Estrogens are produced primarily by the ovaries. They are released by the follicles on the ovaries and are also secreted by the corpus luteum after the egg has been released from the follicle and from the placenta. Thus, if the ovaries of rat are removed, then the level of estrogen is decreased in blood. Related Theory:- Secretion of progesterone from corpus luteum maintains the endometrium which is necessary for implantation of fertilized ovum and other events of pregnancy.