This animation presents the modifications of the uterine wall (or endometrius) during the menstrual cycle that lasts on average, 28 days in the female.
The endometrium is a muquosa which comprises the internal wall of the uterus. During the cycle, it thickens. Then, if no embryon produced from fertilisation of an egg by a sperm cell is implanted into the endometrius, the superficial layer of this muquosa is eliminated: this is menstruation.
This animation allows you to put into context, the uterine cycle with the ovarian cycle: the 14th day of each cycle, one of the ovaries liberates an ovule (an oocyte).