During the [[1]] phase, the Graafian follicle that released the mature egg changes into a structure called the corpus luteum. 

During the luteal phase, the Graafian follicle that released the mature egg changes into a structure called the corpus luteum. 

During the menstrual cycle, the luteal phase is the second half of the cycle that occurs after ovulation. After ovulation, the Graafian follicle regresses and transforms into a temporary endocrine gland called the corpus luteum, a yellow-colored body.

The corpus luteum is responsible for producing progesterone, which is a hormone that helps to prepare the uterus for pregnancy by thickening the endometrium and suppressing the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). 

If pregnancy occurs, the corpus luteum will continue to produce progesterone until the placenta takes over hormone production. However, if pregnancy does not occur, the corpus luteum will break down and degenerate, leading to a decrease in progesterone levels and the onset of menstruation.