I. LIFE PROCESSES 13 



In each generation about fifty individuals of each sex were 

 reared for breeding and for a study of various life processes. 

 The litters selected for this purpose contained an equal, or 

 nearly equal, division of the sexes, and they were alway- of 

 medium size (four to seven young) in order to avoid tin- 

 possible effects of extreme litter size on body growth. Mor- 

 tality among gray rats is often very heavy during the first 

 few weeks of postnatal life, due to a variety of causes. In 

 each generation, therefore, more litters were reared than the 

 requirements of the experiment demanded, and those wen- 

 discarded in which any individual died before reaching sixty 

 days of age. Such a method of selecting breeding stock auto- 

 matically eliminated weak individuals and assured strong, 

 vigorous animals as parents of the following generation. 



All of the captive Grays in the first ten generations lived 

 under the same conditions of housing and of nutrition. Their 

 food was a well-balanced, cooked ration composed of various 

 cereals, meat, and vegetables; raw food, such as lettuce, car- 

 rots, cabbage, and fruit, was given two or three times a week. 

 Variations in temperature to which these animals were sub- 

 jected were not great. The colony room is adequately heated 

 in winter and is relatively cool in summer. The cages that 

 housed these rats were very large and therefore gave plenty 

 of opportunity for exercise. The only unfavorable conditions 

 affecting these rats were those referred to in the introduction. 



All rats in the litters saved for breeding were weighed 

 periodically. Birth weights were recorded if the young were 

 obtained before they had suckled, otherwise the first weigh- 

 ing came at the age of thirteen days. The rats were weighed 

 again when weaned, at thirty days of age, and thereafter at 

 intervals of one month until they attained the age of twenty 

 months. At this time a definite number of males wa> triv.-n to 

 Doctor Donaldson for dissection, together with any females 

 that had not cast a litter for at least two months I-', males 

 that were still breeding were kept for a lonirer time, in order 

 that the litter records might cover tin- entire reprodnetive 

 period. 



