Of the approximately 300 million sperm cells released in an ejaculation, only 1% will reach the egg and only a single sperm will penetrate and fertilize the egg. After the union of sperm and egg, the fusion of genetic material takes place. The fertilized egg, called a zygote, then divides into two cells (called blastomeres) after about 30 hours and four cells after 40 hours. As it divides, it is slowly carried down the fallopian tube. When it reaches the 16-blastomere stage, it is called a morula, and approximately 72 hours after fertilization, it reaches the uterus.
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