Simple villi start forming over the central third of the chorionic sac such that they form a transverse belt incorporating the villi of the vitelline/choriovitelline placentae; this belt of villi is referred to as the __1__. On either side of the __1__, the cranial and caudal thirds of the chorionic sac remain free of villi (despite the fact that the __2__ grows into and vascularizes the ends of the chorionic sac); these smooth regions are referred to as the __3__.