Fertilization and DevelopmentFor fertilization to occur, sperm cells must be released in the vagina during the period that the egg cell is alive. The sperm cells move through the uterus into the Fallopian tube, where one sperm cell may fertilize the egg cell. The fertilization brings together 23 chromosomes from the male and 23 chromosomes from the female, resulting in the formation of a fertilized egg cell with 46 chromosomes. The fertilized cell is a zygote.The zygote undergoes mitosis to form two identical cells that remain attached. This takes place about 36 hours after fertilization. Mitosis then occurs more frequently. Soon a solid ball of cells, a morula, results. Morula formation occurs about six days after fertilization. During that time the cells are moving through the Fallopian tube. Within the next two days, a hollow ball of cells called a
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