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The chorion or choroid.

Greek khorion, the membrane enclosing the foetus.

The chorion is the outer membrane surrounding an embryo, which in mammals develops into the placenta. Hence choriocarcinoma, a malignant tumour of the uterus originating in the chorion of a foetus. The related adjective is chorionic (as in human chorionic gonadotrophin (HCG), a hormone secreted during pregnancy, and chorionic villus sampling (CVS), a test made to detect congenital abnormalities in the foetus).

The choroid is the pigmented layer of the eyeball in front of the retina, so named because it also contains many blood vessels. Some words in chorio‑ refer to this: chorioretinopathy, an eye disease that affects both the choroid and the retina; choriocele, a hernia or protrusion of the choroid layer.

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