388 HELEN DEAN KING. 



either runts or embryos that have resulted from superfecunda- 

 tion, in the majority of cases they are pathological, probably 

 because of faulty implantation of the ova. If such pathological 

 embryos are ever born with the rest of the embryos they are 

 destroyed at once by the mother, as they are never found among 

 the normal newborn young. 



3. CASES OF SUPERFCETATION. 



It has been maintained by many physicians that ovulation 

 does not occur during pregnancy and therefore that the concep- 

 tion of a second foetus by a pregnant woman is impossible. 

 Cases seemingly those of superfcetation have been ascribed to 

 the presence of a bifid uterus, or they have been assumed to be 

 the result of a twin pregnancy in which one foetus was blighted. 

 Undoubtedly many so-called cases of superfcetation can properly 

 be attributed to one or to the other of these causes, but there are 

 a number of well-authenticated cases, such as those cited by 

 Bonnar ('65), which seem explicable only on the assumption 

 that superfcetation can occur in woman under exceptional 

 conditions. 



In the lower mammals superfoetation is seemingly of rare 

 occurrence. An examination of the evidence shows that many 

 cases that have been reported as due to superfcetation are un- 

 questionably instances of superfecundation or of blighted ova. 

 A very probable case of superfcetation in sheep was reported by 

 Arrowsmith in 1834, while the observations of Christopher ('86) 

 show that in the cat ovulation can occur during pregnancy and, 

 therefore, that superfcetation in this species is possible, although 

 it is not known to have taken place. 



In the course of my investigations on the rat I have found two 

 cases which are seemingly due to superfcetation. Female 4^29 

 had a litter of four young born on December 20, 1910. The male 

 was removed when the litter was discovered and the nest was 

 left undisturbed for fourteen days, when the litter was examined 

 for the sex ratio. At this time the four rats born on December 

 20 were well developed, and they weighed from 15.5 gm. to 16.4 

 gm. each. In addition to these rats the nest was found to con- 

 tain seven very small rats that had apparently been born only a 



