SUPERFETATION AMONG MICE. 28 1 



that the fetuses developed at the normal rate, at least one sper- 

 matozoon must have retained its fertilizing power for 25 days or 

 more. In cases 7 and 8, even if we admit the probability here 

 of coition after parturition, the spermatozoa must have remained 

 alive for 15 and 14 days respectively. 



An alternative hypothesis would be to assume that gestation 

 rather than fertilization had been retarded in these cases. Thus 

 Schultze (1866), while not denying the theoretical possibility of 

 superfetation, explains all of the reported cases in man on the 

 basis of either the death or the retardation of the less advanced 

 fetus. This would also seem to be the opinion of Godlewski 

 (1914), who, after pointing out what he believes to be the physio- 

 logical impossibility of this process, makes the following dogmatic 

 assertion: "Wir mussen demnach annehmen, dass die Eier, 

 welche den Mehrgeburten den Ursprung geben, gleichzeitig 

 befruchtet werden" (781). On this assumption, we should have 

 to suppose that, at least in cases I and 5, the ova which gave 

 rise to the second brood had been liberated and fertilized simul- 

 taneously with those which gave rise to the first. In the remain- 

 ing cases, on the other hand, they may have been liberated and 

 fertilized after the birth of the first brood. 



Now Daniel (1910) has shown for white mice and Miss King 

 (1913) for white rats that parturition may be considerably 

 deferred when the mother is still suckling an earlier brood. This 

 retardation was found, in some cases, to be as much as 10 days 

 for the mouse and 12 days for the rat. Both of these writers 

 assume that fertilization ensues fairly promptly after successful 

 insemination, and that the delay in parturition is due entirely to 

 a prolongation of embryonic development. 



If we attempt to apply this interpretation to some of the 

 present examples, we are led into difficulties even greater than 

 those which confront the hypothesis of deferred fertilization. 

 For example, in case 5 (Table I.), the last opportunity for 

 copulation was 19 days before the birth of the first litter, and 

 about 47 days before the birth of the second. Even on the 

 assumption that the fertilization of the second lot of eggs actually 

 took place as late as this last day, the period of gestation was 

 prolonged by about 25 days, i. e., the normal term must have 



