Sex is fundamentally the exchange of genes between two different individuals. It gives rises to new combinations, some of which may be superior in their fitness to the original combinations. Sexual reproduction in animals anvolves the formation by meiosis of the egg and sperm, collectively called gametes, which contain the haploid chromosom number. The egg is typically large and non motile, with a store of nutrients to support the development of an embryo. The sperm is small and non-motile, adapted to swimming actively by beating its long, whiplike tail. The normal diploid chromosome number of the adult is restored when an egg and sperm unite at fertilization. In most species the sexes are separated, with distinct male and female individuals. so the genetic traits of two individuals also are combined at fertilization.
During copulation, the sperm that have been stored primarily in the epididymis are ejaculated by the sudden contraction of muscles in and around the male ducts, and the accessory sex glands currently discharge their secretion. The seminal fluid that is deposited in the vagina may contais as many as 400,000,000 sperm in humans. Mucus in the seminal fluid serves as a conveyance for the sperm, proteolytic enzymes break it down into a more watery fluid after the semen has been deposits in the vagina, permitting the sperm to become highly motile. Fructose provides a source of energy, alkaline materials prevent the sperm from being killed by fatty acids promote the contaction of the smooth muscle in the walls of the uterus and uterine tubes.
Sperm move from the vagina through the uterus and up the uterin tube in a little over one hour. Although sperm can swim ny beating the tail, muscular contraction of the uterus and uterine tubes is primarily responsible for carrying them upward. Fertilization occurs in the upper part of the uterine tube, but the arrival of an egg and the sperm in this region need not coincide exactly. Sperm retain their fertilizing powers for a day or two, and the egg moves slowly down the uterine tube, retaining its ability to be fertilized for about a day.
Pathway of the egg: When the oocyte is released from the follicle at ovulation, it is swept into the adjacent oviduct (sometimes called a Fallopian tube) by the movement of the funnel-shaped opening of the oviduct over the surface of the ovary and by the beating of cilia that line the fingerlike projections surrounding thid opening. The smooth muscles in the walls of the oviduct contract in continous peritaltic waves that move the oocyte from the ovary to the uterus in about three days. An unfertilized oocyte, however, lives only about 24 hours after it is ejected from the follicle. So, fertilization, if it is to occur, must occur in an oviduct. If the egg cell is fertilized, it becomes implanted in the endometrium three to four days after the young embryo reaches the uterus, six or seven days after the egg was fertilized. If the egg cell is not fertilized, it dies, and the endometrial lining of the uterus is shed at menstruation. Fertilized eggs implanted in the endometrium are sometimes lost in abnormal menstrual flow, but it is difficult to estimate the number of such very short-live pregnancy.
The uterus is a hollow, muscular, pear-shaped organ about 7.5 centimeters long and 5 centimetes wide. It is lined by the endometrium, which has two principal layers, one of which is shed at menstruation and another from which the shed layer is regenerated.
The vagina is a muscular tube about 7.5 centimeters long that leads from the cervix of the uterus to the outside of the body. It is the receptive organ for the penis and also the birth canal. The lining of the vagina is rich in glycogen, which bacteria normally present in the vagina convert to lactic acid. As a consequence the vaginal tract is mildly acidic, with a pH between 4 and 5.
The external genital organs of the female are collectively known as the vulva. The clitoris, which corresponds to the penis in male, is about 2 centimeters long and, like the penis, is composed chiefly of erectile tissue.
Curtis, Helena. 1979. "Biology." New York: Worth Publisher, Inc.kk
Dorit, Robert L; Warren F. Walker; Robert D. Barnes. 1991. "Biology". USA: Saunders Collage Publisher.