5i8 Journal of Comparative Neurology and Psychology. 



mated with the largest males in the laboratory. (These males 

 weighed about 400 grammes). The males were kept in 

 the cages until the females showed signs of pregnancy, 

 and were then removed. Twenty one days is the usual 

 length of the period of gestation. In nearly every case large 

 litters were born — in some cases the litters contained as many 

 as 14 young. At the end of twenty days the young were remov- 

 ed from their mother. She was then allowed to recover com- 

 pletely. This took, usually, from 30 to 50 days. When re- 

 covery was complete, the males were again introduced, and the 

 same routine followed, until each mated rat had born three lit- 

 ters. When recovery was complete from the birth of the third 

 and final litter, the experiment, so far as the body-weight was 

 concerned, was at an end. 



The unmated rats, of course, kept to the even tenor of 

 their way. The records previously referred to, show the nor- 

 mal rate of the body growth in the unmated females. In the 

 present experiment we again recorded the weight of unmated 

 females, in order to have a control series with which to com- 

 pare the mated rats. By subjecting both the mated and the 

 unmated rats to exactly the same conditions as regards food, 

 temperature, etc., we hoped to be able to isolate the influence 

 of breeding on the growth of the body. However, if one com- 

 pares the growth-curve of the unmated rats in this paper with 

 the records referred to above, it will be found that, notwith- 

 standing the better quality of food used in the present case, no 

 very great difference in the body growth in the two cases can 

 be observed. 



The following table presents for comparison the record of 

 the weights, at the ages given, of the mated and unmated indi- 

 viduals of Group C. This table gives a good illustration of 

 what occurred in the four other groups. 



Plate VI shows separately in graphic form the changes 

 in the body weight of each of the above mated and unmated 

 rats of Group C. 



A study of this table and plate shows the fluctuations in 

 the body weight of the mated rats, due to pregnancy, suckling 



