chorion - outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals)amniote - any member of the Amniota chorionic villus - one of the tiny villi that stick out of the fetal chorion and combine with the mother's uterine tissue to form the placenta mammal - any warm-blooded vertebrate having the skin more or less covered with hair; young are born alive except for the small subclass of monotremes and nourished with milk fetal membrane - any membrane that functions for the protection or nourishment of respiration or excretion of a developing fetus sac - a structure resembling a bag in an animal |