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Essentials of Human Embryology: "In this text, we begin our description of the developing human with the formation and differentiation of the male and female sex cells or gametes, which will unite at fertilization to initiate the embryonic development of a new individual. ... Fertilization takes place in the oviduct ... resulting in the formation of a zygote containing a single diploid nucleus. Embryonic development is considered to begin at this point... This moment of zygote formation may be taken as the beginning or zero time point of embryonic development."

Larsen, W.J. 1998. Essentials of Human Embryology, Churchill Livingstone, New York, pp. 1-17.