The initial stages of<unk>  (embryogenesis)<unk> , or<unk>  embryogenesis, is the development and formation of the<unk>  embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the<unk>  body entails growth from a one-celled zygote to an adult<unk>  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 development commences.<unk>  development in the<unk> , covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus.<unk>  embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is about nine months or 40 weeks. 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. 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.
Human embryonic development
Prenatal development
Environmental toxicants and fetal development
Fetal viability
Human reproduction
Reproductive technology
Infant visual development
Embryo quality