[97] In the different experiments of Nathansohn (*Jahrb. wiss. Bot.* 38, 1902, and 39, 1903) the salinity of the medium was changed in such a way that there was in each case either an abnormal increase or an abnormal decrease in the concentration of one single ion necessary for metabolism. The cell was found to stand these abnormal changes in such a way that in the case of the increase of the concentration of the medium it did not allow more than a certain amount of the ion in question to come in, and that in the case of the decrease it did not allow more than a certain quantity of the ion to go out. It thus seems as if the permeability of the surface were adjusted to a certain minimum and to a certain maximum of every single ion or salt, the permeability being stopped from within to without, whenever the minimum, and from without to within, whenever the maximum is reached in the cell sap; both irrespective of proper physical osmotic equilibrium (“Physiologisches Gleichgewicht”). Thus, in fact, there only would be a case of primary regulation, nothing more. It would all appear rather similar to what occurs in the kidney. Of course we do not assert that our explanation is right, but it is possible and is at the same time the most simple, and it is our general practice always to prefer the most simple hypotheses. There are many regulation phenomena connected with osmotic pressure and permeability in animal physiology also, though at present they are not worked out as fully as possible. The works of Frédéricq, J. Loeb, Overton and Sumner[98] would have to be taken into account by any one who wished to enter more deeply into these problems. We can only mention here that permeability to water itself also plays its part, and that, according to Overton’s experiments, it is a kind of solubility of the media in the very substance of the cell surface on which all permeability and its regulation depend. [98] Many fishes are able to withstand great changes in the osmotic pressure of sea-water; the osmotic pressure of their body fluids, though never in a real physical equilibrium with the pressure of the medium, nevertheless may vary whenever the abnormal conditions of the latter exceed certain limits.