the membranous structures that surround the embryo and contribute to the placenta and umbilical cord. They include the amnion, chorion, allantois, and yolk sac. In humans the allantois is always very small and by the end of pregnancy the amnion and chorion have fused into a single membrane and the yolk sac has disappeared.
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the trophoblastic parts of the conceptus that provide for the support of the embryo or fetus by attachment, mechanical protection, endocrine action, and the mediation of chemical exchange with the maternal circulation. They include the yolk sac (umbilical vesicle), allantois, amnion, umbilical cord, and chorion, including the placenta. Called also fetal m's.Medical dictionary. 2011.