Gastrulation

Gastrulation is an early stage of an animal's embryonic development. During this stage, theembryo is called a gastrula. Important cellular movements create three layers (fundamental tissues of the embryo) :

• the ectoblast (or ectoderm);
• the endoblast (or endoderm);
• (at least in triploblastic animals), the mesoblast (or mesoderm).

Spherical until then, the embryo folds inwards to form a new inner cavity (in addition to the pre-existing blastocele) : the archenteron, which will later turn into the digestive tube. At the end of this stage, the gastrula adopts the animal's characteristic symmetry (bilateral, for example).