The attempt is made to reconcile the two existing theories on the mechanism of selective ion permeability of the cell membrane by taking into account the molecular arrangement in the substance of the membrane. It is shown that the pore theory and the solubility theory are not contradictory, but two aspects of the same problem, one from the thermodynamic point of view, the other from the point of view of molecular theory. The dried collodion membrane is used as a model in these studies. Its different behavior towards anions and cations is explained on the ground of a quasi-crystalline structure of collodion, the NO(3)-groups acting as dipoles with the negative charge directed towards the intermolecular spaces, no matter whether these pores are of molecular dimensions or larger. In this way a continuity in the behavior of the ordinary large pored collodion membrane and the dried membrane is established, both theoretically and experimentally. Experiments, with membranes of other cellulose derivatives agree with the mechanism suggested. Membranes of cellophane and ethyl cellulose are negatively charged, membranes of cellulose acetate positively. From solutions of collodion mixed with basic dyestuffs or alkaloids membranes can be obtained more permeable to anions than to cations in contrast to the ordinary collodion membrane. Membranes can be built which give high potential differences even between two identical electrolyte solutions. The asymmetry lies here within the membrane and is artificially produced by gluing together an ordinary collodion membrane with one previously impregnated with a basic dyestuff or an alkaloid.