The chorion acts as mediator between the embryo/ fetus and its environment. The early development of the blastocyst is thus modified in such a way that a primary chorion (i.e. the trophoblast) is already present just after the ____ stage. The continuity of the trophoblast/chorion is temporarily broken by the disappearance of the _____________, but is restored once the ____ ____ fuse dorsal to the embryo.