Definition of chorion in English:



  • The outermost membrane surrounding an embryo of a reptile, bird, or mammal. In mammals (including humans), it contributes to the formation of the placenta.

    • ‘In insects, the genes that encode the proteins of the egg membrane, the chorion, become amplified in the surrounding follicle cells.’
    • ‘A needle was used to roll the embryo out of the chorion.’
    • ‘Each embryo is surrounded, from exterior to interior, by the chorion, the vitelline membrane and the embryonic membrane.’
    • ‘Care was exercised not to separate the amniotic sac into its components, the chorion and amnion.’
    • ‘Together with part of the chorion, these membranes make up the placenta, which physically attaches the embryo to the uterine wall of its mother.’


Mid 16th century: from Greek khorion.