<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> 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 . Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). 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 . Embryogenesis covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus . Human embryology is the study of this development during the first eight weeks after fertilisation . The normal period of gestation (pregnancy) is nine months or 38 weeks . </P> <P> The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus . The germinal stage takes around 10 days . During this stage, the zygote begins to divide, in a process called cleavage . A blastocyst is then formed and implanted in the uterus . Embryogenesis continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow . </P> <P> In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs . The entire process of embryogenesis involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation . A nearly identical process occurs in other species, especially among chordates . </P>

What is the first stage of an embryo