If egg is not fertilized the degeneration of the corpus luteum takes place, then menstruation is initiated. The corpus luteum is a temporary endocrine structure in female mammals that is involved in the production of relatively high levels of progesterone and estrogen. Corpus luteum secretes the female hormone progesterone, which helps to keep the uterine wall receptive to a fertilized egg. If the egg is not fertilized, the corpus luteum stops secreting progesterone about nine days after ovulation.

Explanation:

Menstruation is the process by which the lining of the uterus, known as the endometrium, is shed in response to hormonal changes in the female reproductive system. The initiation of menstruation is primarily influenced by a lack of estrogen and progesterone due to degeneration of the corpus luteum.

1. The role of the corpus luteum:
- After ovulation, the ruptured follicle in the ovary forms a structure called the corpus luteum.
- The corpus luteum secretes hormones, primarily estrogen and progesterone, which prepare the uterus for potential implantation of a fertilized egg.
- These hormones help to thicken the endometrium and maintain its blood supply.

2. Degeneration of the corpus luteum:
- If fertilization does not occur, the corpus luteum begins to degenerate.
- As the corpus luteum degenerates, it produces less estrogen and progesterone.
- The decrease in these hormones leads to a decrease in the blood supply to the endometrium, causing it to become ischemic (lack of blood flow) and undergo necrosis.
- The necrotic endometrial tissue is then shed, resulting in menstruation.

3. Lack of estrogen and progesterone:
- Estrogen and progesterone play a crucial role in maintaining the endometrium and preventing its shedding.
- When the corpus luteum degenerates and the levels of estrogen and progesterone decrease, the endometrium is no longer supported and starts to break down.
- This breakdown leads to the release of blood and tissue from the uterus, resulting in menstrual bleeding.

4. The role of FSH:
- Follicle-stimulating hormone (FSH) is released by the anterior pituitary gland and plays a role in the development of ovarian follicles.
- FSH stimulates the growth of follicles in the ovary, which eventually leads to ovulation.
- However, FSH does not directly initiate menstruation. It is primarily involved in the maturation of the follicles and the release of an egg during ovulation.

In conclusion:
Menstruation is initiated by a lack of estrogen and progesterone due to degeneration of the corpus luteum. The decrease in these hormones causes the endometrium to shed, leading to menstrual bleeding. FSH does not directly initiate menstruation but is involved in the maturation of ovarian follicles and ovulation.