40 HELEN DEAN KING 



effect on a large series of litter data, since the stillborn form 

 but a small percentage of the young cast, as a rule. In 968 

 litters of gray rats obtained at birth and known to be com- 

 plete, the stillborn comprised but 1.2 per cent of the 6295 

 individuals (King, '35). This finding accords well with that 

 of 1.3 per cent stillbirths in 31,670 albino rats (King, '21). 

 Since only 0.05 per cent stillbirths were recorded through the 

 first twenty-six generations of gray rats, it is possible many 

 stillborn young were devoured by adults in the cage before 

 the litters were examined. In a former paper (King, '21) it 

 was estimated that the normal percentage of stillbirths in any 

 strain of rats reared under favorable conditions of environ- 

 ment and nutrition is probably not greater than 2 per cent. 

 If, therefore, the number of young in the final summary of 

 table 11 is increased to include 796 (1.5 per cent) stillborn 

 young that, conceivably, might have been omitted from the 

 records, with the sexes proportioned to give approximately 

 the ratio among the 268 stillborn young recorded for the strain 

 (111.8 males to 100 females), the data so adjusted would raise 

 the final sex ratio in table 11 from 97.3 to about 97.5 males to 

 100 females. Obviously the low sex ratio found in gray rats 

 cannot be ascribed to the omission of stillborn young from 

 the records, unless the number of such individuals greatly 

 exceeded 2 per cent of the total and the sex ratio among them 

 was unusually high. There are no valid grounds for either of 

 these assumptions. 



In an investigation of the factors controlling fertility in 

 the rabbit, Hammond ( '14) found that the number of atrophic 

 fetuses in females of various domesticated breeds greatly ex- 

 ceeded that found in gravid wild females. He suggested that 

 possibly one of the effects of domestication on this animal has 

 been to increase the number of ova shed at each period, and 

 at the same time to reduce the proportion of those which 

 develop. In the gray rat, captivity over a considerable period 

 of time has greatly increased fertility, but has not changed 

 average litter size (table 8). If the number of ova shed at 

 each period has been increased by domestication, then fetal 



