In humans, the remaining egg cells deteriorate and disappear entirely when women near the age of fifty. Each oocyte is surrounded by a cellular jacket or follicle. An oocyte nearly fills its immature follicle. During ovulation, however, the cellular jacket around the egg cell swells creating a large fluid filled cavity. The ooctye, attached to one wall of the follicle by follicular epithelial cells, projects into this cavity. At maturation, the follicle bulges from the side of the ovary, and, at the moment of ovulation, the wall of the follicle bursts expelling both the liquid and the ooctye into the abdominal cavity. Though there is no regular pattern that determines which of the two ovaries will ovulate, only one oocyte is released per menstrual period. Oocytes normally enter the large, funnel-shaped ends of the oviducts or Fallopian tubes that surround but are not continuous with the ovaries. The action of cilia that line the upper ends of the oviducts help guide oocytes into the tubes. The other ends of the oviducts enter into the upper end of the uterus. The size of a fist, the uterus lies behind the bladder in the lower abdominal cavity. The uterus has thick muscular walls, a muscous lining in which fertilized eggs embed themselves, and many blood vessels which contribute to the formation of the placenta. The muscular ring of tissue at the lower end of the uterus is called the cervix which protrudes into the vagina. An elastic organ, the vagina expands to receive the male penis during copulation and to allow the fetus passage out during childbirth. In young women, the hymen partially closes the vagina. The vulva is the collective term for all the external female
