5^4 



Prolongation o£ Pregnancy 



by assuming that each siickhng young would prolong gestation by one day. 

 From this he concluded that the delay in implantation was due to an excessive 

 corpus luteum secretion during lactation. 



The present study is an attempt to elucidate further the hormonal mechan- 

 ism of delayed implantation in the lactating mouse. Since the data of earlier 

 workers on the effect of the number of young on the prolonged pregnancy 

 during lactation were in certain respects somewhat conflicting, it was deemed 

 necessary to study first this phenomenon as such. 



b 7 8 



Number of young lactated (x) 



Fig. 1. Dot diagram showing the relationship between prolongation 

 of pregnancy and number of young lactated. 



y = i.2o.\' + 0.30 



Methods. Albino mice, from two to four months old, were used. They were 

 kept in glass jars and given a milk-cereal diet and water ad libitum. Near term 

 the pregnant female was isolated with a male into a separate jar. When the 

 litter was found at the daily inspection, the date of the birth and the number of 

 young were recorded. The vaginal content was then examined for positive 

 mating on the two following days. When sperm were found the male was re- 

 moved and the female was left to lactate her litter and observed for the second 

 litter. The length of gestation was regarded as the time interval between the 

 date of the positive copulation (the finding of plug or sperm) and the date when 

 the following litter was found. The normal length of pregnancy in the non- 



