Gastrulation
The gastrulation is the third stage of the development of the Métazoaire S and the first of the Morphogénèse. It consists of the installation of fundamental fabrics of the embryo (or layers), by cellular migration migration, with possibly the differentiation of a third cellular layer, the Mésoblaste (or mésoderme), between the two preexistent embryonic layers, the Entoblaste (or endoderm) and the Ectoblaste (or exoderme).
At the beginning of the gastrulation, the embryo has an almost spherical symmetry. At the end, the fundamental symmetry of the animal group (radiate, bilatérien) is put does not place. The way in which the gastrulation is held will thus have consequences on the morphology of the adult, it makes it possible to differentiate the four large Phylum animal.
Gastrulation of the vertebrate ones
The Vertébré S are divided into two groups:
- the Anamniote S (Poisson S and Amphibien S), of which the egg S, laid in the Water, do not have embryonic additional ;
- the Amniote S (Sauropsides and Mammifère S), of which eggs, laid with the free Air, are equipped with embryonic appendices.
Gastrulation of the anamniotes
If the methods differ between the Diploblastique S and the Triploblastique S, the principle of the gastrulation remains the same one: once the stage Morula completed, the embryo is invaginated to form a pocket in its center, thus giving him the shape of a hollow sphere, opened to the level of the Blastopore which has just opened. This, at the watery species, will make it possible the embryo to be nourished: the blastopore acts as mouth, and the cavity inside the embryo, of primitive Intestin (Archentéron). The layer which remainder outside becomes the Ectoderme, that which is found inside, the Endoderme, and that which are wedged between the two precedents, the Mésoderme or the Mésoglée according to the type of métazoaire.
Gastrulation of the amniotes
The gastrulation of the Amniote S is characterized by the installation of a thickening on the level of the ectoblast which forms the primitive Ligne. Cells migrate towards this primitive line, are délaminent of the ectoblast (pass from an epithelial state to a mésenchymateux state) then migrate individually through the primitive line. These cells are at the origin of the Endoderme and the Mésoderme. The primitive line is the equivalent at the Amniote S of the Blastopore of the anamniotes. The former area of the primitive line is called node of Hensen and has inductive functions of the primitive line and structures neurales (homologous with the organizer of Spemann of the anamniotes).
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