The observed embryology of human single-ovum twins and other multiple births

Abstract

ONG ago it was noticed that in some cases of human twinning the two inL dividuals resemble each other very closely, whereas in other cases they differ in features and in coloring, and may be of different sex. From about 1866 to 1880, several physicians and biologists in different countries independently developed the concept that the “identical” twins come from a single fertilized egg and the nonidentical or fraternal twins from two separate eggs.l Three circumstances obviously support this conjecture, and account for the ready way in which it has been accepted as a working hypothesis by physicians, biologists, and laymen, in spite of the slowness of the embryologists to confirm it by actual specimens of early embryos in which the process of duplication can be seen. The first circumstantial evidence is that single-ovum twinning has been seen in the developing eggs of fishes and birds. The second has long been available; this is the observation by physicians and midwives that some pairs of twin babies have only a single placenta and are enclosed, before birth, in a common, single chorioamniotic membrane. From what we know about the embryology of the membranes, the presence of two embryos in one chorion is most easily explained by assuming that they originated from a single egg cell. The third kind of circumstantial evidence is that given us by the young science of genetics. From the standpoint of inheritance theory, if two individuals born at the same time are exactly alike, *The Annual Joseph L. Raer Lecture of the Chicago Gynecological Society, delivered