In this reaction there is an alteration only in the absolute quantity of the individual constituents but never in the relative amount. In the living system we have a completely analogous instance, which apart from its course differs from the inanimate example merely in the following points: In the first place, certain quantities of substances reacting on each other are continually introduced into and certain reaction products continually removed from the living system. Secondly, the reacting mixture of the living substance is not homogeneous, and at the same time is more complicated than that of the inanimate example. Thirdly, the sum total of the reaction is not reversible in its entirety. The question arises, should any essential difference between metabolic self-regulation and the maintenance of chemical equilibrium be assumed upon this statement? I must confess that this does not appear to me to be the case. The fact that organisms exist in a stream of substances by which their nutrition is introduced and the metabolic products removed, cannot have any influence on the state of equilibrium so long as the conditions are again and again replaced in the same manner. The equilibrium can only be influenced when the introduction of foodstuffs or the output of metabolic products is changed in value. Then they occur as the inanimate example, when various amounts of material are brought together. A new equilibrium takes place, having a higher or a lower mass level. This is also true in the living substance, in growth and in atrophy. The equilibrium is disturbed as happens in the inanimate reacting mixture, where different quantities of reacting substances are brought together. In both instances we have in principle a conformity of behavior of the inanimate and the living system. Secondly, as far as the greater complexity and inhomogeneity of the living reacting mixture is concerned, it is self-evident that this likewise does not constitute an essential difference, for we are acquainted with conditions of equilibrium in chemical reactions possessing a number of members and in inhomogeneous mixtures. Finally, the fact that the reaction in the living system is not totally reversible, forms no barrier to the assumption in principle of metabolic self-regulation as a chemical equilibrium. It is quite possible to conceive of a chemical equilibrium in a reacting mixture, of which only certain constituent processes are reversible, without the totality of the reactions as a whole being necessarily so. Let us assume, by way of example, that the assimilative processes of the metabolic chain are reversible, then under constant quantitative relations of foodstuffs, following every disintegration of assimilative products with removal of the decomposition products, the same amount of assimilatory processes is required for building up. And this is just that which we observe in metabolic equilibrium. Accordingly,