GESTATION OF THE ALBINO RAT. 381 



34 days it is evident that the suckling of more than five young 

 prolongs the period of gestation in the albino rat. The length 

 of the gestation period is not, as a rule, directly proportional 

 to the number of young suckled. For, assuming 22 days to be 

 the average length of the gestation period, only three of the 

 twelve cases fulfill the conditions of Daniel's law. In the 

 majority of cases shown in Table III. the length of the period of 

 gestation seems to have been little affected by the number of 

 young in the second litter. For instance, female 7^75, with six 

 young in her first litter, had a gestation period of 26 days before 

 the birth of a second litter containing three individuals; on the 

 other hand, females 5^1 and 4.640, each suckling six young, also 

 had gestation periods of 26 days when there were nine and ten 

 young respectively in their second litters. In some few cases, 

 however, the gestation period seems to be further prolonged if a 

 lactating female is carrying a very large litter. This is indicated 

 by the fact that the two longest gestation periods yet recorded, 

 33 and 34 days, were found only when the second litter contained 

 an unusually large number of individuals. 



The data given in the above tables show that the period of 

 gestation in lactating albino rats varies from a minimum of 21 

 days to a maximum of 34 days. The length of the gestation 

 period seems to depend, primarily, on factors that affect the 

 nutritive conditions of the embryos, but it is also influenced, 

 to some extent, by the individual peculiarities of the females. 

 Lactating females may have different periods of gestation when 

 the number of young in each of their litters is the same, as is 

 shown in the case of females $A A and <\A 29 . Each of these females 

 had four young in her first litter and seven young in her second 

 litter, yet the former had a gestation period of 28 days before 

 the birth of her second litter, while in the latter female the 

 gestation period was only 25 days. 



A comparison of the data given in Table II. with that shown 

 in Table III. indicates that in different lactating females the 

 period of gestation is not as extended when five or less young 

 are suckled as when this number is exceeded. This rule seems 

 to apply equally well also to the different litters borne by the 

 same female. Female 7^76, when suckling five young, had a 



