8 HELEN DEAN KING 



that were to be reared for breeding purposes. There was no 

 culHng of the less desirable individuals, however, and all mem- 

 bers of every litter were reared and weighed at the ages noted. 

 When the animals became mature the largest and most vigorous 

 pair in each litter was usually used for breeding. 



As the growth of very young rats depends largely on the 

 amount of nourishment that they receive from their mother, 

 litters of medium size, containing from five to eight young, 

 were, as a rule, those selected for weighing. Such litters, more- 

 over, represent the general run of individuals in a colony more 

 fairly than do very large or very small litters in which animals 

 with extreme body weights are often found. It was not always 

 possible to weigh adult animals at exactly the ages designated in 

 the various tables, but the weighing of a litter was omitted if it 

 could not be taken within one week of the time specified. 



In weighing experiments of this kind there is always an un- 

 avoidable error due to the presence of a greater or a less amount 

 of undigested food in the alimentary tract. To obviate this 

 source of error as far as possible the rats were weighed in 

 the morning before they had received their daily food ration. 

 Animals that were obviously ill and females known to be preg- 

 nant were never weighed, while the weight of suckling mothers 

 was not recorded if it was below the previous record. In the 

 rat pregnancy cannot be detected with certainty until about 

 the thirteenth day, so undoubtedly many gravid females were 

 weighed unknowingly during the course of this investigation. 

 The increase in the body weight of a female as the result of 

 pregnancy cannot be very great up to the thirteenth day, how- 

 ever, since Stotsenburg ('15) has shown that the weight of a fetus 

 at this time is only 0.04 grams. Errors in the records due to the 

 inclusion of the weights of pregnant females were doubtless 

 balanced by the weights for animals that were in early stages 

 of pneumonia when there was no external evidence of the disease. 



The present paper gives data showing the weight increase 

 with age in 333 males and in 30)5 females belonging to the first 

 fifteen generations of the inbred group. Altogether these gen- 

 erations comprised a total of 1601 litters, containing 11,057 



