These studies leave no room for doubt that all the amino-acids necessary for the nutrition of an animal are contained in the proteins found in each of these foods. Certain of these are, however, present in such limited amounts as to restrict the extent to which the remaining ones, which are more abundant, can be utilized. It is for this reason that these proteins are of relatively low biological value unless supplemented by proteins from other sources, the constitution of which is such as to make good their deficiencies. The proteins of the cereal grains are of lower value than are those of milk and eggs, or than those of certain mixtures obtained by combining two or more of these seeds. ■

In this chapter an attempt has been made to make clear the types of experiments which have been employed for the study of the differences in the nutritive values of proteins from different sources, and to determine the exact nature of the limiting deficiencies of each. The experimental work on which this discussion is based constitutes the most confusing phase of the literature relating to nutrition, for the reason that, although the specific problems which have been investigated have in great measure been cleared up, the original papers have, in a number of cases, failed to clearly indicate how the data later recorded modified or rendered obsolete earlier observations made by the investigators themselves. Frequent references, by persons who have made no thorough study of the literature to supposedly proven facts which were later disproved by other experiments, have convinced the author that an interpretation was here especially desirable. This is very difficult to make in an entirely clear manner without exceeding the space which could be allotted to it.