GESTATION PERIOD 21 



Lengthening of gestation period. King ('13) found that the 

 gestation period in lactating albino rats is of normal length if 

 the female is suckHng five or less young and is carrying five or 

 less young. 



The gestation period may be prolonged from one to six days 

 if an albino female, suckling five or less young, is carrying six 

 or more young. 



The period of gestation is always prolonged when a female 

 is suckling six or more young. In these cases the number of 

 young in the second litter seems to have less influence on the 

 length of the gestation period than has the number of young 

 suckled; but if both litters are very large the gestation period 

 may be extended to 34 days. 



In the specially well fed series of Greenman and Duhring ('23) 

 the lengthening of the gestation period under these conditions 

 was much reduced. 



5. S'uperfecimdation and superfetation. Superfecundation oc- 

 curs occasionally in the albino rat and causes an interval of 

 two, three or more days between the birth of different members 

 of the litter (King, 13). 



In rare instances ovulation takes place in the albino rat during 

 pregnancy and superfetation occurs. In two cases of this kind 

 Utters have been produced at intervals of about two weeks 

 (King, '13, pp. 388 and 389). 



Parturition. The young tend to be born when the surround- 

 ing conditions are quiet. In many cases birth occurs at night 

 or in the early hours of the morning. The act of parturition 

 may extend from 15-30 minutes up to 24 hours. Naturally 

 the size of the litter affects the time taken. 



There is no separation of the pubic bones (Todd, '23) and the 

 act of parturition does not appear to cause the female discom- 

 fort. 



The placenta is eaten. The placenta probably furnishes a 

 growth promoting substance to the milk, as shown for man by 

 Hammett ('18). 



6. Fertility and birth weight. In considering fertiUty it must 

 always be kept in mind that the reproductive capacity of the 



