page_title: Embryonic development
section_title: Organogenesis
hierarchical_section_title: Embryonic development / Organogenesis
caption_attribution_description: English: Human embryo, 38 mm, 8-9 weeks
context_page_description: Embryonic development, also embryogenesis, is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.
  Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo.
  Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals.
context_section_description: At some point after the different germ layers are defined, organogenesis begins. The first stage in vertebrates is called neurulation, where the neural plate folds forming the neural tube (see above). Other common organs or structures that arise at this time include the heart and somites (also above), but from now on embryogenesis follows no common pattern among the different taxa of the animal kingdom.
  In most animals organogenesis, along with morphogenesis, results in a larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.
caption: Human embryo, 8-9 weeks, 38 mm
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