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Millions of sperm are released during a single ejaculation. Their tails propel on their journey to encounter the single egg cell. Of the millions of sperm, only a few will survive to reach the egg and just one will penetrate the egg cell’s wall to combine it’s genetic material with that of the egg in the process called fertilization. If during the first 13 days of cell division, the fertilized egg cell, or zygote, divides into 2 zygotes, identical twins will form. Each developing embryo contains the same genetic material as the other.

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