chorion

(kōr′ē-on″)

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[Gr. chorion, membrane (surrounding the fetus)]
An extraembryonic membrane that, in early development, forms the outer wall of the blastocyst. It is the thicker, outer layer of the two fetal membranes and is formed from the trophoblast and its lining of mesoderm. From it develop the chorionic villi, which grow into the endometrium and will become the fetal portion of the placenta.
SEE: embryo, SEE: placenta, and SEE: umbilical cord for illus.; SEE: trophoblast
chorionic (kōr″ē-on′ik)
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, adj.

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