302 E. R. HOSKINS 



King, and others. Owing perhaps to their diet and very favor- 

 able environment, most of the animals (especially the younger 

 male groups) are somewhat larger than the average ordinary 

 albino rats at corresponding ages. Twenty-nine litters were 

 used but where litters contained less than four rats of a sex 

 these individuals were usually rejected. A few rats were killed 

 by the mother before weaning time. In all, 59 females and 73 

 males were fed and of these all of the females and 59 of the 

 males were carefully autopsied at the termination of the experi- 

 ment. While the number of observations is not large from 

 the statistical point of view, and more would be necessary for 

 final conclusions, it is believed that the present data are suf- 

 ficient to establish certain points with a considerable degree of 

 probability, and to furnish valuable evidence upon other points 

 requiring further data. Most of the older animals which were 

 born during the summer and early fall were smaller at the be- 

 ginning of the experiment than were those born in the winter 

 and spring. This is in accordance with previous observations. 

 These initial differences generally persisted throughout the 

 period of experiment (as likewise found by King ('15). Of the 

 older litters, two of which were of purely local stock were quite 

 large rats, whereas three litters which were of pure 'Wistar' stock 

 were considerably smaller. Other litters used were of a mixture 

 of these two strains. 



The rats with few exceptions were weaned at 3 weeks of age, 

 and kept in a well ventilated room in fairly large wire net cages, 

 with wire net bottoms which allowed waste matter to drop 

 through. The males and females were of course separated. All 

 were fed (ad libitum) upon whole wheat (Graham) bread soaked 

 in whole milk, a diet which seems to provide abundant nourish- 

 ment, as shown by the rapid growth of the animals. During 

 the first part of the investigation the rats were fed once a day 

 and their water jars washed once a week. Later all of the ani- 

 mals were fed 3 times a day, the water jars washed daily and 

 at all times the cages were kept clean. Each animal was given 

 a mark of identification with picric acid and a separate growth 

 record kept for it. In general, each rat was weighed at wean- 



