These records of nursing mothers and their young make it apparent that the former are limited in the utilization of food for milk production, in the same manner as the growing young are limited in the utilization of food for the construction during growth of new tissues. The mother is to no small degree, however, a factor of safety for the young. It should be remembered that the young rat cannot grow at all when after weaning it is confined to the oat kernel alone or to this cereal supplemented with protein, fat-soluble A, or a salt mixture alone, or on a diet in which oats are supplemented with both protein and fat-soluble A. In order that it may grow even very slowly it is essential that both a salt mixture and fat-soluble A be added to the oat. It is not possible for it to grow normally on such a mixture unless protein is also supplied.