Editing Gastrulation (section)

== Gastrula layers ==
[[File:Gastrulation in 3D.ogg|thumb|A description of the gastrulation process in a human embryo in three dimensions]]
In [[Triploblasty|triploblastic]] organisms, the gastrula is trilaminar (three-layered). These three [[germ layers]] are the [[ectoderm]] (outer layer), [[mesoderm]] (middle layer), and [[endoderm]] (inner layer).<ref name=mundlos2009>Mundlos 2009: [https://books.google.com/books?id=FlfPSpBvKLgC&pg=PA422 p. 422]</ref><ref name="mcgeady-2004-p.34">McGeady, 2004: p. 34</ref> In [[Diploblasty|diploblastic]] organisms, such as [[Cnidaria]] and [[Ctenophora]], the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as the ''hypoblast'' and ''epiblast''.<ref>{{Cite book|title=Essential Developmental Biology|last=Jonathon M.W.|first=Slack|publisher=Wiley-Blackwell|year=2013|isbn=978-0-470-92351-1|location=West Sussex, UK|page=122}}</ref> [[Sponges]] do not go through the gastrula stage.

Gastrulation takes place after [[cleavage (embryo)|cleavage]] and the formation of the blastula, or blastocyst. Gastrulation is followed by [[organogenesis]], when individual [[Organ (anatomy)|organ]]s develop within the newly formed germ layers.<ref>Hall, 1998: [https://books.google.com/books?id=JhSwumfgTQ4C&pg=PA132 pp. 132-134]</ref> Each layer gives rise to specific [[tissue (biology)|tissues]] and organs in the developing embryo.
* The ectoderm gives rise to [[Epidermis (zoology)|epidermis]], the [[nervous system]], and to the [[neural crest]] in vertebrates.<ref name="Gilbert-2016" />
* The endoderm gives rise to the [[epithelium]] of the [[Digestion|digestive system]] and [[respiratory system]], and organs associated with the digestive system, such as the [[liver]] and [[pancreas]].<ref name="Gilbert-2016" />
* The [[mesoderm]] gives rise to many cell types such as [[muscle]], [[bone]], and [[connective tissue]]. In vertebrates, mesoderm derivatives include the [[notochord]], the [[heart]], [[blood]] and [[blood vessels]], the [[cartilage]] of the [[ribs]] and [[vertebrae]], and the [[dermis]].<ref name="Gilbert-2016" /><ref name=Arnold>Arnold & Robinson, 2009</ref>
Following gastrulation, cells in the body are either organized into sheets of connected cells (as in [[epithelia]]), or as a mesh of isolated cells, such as [[mesenchyme]].<ref name="mcgeady-2004-p.34" /><ref>Hall, 1998: [https://books.google.com/books?id=JhSwumfgTQ4C&pg=PA177 p. 177]</ref>