272 F. B. SUMNER. 



brood. In one case of which I have record (Table II., No. i), 

 this may well have been the true explanation, since the second 

 conception must have occurred when the three males which 

 constituted the first brood were about 42 days old. That male, 

 and even female, white mice may become sexually mature at 

 this age I am certain from independent evidence. I have 

 record of at least one case in which a female, mated to a male of 

 the same brood, became pregnant at an age of about 6 weeks 

 (between 38 and 44 days). 



In one instance, however (Table I., no. 6) such an explanation 

 was not applicable, owing to the extreme youth of the first litter, 

 at the time of the second conception. On March u, 1909, a 

 female gave birth to a brood of two (i cf, i 9). On April 8, 

 or 28 days later, a second brood of seven was born. Since the 

 period of gestation of this animal is normally about 20 days, 1 

 the mice of the first litter were scarcely more than a week old 

 when the second lot began their development. It is needless to 

 point out that a blind and helpless young male of this age could 

 not have played the part of a father. Furthermore, the female 

 had been separated from all other adults of either sex, since the 

 day of the birth of the first brood of young, 28 days previously. 

 Under these circumstances, to be sure, the possibility is not 

 excluded that the second litter resulted from a copulation 

 occurring immediately after the birth of the first. In this event, 

 the only anomaly would be the deferred birth of the later brood 

 of young. 



Unfortunately, no further notes on this subject were made 

 during my experiments on white mice. It is my impression 

 that other instances similar to the foregoing were observed, but 

 I find record of only these two cases. 



It is the chief object of the present paper to furnish data 

 relating to superfetation and deferred fertilization which have 

 been recorded incidentally during the past two years, in the course 

 of rearing wild mice of the genus Peromyscus. Unfortunately, 

 my data are in some cases somewhat equivocal, since the experi- 

 ments were not made with a view to studying these particular 

 problems. But definite answers can none the less be given to 

 certain questions, even if not to others. 



1 My own observations confirm those of Daniel (1910) on this point. 



