THE CORPUS LUTEUM OF PREGNANCY IN SWINE. 91 



(2) In the sow, however, I have found a uterus in which there were T> fetuses, hut 

 only 4 corpora lutea in both ovaries. Such a case, if our first hypothesis he excluded, can 

 be explained only by the second, namely, that two of the pigs originated from a polyovular 

 Graafian follicle, and this latter explanation is based on anatomical facts. Follicles con- 

 taining more than one ovum have been known for many years, and have been recorded 

 from man, the cat, dog, sow, certain bats, and other animals. Ancel (1903) thinks they are 

 especially common in the dog, in which he has found follicles with 2, 3, 4, 5 ova. Arnold 

 (1912), in an interesting report with review of the literature, describes a human case in 

 which a large proportion of follicles were polyovular, some of them having as many as 18 

 ova. The anatomical origin of the condition is still obscure. The question arises whether 

 the phenomenon is sufficiently common to account for the fact that in swine 37 per cent of 

 uteri have more pigs in one horn than there are corpora lutea in the corresponding ovary. 

 Schmaltz (1911) states that polyovular follicles are seen in almost every preparation of the 

 sow's ovary, sometimes having as many as 6 ova, 4 to a follicle being frequent, but he 

 thinks that such follicles are usually doomed to become atretic and rarely attain maturity. 

 In sections I personally have found only 2 polyovular follicles, one of them with 2, and the 

 other with 3 ova, and these follicles were both small and undeveloped. The members of a 

 class in histology were requested to watch for the condition when studying the ovary of the 

 sow, and one of the students found and showed me 2 ova which he had discovered in a large 

 ripe follicle. The conclusion is that some of the 43 cases under discussion may have been 

 due to the fertilization of ova from polyovular follicles, but that the latter certainly do not 

 occur frequently enough to explain all the cases, since for this it would be necessary for at 

 least one follicle to be polyovular in one-third of all the groups of follicles maturing at once. 



(3) On the other hand, migration of the ovum is a well-attested fact, in the human 

 subject, and has been produced experimentally in animals by Leopold (1880), who excised 

 one ovary and the opposite tube, and found that the animals could still become pregnant. 

 Howard A. Kelly performed the same operation in a human patient, for the cure of disease, 

 and the woman later became pregnant. Other interesting examples have been mentioned 

 by J. Whitridge Williams, who found the corpus luteum in one ovary and the embryo in the 

 opposite tube, in 5 out of 30 carefully recorded cases of extra-uterine pregnancy. These 

 are all cases of external migration. The occurrence of internal migration has never been 

 conclusively proven. The frequency of external migration of the ovum is difficult to esti- 

 mate in the human subject, since it is demonstrable only in the presence of some abnormal 

 condition, such as the lack of one tube and the opposite ovary, a bicornate uterus, an extra- 

 uterine pregnancy, or the like. Mayrhofer (1876) estimates tentatively that migration 

 must occur at least once in every ten ovulations in the human female. 



That it can take place in the sow I think is proven by four uteri of my series, in each of 

 which one ovary contained no corpora lutea, and yet in two of the cases the side of the uterus 

 corresponding to the ovary without corpora lutea contained 1 fetus, in the other two cases 

 3 fetuses. In all four cases the total number of corpora lutea in both ovaries accounted for 

 the total number of pigs in both sides of the uterus. In all likelihood we can explain most 

 of our cases by external migration. The point which I wish to bring out is not that migra- 

 tion is a possibility — that has been shown many times — but I want to emphasize its great 

 frequency. For every time it can be demonstrated in the sow, there must be many more 

 times when it can not be detected; for instance, when ova migrate from both ovaries, and 

 hence the condition is not apparent. It is a very conservative estimate if we suppose that 

 one or more ova migrate in 50 per cent of all ovulations in the sow. In a sense, the term 



