny the existence of 
such semi-permeable membranes. In the present connection reference 
may be made more particularly to the opinions of Moore and Roaf 
(1908), who state that the membrane theory fails to explain (a) the 
difference in composition of electrolytes in the corpuscles of blood 
and in the serum surrounding them; for instance, the concentration 
of chloride in the serum was found to be about three times the con¬ 
centration of this ion in the corpuscles (and see also Abderhalden, 
1898 and Hober, 1912 a, for further data); ( b ) the physiological effects 
of perfusion by media defective or excessive in certain electrolytes 
and normally present in the cell; and ( c ) the selective uptake of 
certain ions. Moore and Roaf hold that these effects are simply 
explained on the view that the cell proteins or other substances of 
the protoplasm form chemical or adsorption compounds with ions. 
The cell, when functioning normally, is held to be in a state of mobile 
equilibrium so that it can undergo reversible changes involving these 
electrolytic constituents, and presumably others, whereby they be¬ 
come associated with the cell protoplasm or the reverse. These writers 
thus suggest that selective uptake is to be explained, not by the 
selective permeability of a membrane, but by specific affinities of 
the cell protoplasm for certain ions, whereby they are adsorbed or 
combine chemically with the protoplasm, while the quantitative 
relation between the concentration of an electrolyte or ion in the 
cell and in the environment is maintained at a definite equilibrium 
which depends on the adsorption equation or molecular affinities, 
and which will therefore not as a rule correspond to equality of 
concentration inside and out. Similarly it is pointed out by Moore, 
