Immediately after penetration of the head of a sperm into the egg, takes place the merger of the two nuclei. This means that each of the 23 chromosomes that contain both the core of the egg and the sperm, look your partner in the other core finally obtaining 23 pairs of chromosomes, or what is the same, 46 chromosomes which is the normal allocation cromosomial corresponds to a person, whether male or female. When the head of the sperm manages to penetrate into the egg, changes occur in the membrane that prevent the entry of any other sperm. Moreover, even the body and tail for the sperm head that has managed to enter, remain outside. Only the head, with its genetic load, is within. Fertilization occurs when one gets the sperm penetrate the egg membrane. There seems to be somewhere on the surface of the egg, a cone of attraction, something like a false door that facilitates entry of a sperm and then closes tightly. At the core of the egg is contained in the form of chromosomes, inheritance, the mother transmitted to her children. However, as the uterus can not predict whether this egg is fertilized, month after month is preparing for a possible pregnancy. The inside lining of the womb that has evolved in the course of the previous ruler, was rebuilt during the time elapsed since the menstruation to ovulation, by the action of estrogens that are a hormone made by the ovarian follicle. All this preparation will be in vain if no fertilization occurs in this egg, so a few days after the lining of the womb will come off causing menstruation and egg disintegrates. The egg of the mother-cell is the other major provided the necessary elements to survive until it is fertilized. Each month, thanks to action on the ovary of the so-called gonadotropic hormones from the pituitary gland (which located at the base of the brain), takes place 14 days after menstruation, ovulation. This consists of ruptured ovarian cover to output an egg from a mature follicle. In order not to lose in the womb, fallopian tube ovary on the same side as completely engulfed by a sucking motion. Once deposited in the vagina, the path that the sperm must travel to reach the egg, is long and difficult. Long since, given its small size, the 15 cm. they separated from the egg, equivalent to a journey of hundreds of miles to us. And difficult because, apart from that there are a number of difficulties and barriers to their advancement through the woman's genitals, have to make this shift soon. The body of the sperm is like a store of fuel will be consumed by the tail, the real engine of sperm, which compels and moves into high gear. A sperm is a tiny single-celled element, whose length is less than 50 microns which nucleus is an aggregate of chromosomes where is deposited the inheritance of future father, ie the traits and features special organic and psychological man are transmitted to offspring deposited in this core, the true reservoir of genetic materials. This semen, also called sperm, contains about 100 million sperm per cubic centimeter, ie, in each sex or intercourse, 200-600 million sperm externos.Durante invade the genital intercourse man deposits his semen in woman's vagina.