The stage in the development of an animal embryo that succeeds the blastula. The process, called gastrulation, is characterized by division and migration of cells to form the primary germ layers—the ectoderm, mesoderm, and endoderm. The embryo becomes converted to a cup-shaped structure containing a cavity (the archenteron), with an opening called the blastopore. An area of tissue in the dorsal lip of the blastopore organizes gastrulation and determines the subsequent developmental fate of the germ layers (see organizer). Whether the blastopore subsequently becomes the mouth or the anus divides triploblastic animals (i.e. all except cnidarians, ctenophores, etc.) into two major evolutionary groups—respectively the protostomes (‘mouth first’) and the deuterostomes (‘mouth second’).