The fertilization occurs when binds one egg (female gamete) with sperm (male gamete) to form an egg cell or a zygote. In humans, this fertilization is internal, since fertilization occurs within the female reproductive system , specifically in the fallopian tubes. 
Fertilization is divided into three stages:
To the sperm enter the female reproductive tract is necessary that occur copulation or coitus, consisting of the introduction of the erect penis into the vagina, and subsequent ejaculation (output the sperm). About 300 million sperm are released into the vagina, of which only a few thousand sperm make it through the uterus and up the fallopian tubes. Very few (about 100) will be able to reach the egg, chemically attracted, and only one will be able to break the plasma membrane of the eggand fertilize it. 
The rest of the sperm will not have achieved their goal and will be destroyed.
The fertilization occurs when the sperm, with the enzymes contained in acrosome, manages to break the protective covers of the egg and introducing the core, with 23 chromosomes, which will merge with the core of the egg, also with 23 chromosomes, to form the zygote, the first diploid cell (with 46 chromosomes) of the new individual.
Once fertilized, the ovum generates the fertilization membrane, and no other sperm will be able to fertilize it again. The zygote will begin to divide into more diploid cells and travel to the uterus where it will implant.
As fertilization has occurred, the corpus luteum does not degenerate and continues to secrete progesterone and the endometrium continues to thicken, without menstruation occurring.
On the way to the uterus, the zygote divides numerous times to become a morula, a blackberry-like mass of cells. It continues dividing and displacing, passing into a state called blastula. Thus it reaches the uterus and implantation or nesting occurs.
By Ttrue12 (Human Fertilization-es.png) [CC BY-SA 3.0], vía Wikipedia
Activity: Fertilization.
Activity: Fertilization.
Video: Fertilization (in English but you can put subtitles).
Animation: La fécondation.
11.6.- In what part of the female reproductive system does fertilization take place?
11.7.- Explain the difference between fertilization, menstruation and ovulation.
11.8.- Why do children resemble their parents? Why aren't all siblings the same, children of the same parents?
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