Human embryogenesis is the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell. The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation is nine months or 38 weeks.