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

The cages being bottomless are readily cleaned. They are set on circles of
wire mesh over galvanized iron funnels permitting urine and feces to pass
through. A second screen over the collecting cup and of fine mesh
separates the feces from urine and also collects scattered food.]

In building up such a diet many experiments have been combined and thanks
largely to the efforts of Osborne and Mendel and McCollum in this country,
we have a thoroughly standardized procedure even extending to types of
cages and care best suited to normal growth and development. For clearer
appreciation of the nature of these diets and their preparation we have
summarized in the following pages the combinations used by the principal
contributors to the subject in this country.

[Illustration: FIG. 5. ILLUSTRATING THE USE OF THE CHATILLON SCALE FOR
RAPID WEIGHING OF ANIMALS

The dial is so made that it can be set to counterbalance the weight of the
cage and the weights read directly. This is also used for weighing food.]

[Illustration: FIG. 6. SAMPLE LABORATORY RECORD]

It is at once obvious from the table that the testing value of these basal
diets demands the absence of the two vitamines in the protein,
carbohydrates and fat fractions. To make sure of this absence various