McCollum and Simmonds (15) have studied the effects on the rate of growth of young rats, of different types of deficiencies in the diet of the lactating mother. The results are shown in Tables XI, XIII and XV. In Table XV are shown the weights of litters of young containing different numbers and at various ages during the nursing period, as well as later after they became able to supplement the mother's milk with the diet on which she had nursed her young. These weights serve as normals for comparison with the weights of the young of other mothers whose diets were faulty in some respect. Table XIII shows the effects on the young of depriving the lactating mother of sufficient calcium.

Although the data in the tables are self-explanatory, a few cases of marked contrast may be mentioned. Rat 2153 had five young and was given a satisfactory diet while nursing. At fifteen days from birth their collective weight was 137 grams. Rat 2365 likewise had five young. She was fed a diet containing but 9 per cent of protein, two-thirds of which was derived from barley and one-third from navy beans. When the young were eighteen days old they weighed collectively but 70 grams. The protein moiety of this mother's diet was of rather poor quality and she was unable for this reason to secrete a satisfactory milk supply.

Rat 2767 on a normal diet of good quality had a litter of six young. At the age of nineteen days they weighed collectively 163 grams. On the other hand, rat 2785 on a diet too poor in calcium had six young, which at the age of twenty days weighed altogether but 73 grams. The tables afford a number of such contrasts in the state of development of the young where the mothers ate all their appetites called for of diets comparable in certain cases in composition to diets employed by people in the United States. It is a highly significant fact that a number of these diets were unsatisfactory for the proper nursing of young.