In America the comparison of vitamine content has been largely based on feeding experiments with the white rat. No other animal has been so well standardized as this one. Dr. Henry Donaldson of the Wistar Institute of Philadelphia has brought together into a book entitled _The Rat_ the accumulated record of that Institution bearing on this animal. This book provides standards for animal comparisons from every view point; weight relation to age, size and age, weight of organs and age, sex and age and weight, etc. This book together with the experience of many workers as they appear in the literature and especially the observations of Osborne and Mendel have made the rat an extremely reliable animal upon which to base comparative data. The omnivorous appetite of the animal, his ready adjustment to confinement, his relatively short life span, all contribute to his selection for experimental feeding tests. Another important reason for his selection is that being a mammal we may reasonably consider that his reactions to foods will be more typical of the human response than would another type, the bird for example. It is perhaps necessary to sound a warning here, however, and point out the danger of too great faith in this comparability of rat and man or in fact of any animal with man. In the case of the rat he has been found useless for the study of "C" vitamine for the simple reason that rats do not have scurvy. In general however his food responses to the vitamines, at least of the "A" and "B" types, have proved, so far as they have been confirmed by infant feeding, to be reasonably comparable. Provided with the experimental animal the next step was to devise a basal diet which should be complete for growth in every particular except vitamines. Such basal diets have been a process of development. The requirements for such a diet are the following factors: 1. It must be adequate to supply the necessary calories when eaten in amounts normal to the rat's consumption. 2. It must contain the kinds of nutrients that go to make up an adequate diet and in the percents suitable for this purpose. 3. It must contain proteins whose quality is adequate, for growth, i.e., which contain the kinds and amounts of amino acids known to fulfil this function. 4. It must be digestible and palatable. [Illustration: FIG. 3. TWO TYPES OF EXPERIMENT CAGES DEVISED BY OSBORNE AND MENDEL These are manufactured by the Herpich Co. of New Haven, Conn.] 5. It must be capable of being supplemented by either or both vitamines in response to the particular test it is devised to meet and when both are present in proper amounts it must produce normal growth and serve as a control. [Illustration: FIG. 4. A METABOLISM CAGE DEVISED FOR USE IN THE AUTHOR'S LABORATORY