Question

Basal part of the endometrium remaind intact during mensturation

Answer 1

During menstrual cycle ovary goes through the follicular phase and luteal phase. Follicular phase is a developing phase of egg inside the follicle. After ovulation, ovary enters the luteal phase where it remains as such until the next cycle.
During the development of egg it secretes estrogen that helps in thickening the uterine lining. After ovulation,  glands and blood vessels in the endometrium prepare the uterine lining for implantation. If implantation does not occur, progesterone and estrogen drop cause the disintegration of uterus lining and flow of egg while the menses.