2j8 F. B. SUMNER. 



Miss King (1913, p. 385) remarks that "superfoetation . . . 

 is applied to cases in which ovulation, followed by copulation, 

 occurred during pregnancy and led to the simultaneous develop- 

 ment in the uterus of two sets of ova belonging to different 

 periods of ovulation." In discussing the two instances observed 

 by her in the rat she says: "The most plausible explanation for 

 these cases seems to me to assume that the two ovaries acted 

 independently, ovulation occurring in one ovary some little 

 time before it took place in the other. If copulation followed 

 each ovulation two sets of embryos would develop in the uterus 

 simultaneously, and they would be born at different times, 

 depending on the interval between the two periods of ovulation" 



(p. 390). 



I believe, with the writers quoted, and in opposition to certain 

 others, that these younger fetuses owe their origin to later periods 

 of ovulation, which may even occur during gestation. I cannot 

 see the necessity, however, for assuming an independent action 

 of the two ovaries, as does Miss King. Furthermore, I regard 

 it as highly improbable that some of the cases which I have 

 recorded resulted from a copulation occurring at the time of 

 the later ovulation, whether this last took place during pregnancy 

 or after the birth of the first brood. 



My own observations point, with considerable probability, to two 

 facts: (i) to a definite periodicity in ovulation, continuing in some 

 cases throughout pregnancy; and (2), with even greater probability, 

 to the retention by the spermatozoa of their fertilizing power for days 

 or even weeks after reception into the uterus or fallopian tubes. 



In support of the first proposition I will point to the rather 

 surprising numerical relations among the figures (Table I.) 

 representing the intervals between the delivery of the two broods 

 by the same mother. These eight numbers are all nearly or 

 quite multiples of 13, viz: 



1 X 13 () 



2 X 13 



2 X 13, (+ I?) 

 2 X 13, (+ I?) 



