Methyl guanidin is a decomposition product of creatin, while guanidin, as can be seen from the formula, is like urea, excepting that the group NH replaces the oxygen of urea. These simple statements will suffice for our present purpose, viz., to indicate the more or less close chemical relationships existing between many of these nitrogenous decomposition products resulting from proteid katabolism; also to suggest how by slight chemical alteration one decomposition product may be resolved into another related substance in the processes of katabolism. Our conception of the processes involved in proteid katabolism is that of a series of progressive chemical decompositions, in which intracellular enzymes play the all-important part. The intermediary products formed are definite bodies because of the specific nature of the active enzymes, and, secondly, because of the chemical nature of the substances acted upon. In other words, oxidation in the animal body takes the shape of a series of well-defined chemical reactions, in which chemical constitution and specific enzyme action are the predetermining cause. In the absence of the particular chemical groups, the oxidase is unable to bring about oxidation, or, given the proper compound or mother substance in the absence of the specific oxidase, there is no oxidation. Hence, oxidation in the animal body is not the result of simple combustion, but, on the contrary, it consists of a series of orderly chemical processes, each one of which is presided over by an intracellular enzyme, specific in its nature, in that it is capable of acting only upon substances having a certain definite constitution, and leading invariably to a certain definite result. The processes which years ago were considered as due to the peculiar vital properties of the tissue cells, and which were supposed to be entirely dependent upon their morphological and functional integrity, are now seen to be due primarily to a great variety of enzymes, manufactured indeed by the living cells, but capable of manifesting their activity even when free from the influence of the living protoplasm. The varied processes of tissue katabolism are the result of orderly and progressive chemical changes, in which cleavage, hydrolysis, reduction, oxidation, deamidization, etc., alternate with each other under the influence of specific enzymes, where chemical constitution and the structural make-up of the various molecules are determining factors in the changes produced. CHAPTER III THE BALANCE OF NUTRITION