FUNCTION OF THE CORPUS LUTEUM Speaking broadly, the endocrine function of the corpus luteum is now well understood. The gland is a part of the mechanism of pregnancy. When an ovum begins its journey through the fallopian tube, the follicle from which it took origin gives place to the corpus luteum, and this organ thereupon delivers into the blood stream a substance, progesterone, that has the property of causing extensive development of the endometrium, preparing the uterus for the reception and nutrition of the embryo. Action on the Uterine Mucosa.— The action of the corpus luteum on the uterus is best known in the rabbit, because that species especially has been used for experiments with progesterone. About two days after the formation of the corpora lutea or after the beginning of a course of treatment with progesterone, rapid cell division begins in the epithelium, resulting in a downgrowth of