GASTRULATION (late lat. gastrula, grech, gaster a stomach) — process of transformation of a single-layer germ — a blastula — in two-layer (at invertebrates) or at first in two-layer, and then in three-layered (at vertebrata) — a gastrula. The cavity of a gastrula is called gastrotsely, the opening conducting in gastrotsel — to blastopores (see). At the same time from a wall of a blastula — blastoderms (see) — there are at first primary germinal leaves: outside — primary ectoderm (epiblast) and internal — primary entoderm (hypoblast). The third — an average germinal leaf — mesoderm (see) — it is formed at invertebrates (including the lowest chordates, napr, at a lancelet) after G. whereas at vertebrata — in the second Phase. The internal leaf of a germ at coelenterates represents at the same time early laying of a definitivny intestinal tube. At the majority of more high-organized animals the internal leaf contains, except cellular material actually of an entoderm, i.e. future vystilka of intestines, also cellular material of future mesoderm (at chordates also a chord) and therefore is called primary gut or arkhenterony. At birds and mammals two Phases clearly are isolated. In the first phase from a uniform blastoderm it is chipped off only entoderm (see) and the germ becomes two-layer. In the second phase from structure of an epiblast from actually ectoderms (see) the mesoderm and cellular material of a chord stand apart. At all chordate G. not only formation of a three-layered germ, but also formation of an axial complex of rudiments is result: laying of a medullary plate (later — a neurotubule) as a part of an ectoderm, the chord located under it and the mesoderm which is on the right and to the left of a chord.
Ways G. are various. Invagination, e.g. at a lancelet, erinaceouses (emboly of a vegetative half of a blastula in animalny), opened by A. O. Kowalewski, Gekkel (E. The N of Haeckel) mistakenly considered phylogenetic in primary Way. As I. I. Mechnikov showed, at the most primitive multicellular — coelenterates — G. comes by eviction of cells from a wall blastulas (see) in blastotsel (multipole immigration) or splittings of a blastoderm on two layers (delamination). Further evolution of G. — concentration of process of immigration on one of poles of a blastula — polar immigration, edges gave rise to an epiboliya (overgrowing by an animalny half of a blastula of its vegetative half), invaginations, etc. At amphibians invagination is combined with an epiboliya. At reptiles and birds in the huge eggs overflowed with a yolk, the first phase G. occurs by a delamination (eliminating of an entoderm), the second — by the difficult movements of cellular material bringing (at birds) to education in an epiblast of primary strip, cover cells, plunging into an interval between an ectoderm and an entoderm, form a mesoderm. At the same time kpered from primary strip through a primitive pit — a dorsal lip the blastopore — plunges inside and cellular material of a chordal plate, forming a head shoot — a rudiment of a back string, or a chord. The same way G. is carried out at mammals and the person, despite sharp reduction of quantity of a yolk at them in an ovum and a total cleavage.
See also Germ .
Bibliography: Knorre A. G. A short sketch of embryology of the person with elements of comparative, experimental and pathological embryology, L., 1967; Siewing R. Lehrbuch der vergleichen-den Entwicklungsgeschichte der Tiere, Hamburg - B., 1969.