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.
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 between the albumin, which is the white of the egg, and the eggshell. 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.
- Chapman, R.F. (1998) "The insects: structure and function", Section The egg and embryology. Previewed in Google Books  on 26 Sep 2009.
- Chapman, R.F. (1998) The insects: structure and function. (4th Ed). Cambridge University Press. ISBN 0-521-57890-6, ISBN 978-0-521-57890-5.
- "chorion (embryology) -- Britannica Online Encyclopedia." Britannica Online Encyclopedia. N.p., n.d. Web. 14 Oct. 2012. <http://www.britannica.com/EBchecked/topic/114523/chorion>.
- Marieb, Elaine Nicpon. Essentials of human anatomy and physiology. 5th ed. Menlo Park, Calif.: Benjamin/Cummings Pub. Co., 1997. Print.
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- Hickman, Cleveland P.. Integrated principles of zoology. 13th ed. Boston: McGraw-Hill Custom Pub., 2005. Print.
- "Brit_chorion">Entry of 'chorion' in the Encyclopædia Britannica website . Retrieved on 26 Sep 2009. In insects, the chorion is the outer shell of the insect egg "Brit_chorion"
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