gastrulation

gastrulation [‚ga·strə′lā·shən]

(embryology)

The process by which the endoderm is formed during development.

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Gastrulation

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