Form during animal embryonic development possessing a gut and single opening to outside of the embryo.
In the gastrula the primitive gut is called the archenteron and the single opening is called the blastopore. The formation of the gastrula is called gastrulation.
Upon formation the gastrula, essentially by definition, contains both endodermal and ectodermal primary germ layers. In those animals in which mesoderm also forms, this formation occurs within the context of the gastrula.