GESTATION OF THE ALBINO RAT. 



379 



i. PROLONGATION OF THE PERIOD OF GESTATION. 



According to a series of unpublished records kindly furnished 

 me by Dr. J. M. Stotsenburg, of The Wistar Institute, the time 

 between the copulation of a non-lactating albino rat and the 

 birth of her litter varies from 21 days and 15 hours to 22 days 

 and 1 6 hours. The normal period of gestation for the albino 

 rat, therefore, can roughly be estimated at from 21 to 23 days. 

 In the brown rat kept in captivity the period of gestation seems 

 to be somewhat longer than in the albino rat, as Miller found 

 that it varies from 231/2 days to 25^ days in different cases. 



Daniel's ('10) investigations show that in the mouse the 

 period of gestation, which is normally 20 days, is considerably 

 prolonged if the female is suckling young. From data obtained 

 in ten cases he formulates the following law: "The period of 

 gestation, in lactating mothers, varies directly with the number 

 of young suckled." In the course of my experiments records 

 have been made of the births of 31 litters borne by lactating 

 albino rats. These records, as shown in the three following 

 tables, indicate that the length of the period of gestation is 

 affected by the number of young suckled and by other factors 

 as well. 



Table I. shows the length of the period of gestation when 

 lactating females were suckling five or less young and carrying 



five or less young. 



TABLE I. 



In each of the eight cases cited above the period of gestation 

 can be considered as normal since it did not exceed 23 days. It 

 appears, therefore, that the period of gestation in lactating 

 albino rats is not extended if the number of young suckled and 

 the number of young in the second litter does not exceed five 

 in either case. 



