Twin foetuses in utero, illustration. These are dizygotic (DZ) twins that have developed from the fertilization of two ovulated oocytes which form two zygotes. DZ twins may be of the same sex or different sexes. About 60 percent of DZ twins have separate placentas (red), separate amniotic cavities (grey) and separate chorionic sacs (red boundaries), with 40 percent sharing a fused placenta. From Kollmann, J. (1907) Handatlas der Entwicklungsgeschichte des Menschen, vol 1, Fig. 178. G. Fischer, Jena.