At implantation, the ventral portion of the yolk sac lies directly against the ____; the expanding extra-embryonic mesoderm and associated vitelline circulation have not yet reached the ____ pole. The early placenta is thus a bilaminar vitelline placenta (see Fig. 35). Later, once the extra-embryonic mesoderm has grown between the ____ and ____, but before the region is vascularized, the bilaminar vitelline placenta is converted to a trilaminar vitelline placenta.