After a female egg is fertilized, the resulting one-celled organism becomes known as a zygote. Once this has occurred, 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.

In most cases, each male and female sex cell contain 23 chromosomes. When these two haploid cells join, they form a single diploid cell that contains a total of 46 chromosomes. The zygote begins a journey down the fallopian tube to the uterus where it must implant in the lining in order to obtain the nourishment it needs to grow and survive.

The period of the zygote is the first stage of the prenatal period. And, it lasts for about four days. Around the fifth day, the mass of cells becomes known as a blastocyst. The germinal period will last for fourteen days, after which the embryonic period will begin.

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Sources:
http://psychology.about.com/od/zindex/g/def_zygote.htm

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