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The formation of the primordial gut, the archenteron, or digestive cavity of an early animal embryo. More generally, and originally, the term gastrulation referred to the process by which the gastrula stage of the embryo is formed. Thus to nineteenth-century embryologists, gastrulation was the process by which the single-layered blastula, a hollow ball of cells, is converted into the double-layered gastrula. The term has now come to have a still more general meaning, namely, the process by which the three germ layers, or primordial tissues of the embryo, are brought into the positions and relations characteristic of the late gastrula stage, with ectoderm (outer skin), mesoderm (middle skin), and endoderm (inner skin) from the outside to the inside. The terms epiblast, mesoblast, and hypoblast are also used to denote ectoderm, mesoderm, and endoderm, respectively. See Blastulation
Two general but not mutually exclusive methods of gastrulation have been recognized: epiboly and emboly. Epiboly is the growing or extending of one part, such as the upper hemisphere of a spherical blastula, over and around another part, such as the lower hemisphere. Emboly is the pushing or growing of one part into another. In many embryos, both types of cell movement may occur; in certain invertebrate embryos, one type may predominate almost to the exclusion of the other. Generally speaking, epiboly tends to be the major, but not the only, method of gastrulation in forms with large, yolky eggs. See Germ layers
the process in the early embryonic development of multicellular animals that leads to the formation of an embryo with a two-layered and later, in the majority of animals, to a three-layered body wall. Complex shifts of cellular material occur during gastrulation; as a result, part of that material enters the interior of the previously single-layered embryo and lines its wall, which consequently becomes two-layered.
Four basic means of gastrulation are distinguished. Invagination, or intrusion, is the process by which the wall of a one-layered embryo gradually turns inward and forms an internal layer. In epiboly, or overgrowth, relatively large cells rich in yolk are overgrown by the small ones and find themselves inside, forming an internal layer. In immigration, or settlement, certain cells move to the interior of the embryo and settle under the superficial layer; immigration may be unipolar (settlement from one place) or multipolar (from various places). Delamination is the process in which the cells split transversely, converting the one-layered wall of the embryo to a two-layered one. Gastrulation by delamination and immigration is found mainly in coelenterates. In the majority of animals a combination of means of gastrulation occurs, predominantly by invagination and epiboly but also by immigration. The greater the yolk content of an egg, the longer the epibolic process.
In echinoderms, tunicates, acraniates, and lower vertebrates (cyclostomes, fishes, and amphibians), gastrulation generally proceeds in the following manner: the material that actively turns inward in the process of gastrulation forms the covering of the primitive gut, or the dorsal portion of the internal layer, and subsequently, separating from the rest of itself, forms the middle germ layer, the mesoderm. The more passive material forms the base of the primitive gut. Gradually the cells of the base undergrow the material of the covering of the primitive gut and, joining together, close the cavity of the definitive gut. Thus, the two-layered embryo is converted to a three-layered one (the so-called enterocoel method of mesoderm formation).
In higher vertebrates (reptiles, birds, and mammals), the endoderm forms first and does not include material of the future middle layer. The latter separates from the outer layer and is disposed between the ectoderm and the endoderm. In protostomia (worms, mollusks, and arthropods) the mesoderm is formed by the proliferation of certain isolated cells, teloblasts (so-called teloblastic mesoderm formation), whose derivatives position themselves between the ectoderm and the endoderm. The modes of gastrulation, including formation of the mesoderm, are extremely varied and are often very complex.
T. A. DETLAF