SUPERFETATION AMONG MICE. 283 



have appeared in females which had earlier merely undergone 

 insemination, without a normal conception having first ensued? 

 As a matter of fact, I have never found an instance, among all the 

 births recorded, in which a first brood 1 is known to have been 

 born more than 22 days after the last possible opportunity for 

 coition. Why should not the spermatozoa have occasionally 

 been held in reserve for a period of ovulation occurring some 

 time after this last opportunity for copulation? In reply, I can 

 only point out that coition probably occurs only when the 

 female is in a condition of "heat." If, on the advent of the first 

 "heat" period after the introduction of the male, fertilization 

 did not follow insemination, it would, in a large proportion of 

 cases, point to a sterility (temporary, at least) on the part of one 

 or both sexes. As a matter of fact, my records show that the 

 great majority of the females which conceived at all did so 

 during the first half of the sojourn of the male in the cage. 2 

 As stated above, only one female in the course of my experiments 

 is known to have become pregnant as late as the last possible 

 date of copulation. 



To what degree the term superf elation is applicable to the cases 

 comprised in my first table is a matter of definition. Case no. I 

 (as well as the two cases described by Miss King) would doubt- 

 less be covered by the term as ordinarily understood, since one 

 period of gestation, with little doubt, was superimposed upon 

 another. For reasons stated, it does not seem likely, however, 

 that two sets of developing fetuses were simultaneously present 

 in any of the other cases. But in at least one of these instances 

 (no. 5), the sperm which fertilized the second lot of eggs must 

 have been received during or prior to the first pregnancy. Per- 

 haps the term superfetation might conveniently be extended so 

 as to cover such cases as these last. But the word is plainly 

 inapplicable to instances in which the second effective copulation 

 occurred .subsequently to the delivery of the first brood. 



Admitting the last named possibility for most of the cases 

 comprised in my Table I, we none the less find in them instances 



1 First, as distinguished from the supernumerary broods here discussed. I do 

 not refer necessarily to the first offspring of a given mother. 



2 This commonly covered 20 days. 



