Scheme of the foam structure of living substance. A--In undifferentiated protoplasm. B--In fibrillae protoplasm.] On the basis of investigation in the physical chemistry on the properties of semi-permeable membranes, we know that such membranes produce an elective effect on the diffusion of dissolved substances. This is in the way that the two different solutions, separated by a semi-permeable surface, do not follow the known laws of diffusion, but are altered in that certain substances in contrast to their rapidity of diffusion pass through the membrane or are prevented from entering by the latter. This applies likewise to the two kinds of ions, which are dissociated in diluted substances. If the surface exercises a selection in the way, for instance, that the positive kations are allowed to pass through, whilst the negative anions are held back, a difference of potential must exist between the two. In this manner, wherever two different solutions are separated from each other by a semi-permeable surface, an opportunity occurs for the taking place of galvanic currents. As we know, living protoplasm by reason of its colloidal components possesses, in common with all colloidal substances, on its surface the properties of semi-permeable membranes. Between the cell and the medium, therefore, there is always the opportunity for the occurrence of differences of electric potential. But more. We likewise know that protoplasm itself represents a mixture of colloid substances and actual solutions. Frequently, if not always, living structure presents a morphological differentiation of two types, when seen under the microscope, in the form of a foam structure described by _Bütschli_. (Figures 27 and 28.) If we suppose that with the disintegration of complex molecules, which we must assume as taking place in the material of the walls of the protoplasm network, substances are formed which are subjected to electrolytic dissociation, the anions and kations hereby liberated must be diffused from the place of their separation into the surroundings. Their diffusion, however, is restricted by the protoplasmic network. The positive ions may pass through, but the negative ions may not. As a result: the reticulated substance is the seat of electric discharge, which in turn gives the impact to the breaking down of new molecules and with this to the occurrence of new potential differences, and so on, consequently the disintegration is extended further and further through the connected masses of the protoplasmic framework. [Illustration: Fig. 28. Protoplasm of different cells, showing foam structures. A--Pseudopod of a marine rhizopod. The protoplasm only shows foam structure at the point of stimulation. B--Epidermic cell of lumbricus. C--Nerve fiber. D--Part of the cell body of a ganglia cell. (A-C after _Bütschli_, D after _Held_.) ]