Eutherian fetal membranes. The embryo is enclosed in the amnion. Trophoblast (blue) and mesoderm (red) form the chorion. Trophoblast and yolk sac endoderm (yellow) together constitute a bilaminar omphalopleure. A choriovitelline or yolk sac placenta is then formed by interposition of mesoderm containing fetal blood vessels. Later the allantois expands into the exocoelom and the allantoic and chorionic mesoderm fuse to form a chorioallantoic placenta. The allantois will continue to expand into the exocoelom and eventually displace the yolk sac. The full sequence of events is seen most clearly in species with epitheliochorial placentation, such as horse and pig, although this cartoon is based on the rock hyrax (reference ).