LIFE PROCESSES IN CAPTIVE GRAY RATS 31 



Females in the first nine generations of captive grays cast, 

 on the average, 3.69 litters each, and the average litter con- 

 tained 6.17 young (King and Donaldson, '29: table 7). Data 

 for litter production and for litter size in later generations 

 are given in table 8. 



The average number of litters produced by females of the 

 tenth generation (4.81) was greater than that of females in 

 any preceding generation. Subsequently, litter production 

 increased slowly until the nineteenth generation, when females 

 cast an average of 10.18 litters (table 8). The slight decline 

 in litter production during later generations can be attributed 

 mainly to various changes in the location of the colony which 



Numt>' Mtrs per Uma'e 



Fig. 9 Average litter production of females in different generations. 



adversely affected fertility as well as the body growth of the 

 rats. With this rise and fall of litter production there was 

 a corresponding change in the number of young cast, conse- 

 quently average litter size in any generation did not differ 

 materially from the mean (6.10) for the entire series (table 8). 

 The general trend in litter production during the course of 

 this investigation is depicted in figure 9. 



STERILITY 



The infertility of wild rats brought to the colony at the 

 beginning of this investigation and the high percentage of 

 sterility in females of the early generations of this strain 

 were discussed in the former report. 



Later work has tended to support the hypothesis, previ- 

 ously advanced, that sterility in wild rats when first captured, 



