Puberty occurs in girls between eight and fifteen years of age, and by this time each ovum is encaused in a mass of cells called a follicle. Once a month the pituitary gland secretes a hormone which causes one of these follicles to absorb liquid, expand, and burst, releasing its ovum from the ovary. This event is called ovulation.Ovulation occurs once a month, unless interruped by prenancy, until about the age of fifty.