Degeneration of corpus luteum causes

Degeneration of corpus luteum in the ovary causes disintegration of the endometrium (inner glandular layer) of the uterus leading to menstruation. Degeneration of corpus luteum occurs in the absence of fertilisation.

As the corpus luteum degenerates, progesteron declines and triggers the release of FSH. So there is slow rise of FSH.

Myometrium is the muscular middle layer of uterus which remains intact during menstruation.