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Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three ''germ layers'' are known as the ectoderm, mesoderm, and endoderm.〔Mundlos 2009: (p. 422 )〕〔McGeady, 2004: p. 34〕 Gastrulation takes place after cleavage and the formation of the blastula. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers.〔Hall, 1998: (pp. 132-134 )〕 Each layer gives rise to specific tissues and organs in the developing embryo. The ectoderm gives rise to epidermis, and to the neural crest and other tissues that will later form the nervous system. The mesoderm is found between the ectoderm and the endoderm and gives rise to somites, which form muscle; the cartilage of the ribs and vertebrae; the dermis, the notochord, blood and blood vessels, bone, and connective tissue. The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas.〔Arnold & Robinson, 2009〕 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.〔〔Hall, 1998: (p. 177 )〕 The molecular mechanism and timing of gastrulation is different in different organisms. However, some common features of gastrulation across triploblastic organisms include: (1) A change in the topological structure of the embryo, from a simply connected surface (sphere-like), to a non-simply connected surface (torus-like); (2) the differentiation of cells into one of three types (endodermal, mesodermal, and ectodermal); and (3) the digestive function of a large number of endodermal cells.〔Harrison 2011: (p. 206 )〕 Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work ''"Biology of Calcareous Sponges"''.〔Ereskovsky 2010: (p. 236 )〕 Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation: 1) invagination 2) involution 3) ingression 4) delamination 5) epiboly.〔Gilbert 2010: p. 164.〕 ==In amniotes== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gastrulation」の詳細全文を読む スポンサード リンク
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