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The chorionic membrane is a membrane composed of the trophoblast layer and the parietal layer of the extra-embryonic mesoderm. As the embryo develops, the dense chorionic membrane forms the placenta with the basal decidua, while the smooth chorionic membrane gradually merges with the decidua parietal membrane.
The development of villi enlarged the contact surface between the villi and the decidua, which was conducive to the material exchange between the embryo and the mother.