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The Developing Human: Clinically Oriented Embryology: "Zygote: this cell results from the union of an oocyte and a sperm. A zygote is the beginning of a new human being (i.e., an embryo). Human development begins at fertilization, the process during which a male gamete or sperm ... unites with a female gamete or oocyte ... to form a single cell called a zygote. This highly specialized, totipotent cell marks the beginning of each of us as a unique individual."

Moore, K. and T.V.N. Persaud. 1998. The Developing Human: Clinically Oriented Embryology (6th ed.), W.B. Saunders Company, Philadelphia, pp 2-18.