Studies of this character must of necessity be carried out with small laboratory animals, because it is difficult and laborious to prepare the purified foodstuffs in sufficient amounts for the conduct of feeding experiments. It is, however, highly desirable to shorten the length of such experiments as far as possible by the employment of short-lived omnivorous animals which grow rapidly and which quickly reach maturity. In this way it is possible to accumulate data sufficiently fast to make progress reasonably satisfactory. The domestic rat serves this purpose admirably. It has a gestation period of but 21 days and, under normal conditions, the young are ready to wean at the age of 25 days. The female, when well nourished, usually produces her first litter at the age of 90 days, and will, if properly handled, have five or more litters by the time she reaches an age of fourteen months, which age ordinarily marks the end of her period of fertility. The extreme span of life of the well-fed rat is about thirty-six months. By using such an animal it is possible to accomplish within a relatively short time the accumulation of data regarding growth and reproduction which it would take years to secure with domestic animals of large size, long period of gestation and long span of life. With the latter the expense would be prohibitive. In 1907 the author took up the study of the cause of the failure of animals to grow on mixtures of purified foodstuffs and employed the domestic rat as the experimental animal.