Zygote is a diploid cell resulting from the fusion of two haploid gametes; a fertilized ovum. A zygote is the union of the sperm cell and the egg cell. Also known as a fertilized ovum, the zygote begins as a single cell but divides rapidly in the days following fertilization. After this two-week period of cell division, the zygote eventually becomes an embryo. If it goes well, the embryo becomes a fetus.