From Big Medical Encyclopedia

CHORION [chorion (LNE); Greek chorion cover, afterbirth; a synonym a fleecy cover] — the outside cover of a germ of mammals and the person developing from a trophoblast and a mesenchyma, adjacent to it; it is supplied with outgrowths (fibers) growing into a mucous membrane of a uterus and creating a placenta — see the Germ, Fetal membranes.