39O HELEN DEAN KING. 



they were born only after a gestation period of about 35 days 

 also seems highly improbable. The most plausible explanation 

 for these cases seems to me to assume that the two ovaries acted 

 independently, ovulation occurring in one ovary some little time 

 before it took place in the other. If copulation followed each 

 ovulation two sets of embryos would develop in the uterus si- 

 multaneously, and they would be born at different times, depend- 

 ing on the interval between the two periods of ovulation. 



Conditions which retard ovulation from one ovary for a greater 

 or a less period of time and so make superfcetation possible must 

 occur very rarely in the rat, since the two cases described are the 

 only ones that have been found in the course of my experiments. 



SUMMARY. 



1. The normal period of gestation in non-lactating albino rats 

 ranges from 21 to 23 days. 



2. The gestation period in lactating albino rats is of normal 

 length if the female is suckling five or less young and is carrying 

 five or less young. 



3. 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. 



4. 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. 



5. In the albino rat ovulation takes place within a few hours 

 after parturition. 



6. Lactation does not delay ovulation, but the suckling of a 

 litter that contains six or more young seems to lessen the food 

 supply to the fcetal young and so retards their development. 



7. Superfecundation occurs occasionally in the albino rat and 

 causes an interval of two or three days between the birth of 

 different members of the litter. 



8. In rare instances ovulation takes place in the albino rat 

 during pregnancy and superfcetation occurs. In two cases of 



