A minute study of twins was made at the time of parturition, with special attention to the sex, the placenta, the membrane of the ovum, and the difference between the mono-oval and the bi-oval; in addition the type of the blood groups in the twins was examined from the standpoint of inheritance. The following is the general result: The type of blood group in twins is inherited from the parents as regularly as in the ordinary case.
Mono oval twins have the same type of blood group in all cases and are of the same sex, while bi-oval twins are in most cases of the same type but a minority are of different types. Bi-oval twins are in most cases of the same in sex and in blood group type. It has been proved that when the type of the blood groups in twins is different the circulation of the blood in the twins has been independently carried on in the fetal stage. It cannot, however, be concluded that twins have had a common blood-circulation in the fetal stage merely from examination of the blood-group type, even when the blood-group type in the twins is identical. 12 pairs of mono-oval twins, and 23 pairs of dual-oval twins, 35 in all, were examined. Of these 35 pairs of twins, 31 had the same blood type while 4 had different blood-group types. Of the twins of the same sex, 27 pairs had the same blood-group type, while only 2 pairs differed in this respect. Among the instances of twins of different sex, two pairs differed in the blood type while 4 pairs had the same type.
When the type of the blood group in twins was different, it could be recognized that two kinds of ova or spermatozooids had been concerned in the impregnation. When the blood group type in twins was different, both the parents, or one of the parents, was heterozygotic. The blood group type in twins is an interesting subject for investigation from the standpoints of biology, obstetrics and forensic medicine.