necessary for its formation is always at its disposal in sufficient quantity. Otherwise the capability of action of the organism would be impaired at every moment or at least suffer great fluctuations. [74] _Max Verworn_: “Ermüdung, Erschöpfung and Erholung der nervösen Centra des Rückenmarks.” Arch. f. Anat. u. Physiol. physiol. Abt. Suppl. 1900. In accordance with this we must suppose that under physiological conditions all those substances required to replace the disintegrated molecules are always present in the cell in sufficient quantity and suitable form to replace at once those lost by excitation. Further, without doubt, in the organism which is always aërobic, oxygen must be present in certain quantities to assure at any moment oxygen replacement following oxydative disintegration, to guarantee sufficient amount for succeeding stimulation. A further question arises: How is it that the material lost in disintegration is always replaced in just sufficient quantity to establish the metabolic equilibrium? In short, how are we to understand in a mechanical sense the self-regulation of metabolism? In the preservation of metabolic equilibrium, we have a process before us, the principle of which is nowadays restricted to living substance. In my “Biogen hypothesis,”[75] I have associated the self-regulation of metabolism with the chemical equilibrium in interreacting masses. I have considered the metabolic self-regulation as the expression of the formation of a mass equilibrium between the quantity of foodstuffs and the quantity of a hypothetical combination of living substance, the _biogen_, which continuously disintegrates and builds up again of its own accord. In fact, however, we have in the chemical equilibrium of reacting mixtures in the non-living world, a principle which is completely analogous to the self-regulation in living substance. The chemical facts are, indeed, well known. If we take the classical example of the formation of ethylacetat from acetic acid and alcohol, we have a case of an inanimate system, in which the amounts of the reacting substances are in constant equilibrium. The reaction following the mixture of equal amounts of alcohol and acetic acid is as follows: [75] _Max Verworn_: “Die Biogenhypothese.” Jena 1903. Compare also _Max Verworn_: “Allgemeine Physiologie.” V. Aufl. Jena 1909. 1/3 Mol. C_{2}H_{5}OH + 1/3 Mol. CH_{3}COOH = 2/3 Mol. CH_{3}COOC_{2}H_{5} + 2/3 Mol. H_{2}O.