gastrulation In the embryogenesis of a vertebrate, the next stage after the blastula, in which the three germ layers differentiate to form the triploblastic embryo and the basic body plan of the vertebrate is laid down. During this stage there is a complicated movement of cells from various positions in the blastula to form the germ layers in an approximation of their final positions. This process usually occurs at the end of the cleavage period.

gastrula The stage of embryonic development of an animal, following the blastula stage, at which distinct germ layers are present, the archenteron opens to the exterior by the blastopore, and gastrulation movements occur.

gastrula The stage in the development of an animal embryo that succeeds the blastula. It begins with the production of the primary germ layers and the embryo becomes converted to a cup-shaped structure containing a cavity (the archenteron).

gastrula (gas-troo-lă) n. an early stage in the development of many animal embryos. The gastrula consists of a double-layered ball of cells formed by invagination and movement of cells in the preceding single-layered stage (blastula) in the process of gastrulation.