Light and electron microscopic examination of tissues fixed in situ by perfusion of the gravid horn of the uteri of mares between 36 and 38 days of gestation revealed that the equine endometrial cups are composed of trophoblast cells which originate from the discrete annulate portion of the foetal membranes known as the chorionic girdle. This structure consists of closely opposed villous projections of elongated trophoblast cells and it becomes firmly attached to the endometrium around the thirty-sixth day of pregnancy. The specialized girdle cells invade and phagocytose the endometrial epithelium and then migrate through the basal lamina into the endometrial stroma where they develop into endometrial cup cells.