Gastrula n. : An embryonic form having its origin in the invagination or pushing in of the wall of the planula or blastula (the blastosphere) on one side, thus giving rise to a double-walled sac, with one opening or mouth (the blastopore) which leads into the cavity (the archenteron) lined by the inner wall (the hypoblast). See Illust. under Invagination. In a more general sense, an ideal stage in embryonic development. See Gastraea.

Gastrula a. : Of or pertaining to a gastrula.

Example:
Why is the Gastrula considered an important stage in development?

Taxonomy
cells, gastrulation, embryo, blastocoel, hypoblast, archenteron, yolk, endoderm, mesoderm, layers, ectoderm, blastoderm, vegetal pole, epiboly, dorsal, EVL, YSL, mesenchyme cells.

Defination from the web:
... ectoderm, consisting of the cells on the outside of the gastrula that played little part in gastrulation, will develop into the ... Archenteron/Primitive streak (Primitive pit, Primitive knot/Blastopore, Primitive groove) · Gastrula/Gastrulation · Regional specification. Ectoderm ...