SUPERFETATION AMONG MICE. 2J9 



2 X 13, (+2?) 



2 X 13, (+2) 



3 X 13, (-2?) 



3 X 13, () 



It will be seen that some multiple of a quantity lying between 

 13 and 14 would fit every case with the exception of one, in 

 which the figure may have been slightly less than 13. Further- 

 more, let us add Miss King's two cases for the rat. In one of 

 these, the second-born brood was delivered "about 14 days" 

 (but not more than 14 days) after the first. In the other 

 case, the second delivery was, she says, "about 12 days" after 

 the first. It must be mentioned, however, that this "newborn" 

 brood was actually found 13 days after the other. 



Here, then, we have altogether 3 instances in which the broods 

 were born about 13 days apart, 5 instances in which the interval 

 was about twice as great, and 2 in which the interval was about 

 3 times as great. Statistically speaking, the case is not, of course, 

 entirely convincing, and later findings may fail to conform to 

 the scheme here indicated. But the probability seems to be 

 sufficiently high to warrant its provisional acceptance. 



Referring to the second table, it will be found that the inter- 

 vals therein recorded could not all be interpreted as multiples of 

 any one number, though four of them are not far from being 

 multiples of 13. Were one disposed to have recourse to a not 

 uncommon type of argument, he could point out that the normal 

 variability of the single intervals might reasonably be expected 

 to lead to a cumulative divergence in the course of five or six 

 periods. But most of us would be reluctant to draw any con- 

 clusions from this second set of figures. 



Regarding the existence of a definite periodicity in the ovula- 

 tion of mice, there appears to be some difference of opinion. 

 Most recent observers seem to agree that ovulation occurs 

 independently of coition in the mouse and rat. Sobotta (1895), 

 Kirkham (1910, 1913), and Long and Mark (1911) hold that it 

 takes place not long after parturition. According to the latter 

 authors the interval between parturition and the ensuing ovula- 



