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In animals, fertilization is the fusion of a sperm cell with an egg cell. The penetration of the egg cell by the chromosome-containing part of the sperm cell causes a reaction, which prevents additional sperm cells from entering the egg. The egg and sperm each contribute half of the new organism's genetic material. A fertilized egg cell is known as a zygote. The zygote undergoes continuous cell division, which eventually produces a new multicellular organism.

Fertilization in humans occurs in oviducts (fallopian tubes) of the female reproductive tract and takes place within hours following sexual intercourse. Only one of the approximately 300 million sperm released into a female's vagina during intercourse can fertilize the single female egg cell. The successful sperm cell must enter the uterus and swim up the fallopian tube to meet the egg cell, where it passes through the thick coating surrounding the egg. This coating, consisting of sugars and proteins, is known as the zona pellucida. The tip of the head of the sperm cell contains enzymes which break through the zona pellucida and aid the penetration of the sperm into the egg. Once the head of the sperm is inside the egg, the tail of the sperm falls off, and the perimeter of the egg thickens to prevent another sperm from entering.

The sperm and the egg each contain only half the normal number of chromosomes, a condition known as haploid. When the genetic material of the two cells fuses, the fertilization is complete.

In humans, a number of variables affect whether or not fertilization occurs following intercourse. One factor is a woman's ovulatory cycle. Human eggs can only be fertilized a few days after ovulation, which usually occurs only once every 28 days.

In other species, fertilization occurs either internally (as above) or externally, depending on the species involved. Fertilization outside of the animal's body occurs in aquatic animals such as sea urchins, fish, and frogs. In sea urchins, several billion sperm are released into the water and swim towards eggs released in the same area. Fertilization occurs within seconds of sperm release in the vicinity of the eggs. Sea urchins have been used greatly in research on fertilization.

Artificial insemination, in humans or animals, occurs when sperm is removed from the male and injected into the vagina or uterus of the female. In the latter case, the sperm must first be washed to remove the semen. This is a common treatment for human infertility. The development of gamete intra-fallopian transfer (GIFT) technology has further improved the treatment of infertility. In this procedure, sperm and eggs are placed together in the woman's fallopian tube and fertilization progresses naturally.

Fertilization occurring outside of the body is in vitro (in a dish or test tube) fertilization, IVF. Eggs are removed surgically from the female's reproductive tract and fertilized with sperm. At the 4-cell (day 2) stage, the embryos are returned to the female's fallopian tube or uterus where development continues. Mammalian IVF has been performed successfully on animals since the 1950s, and the first human birth following IVF occurred in Great Britain in 1978. This procedure has since become a routine treatment for infertility. If the sperm is too weak to penetrate the egg, or if the male has a very low sperm count, an individual sperm can be injected directly into the egg. Both eggs and sperm can be frozen for later use in IVF. A mechanical "sperm sorter" that Magnified image of a human sperm penetrating a female egg (ovum). D.W. Fawcett/Photo Researchers, Inc. Reproduced by permission.

separates sperm according to the amount of DNA each contains, can allow couples to choose the sex of their child. This is because sperm containing an X chromosome, which results in a female embryo, contains more DNA than sperm with a Y chromosome, which would yield a male embryo.

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