NOTES ON SUPERFETATION AND DEFERRED 

 FERTILIZATION AMONG MICE. 



F. B. SUMNER, 

 SCRIPPS INSTITUTION FOR BIOLOGICAL RESEARCH, LA JOLLA, CALIF. 



The phenomenon of superfetation, or the occurrence of a 

 second conception in a mammal already pregnant, has been 

 reported and discussed by various writers since the time of 

 Aristotle. The latter 1 refers specifically, in this connection, 

 only to man and the hare. Modern writers 2 have described 

 occasional instances for several animals (man, cat, rat, sheep). 

 On the other hand, this phenomenon appears to have been 

 generally regarded as a biological curiosity, while its very 

 existence has been questioned by some, 3 the facts observed being 

 explained on other grounds. Apparently, it has been very 

 rarely observed in the rat, for Miss King records only two cases 

 among the more than 700 normal broods born in her stock. 4 

 For mice, I have thus far found no published records of super- 

 numerary broods, though two well-known zoologists, who have 

 reared varieties of the house mouse in large numbers, assure me 

 that they have encountered cases of this sort. In the absence 

 of exact records, I am unable, however, to report these instances 

 in detail. 



In the course of several years' experience in breeding \vhite 

 mice, the present writer has occasionally noted the birth of a 

 second litter, in cases where an earlier litter had been kept with 

 the mother for some time after weaning. This was at first 

 attributed to the precocious sexual development of the earlier 

 male offspring, one of which was held to be the father of the later 



1 See bibliographic references. 



2 Marshall, 1910; King, 1913 (also papers cited by the latter). 



3 Schultze, 1866; Godlewski, 1914. 



4 1 do not here include those cases (known as superfecundation), in which 

 members of the same litter, derived from the same period of ovulation, are born 

 one or two days apart. 1, have myself observed a few cases of this phenomenon, 

 but shall not discuss them here. 



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