The human embryo at about 5-6 weeks, showing the protective layer of the chorion (white) surrounding the developing embryo. The cells of the chorion arise from the embryo's mesoderm (middle germ layer): the ones which migrate furthest become the chorion, a part of which forms the placenta (the organ which connects with the mother's blood circulation through the umbilical cord). Other layers of mesoderm surround the yolk sac, which supplies the embryo with nourishment until it can obtain its food from the maternal blood vessels. The amnion (the inner, fluid-filled layer surrounding the embryo) is visible at left.
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