The sperm cell (spermatozoon) and the egg (ovum) each have only half the genes of the parent cell — they're haploid cells. When the spermatozoon fertilizes the ovum, the resulting cell has the full complement of genes, so it's a diploid cell. The diploid cell then divides rapidly, becoming first an embryo, then a fetus. The word zygote comes from the Greek word for yoke — joining two things together, like hitching two oxen together to pull a plow.