Illustration of twin foetuses in utero that share a single common placenta (red) and chorionic sac (red) but separate amniotic cavities (grey). These twins arise from duplication of the inner cell mass of the early embryo that develops separately into two foetuses. Among twin pregnancies this arrangement occurs in about 70 percent of cases. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 1, Fig. 177. G. Fischer, Jena.