382 HELEN DEAN KING. 



gestation period of 22 days before the birth of a second litter 

 containing five young; when suckling a litter of six the period 

 of gestation was prolonged to 26 days although there were but 

 three young in the second litter. Why the suckling of six instead 

 of five young should invariably prolong the period of gestation 

 is not at all clear. Records of some 800 litters of albino rats 

 show that the average number of young in a litter is six. Seem- 

 ingly, therefore, the length of the gestation period is prolonged 

 when the number of young suckled equals or exceeds the number 

 that represents the average size of the litter in the species. 



Female 65| 7 had a gestation period of 34 days, the longest 

 period so far observed. This case is rather an interesting one. 

 The nine young born on May 24, 1912, were suckled until June 

 20, when it was noticed that the female was pregnant. As the 

 young rats were well developed and able to care for themselves 

 they were removed from the cage, although, under ordinary 

 conditions, a litter is allowed to remain with its mother for a 

 month. The female was watched closely, and parturition was 

 found to take place on the morning of June 27. There was thus 

 a period of one week after the removal of the first litter before 

 the second litter was born, although the normal period of gesta- 

 tion had been passed before the first litter was taken away. 

 Had the young rats in the second litter reached the proper stage 

 of maturity for birth when the first litter was removed it is very 

 probable that they would have been born immediately. In this 

 instance, therefore, lactation did not merely delay parturition 

 but it must have retarded the development of the embryos. 



The young rats in the second litter were examined shortly 

 after their birth. They were normal in appearance, but they 

 were very small, weighing not more than 4 gm. each. According 

 to Donaldson ('06), the weight of the albino rat at birth varies 

 from 4.2 gm. to 6.5 gm. As the size of the newborn rat depends 

 to a very considerable extent on the number of individuals 

 in the litter, no significance can be attached to the fact that in 

 this very large litter the rats at birth weighed less than 4.2 gm. 

 each. As it was not considered advisable to allow the female to 

 suckle so many young, six of them were killed at once. The 

 others developed normally, and opened their eyes at the usual 



