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amnion

The amnion is the pouch-like tissue present in the early embryo that lies over the dorsal surface of the epiblast portion of the developing inner cell mass. It is formed from cells that detach from the cytotrophoblast layer of the trophoblast stage of the embryo. As gestation progresses, the amnion grows outward and eventually surrounds the entire embryo and, in live-bearing animals, the umbilical cord, for which it forms an epithelial covering. In the latest stages of gestation, the amnion adheres to the inner cell layer of the chorion, forming the inner of the two membranes (the two together serving as the amniotic sac) surrounding the fetus. During later development, the amnion serves as a reservoir for urine, which is mixed with fluid that derives from maternal serum; this mixture cushions the developing fetus and provides a buoyant environment that allows for symmetrical growth and prevents embryonic adherence and growth onto the placental tissues. The amnion remains present at birth in most species but is absent in fish and amphibians altogether.