VOCABULARY: What Is a Zygote? the One-Celled Beginning of Life
After a female egg is fertilized, the resulting one-celled organism becomes known as a zygote. Once this has occured, the zygote begins a two-week period of rapid cell division and will eventually become an embryo.
The zygote divides through a process known as mitosis, in which each cell doubles by dividing into two cells. This two-week stage is known as the germinal period of development and covers the time of conception to the implantation of the embryo in the uterus.
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Researchers estimate that nearly 60 percent of all naturally occurring conceptions fail because the zygote never becomes properly implanted in the uterus.