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The lucky few sperm who reached the egg in the Fallopian tube surround it and begin competing for entrance. The head of each sperm, the acrosome, releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics. This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg in a process called cortical reaction. The cortical reaction ensures that only one sperm fertilizes the egg. The other sperm die within 48 hours.
The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately.
The dividing zygote gets pushed along the Fallopian tube. Approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days and finally implants itself in the uterine wall around six days after fertilization. This signals the beginning of pregnancy. The implanted blastocyst continues developing in the uterus for nine months. As the baby grows, the uterus stretches until it's about the size of a basketball.