The unification of sperm and egg to form a zygote (the earliest stage of human life). The zygote would then become an embryo, and then a fetus. A sperm or an egg contains half of the genetic make-up of its species. A normal human cell contains 46 chromosomes, while an egg or sperm contains only 23 chromosomes. After successful and normal fertilization, the number of chromosomes becomes 46 (23 from the egg and 23 from the sperm).