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Chorion (egg)
The chorion is the outermost membrane around the embryo in mammals, birds and reptiles. It develops from an outer fold on the surface of the yolk sac, which lies outside the vitteline envelope and is developed by the follicle cells while the egg is in the ovary.〔Chapman, R.F. (1998) "The insects: structure and function", Section ''The egg and embryology''. Previewed in Google Books () on 26 Sep 2009.〕
== Function in Amniotes ==
The chorion is one of the four extraembryonic membranes that make up the amniotic egg that provide for the nutrients and protection needed for the embryo's survival. It is located inside the albumin, which is the white of the egg. It encloses the embryo and the rest of the embryonic system. During growth and development of the embryo, there is an increased need for oxygen. To compensate for this, the chorion and the allantois fuse together to form the chorioallantoic membrane. Together these form a double membrane, which functions to remove carbon dioxide and to replenish oxygen through the porous shell. At the time of hatching, the fetus becomes detached from the chorion as it emerges from the shell.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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