ALLEGED OCCURRENCE OF SUPERFETATION. 247 



ing the occurrence of superfetation, even in mammals other than man. More- 

 over, as far as man is concerned, the large amount of literature on the subject 

 furnishes no cases more convincing than that of Calderini (1909). Most of them 

 can be easily recognized, from the reports alone, as probable instances of twin 

 pregnancy, in which one fetus died and then was retained until the birth of the 

 other. Confusion is due largely to the fact that dead fetuses often are retained 

 for a considerable period without showing very evident changes in form or gross 

 structure. This seems to be due, in part at least, to retardation in the lytic proc- 

 esses in embryonic as compared with adult tissues, a phenomenon which has 

 been referred to the slow post-mortem development of acidity in embryonic tissue. 



Since external form is mainly relied on as a criterion by the general practi- 

 tioner, it is easy to see how misinterpretation of the facts arises, and it is significant 

 that it is only the specialist who has come to doubt that superfetation ever occurs 

 in human kind. This assumes, however, that by superfetation is meant the partly 

 simultaneous intrauterine development of two or more fetuses derived from ova 

 liberated by independent ovulations separated from each other by a considerable 

 period of time ; or, in brief, the superposition of one pregnancy upon another. 



Since most reports on superfetation concern fetuses in the later months of 

 pregnancy, it occurred to me that a representation of certain instances of twin 

 pregnancy from the earlier months of pregnancy might be of special interest, 

 because such specimens could more easily and rightly be taken for examples 

 of true superfetation. In the cases under consideration, one fetus evidently 

 died some time before the abortion of both, a circumstance that resulted in con- 

 siderable differences in age and form between the two. An examination of the 

 accompanying cases will make it clear that all the instances here reported easily 

 could be included among the alleged instances of superfetation found in the litera- 

 ture without doing the least violence to that literature. More than that, the in- 

 stances here reported would form fine examples among such cases. 



The youngest specimen contained in the Carnegie Collection which came 

 to my attention, and which might be regarded as one of superfetation, is No. 587, 

 donated by Dr. F. A. Conradi of Baltimore. The cavity of the larger chorionic 

 vesicle in the fresh specimen measured 32 by 23 by 23 mm., and that of the smaller 

 about 6 by 6 by 9 mm. The latter was empty, but the former contained a cylindrical 

 embryo 7 mm. long which Mall found greatly dissociated with a practically solid 

 brain and only a remnant of the intestine. The amnion was present, and as shown 

 in figure 31 (plate 4, Chap. IV), both vesicles formed a single mass which was 

 covered by infiltrated decidua. That these chorionic vesicles both once contained 

 an embryo, no embryologist will doubt. It is evident also that both were retained 

 for some time after life and growth had ceased. The gross and histologic appear- 

 ance of the entire specimen alone is sufficient proof of this. Although the smaller 

 chorionic vesicle was incorporated in the margin of the larger, its death did not 

 lead to its abortion or to that of the surviving twin. Yet the smaller of these 

 twins undoubtedly died a considerable period before the larger. This is indicated 

 also by an examination of the specimens and by the presence of decidua between 



