<Li> Human body portal </Li> <Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <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>

When do cells differentiate during the development of a fetus