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 methods have be devised to attain the maximum purity. The authors recommend the following procedure: _a_. To purify the casein or other protein used. Boil the protein three successive times (it is assumed that the original is already as pure as it is possible to obtain it by the usual methods of preparation) for an hour each time, with absolute alcohol, using a reflux condenser to prevent loss of alcohol. Filter off the alcohol each time by suction. This process will take off all the adherent fat and hence all the "A" vitamine that might be present. The casein is then dried and ready for use. In certain experiments the authors use meat residues instead of a single protein. This they prepare as follows: Fresh lean round of beef is run through a meat chopper and then ground to a paste in a Nixtamal mill, stirred into twice its weight of water and boiled a few minutes. The solid residue is then strained, using cheese cloth, pressed in the hydraulic press and the cake stirred into a large quantity of boiling water. After repeating this process of washing with hot water the extracted residue is rapidly dried in a current of air at about 60°C. This dried residue may then be further purified with the absolute alcohol treatment as described for casein. _b_. To purify the carbohydrate they treat starch in exactly the same way as the casein. _c_. To purify the lard. This is melted and poured into absolute alcohol previously heated to 60°C., cooled over night and filtered by suction. This process is repeated three times and the resulting solids dried in a casserole over a steam bath.