The amnion occupies the ____ part of the central portion of the chorionic sac, while the ____ occupies the rest of the sac (see Fig. 32). As the allantois does not extend dorsally between the amnion and chorion, an ____ is formed in this region. The ____ surface of the amnion and the ____ surface of the central portion of the allantois fuse to form an allantoamnion. The amniochorion and allantoamnion are both vascularized by the ____ circulation; the former forms part of the Pars fetalis of the placenta.