By superfoetation is meant the conception, by a woman already pregnant, of a second embryo, resulting in the birth of two children at the same time, differing much in their degree of maturity, or in two separate births, with a considerable interval between. The possibility of the occurrence of superfoetation has been doubted, but there are well-authenticated cases which countenance the theory of a double conception. It has been shown that the os uteri is not closed, as was once supposed, immediately on conception. Should an ovum escape into the uterus, it may become impregnated a month or so after a previous conception. The most probable explanation is that the case has been one of twins, one being born prematurely; or, on the other hand, the uterus may have been double, and conception may have taken place in one cornu at a later period than in the other cornu.