<P> If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 10 days in humans). It then degenerates into a corpus albicans, which is a mass of fibrous scar tissue . </P> <P> The uterine lining (endometrium) is expelled through the vagina (in mammals that go through a menstrual cycle). In an estrous cycle, the lining degenerates back to normal size . </P> <P> If the egg is fertilized and implantation occurs, the syncytiotrophoblast (derived from trophoblast) cells of the blastocyst secrete the hormone human chorionic gonadotropin (hCG, or a similar hormone in other species) by day 9 post-fertilization . </P> <P> Human chorionic gonadotropin signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus and providing an area rich in blood vessels in which the zygote (s) can develop . From this point on, the corpus luteum is called the corpus luteum graviditatis . </P>

What is corpus luteum in the right ovary