page_title: Chorion
section_title: Other animals
hierarchical_section_title: Chorion / Other animals
caption_attribution_description: English: Idealized amniote embryo. Legend: a=embryo, b=yolk, c=allantois, d=amnion, e=chorion
context_page_description: The chorion is the outermost fetal 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 zona pellucida, known as the vitelline membrane in other animals. In insects it is developed by the follicle cells while the egg is in the ovary.
context_section_description: In reptiles, birds, and monotremes, 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 albumen, which is the white of the egg.  It encloses the embryo and the rest of the embryonic system.  The chorion is also present in insects.  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.
caption: Amniotic embryo. a=embryo, b=yolk, c=allantois, d=amnion, e=chorion
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