Corpus luteum (the yellow body) is the spent follicular cells on the ovary. In other words the cells of the ruptured follicle grow larger and fill the cavity of the follicle, to form a new structure called 'corpus luteum'.
After ovulation and during the cycle’s second half, LH from the anterior pituitary promotes the development of the corpus luteum by converting the follicle.
Corpus luteum produces large amounts of the sex hormones progesterone and some estrogen.
Corpus luteum degenerates if fertilization does not occur in about 12 days. Consequently the estrogen and progesterone levels fall rapidly, and another menstrual cycle begins.
If pregnancy does occur, the corpus luteum does not regress and it is maintained. It secretes increasing amounts of progesterone; as a result, menstruation does not occur and the lining of the uterine is maintained, where the embryo resides.