Q: Passage: Human fertilization is the union of a human egg and sperm, usually occurring in the ampulla of the fallopian tube. The result of this union is the production of a zygote cell, or fertilized egg, initiating prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.

Question: Based on this passage, a cell that is formed from the union of the sperm and ovum is called?
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Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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To answer the above question, the relevant sentence is: The result of this union is the production of a zygote cell, or fertilized egg, initiating prenatal development. So the answer is a zygote cell , or fertilized egg.