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