The opening of the archenteron, central cavity of gastrula, which is formed by invagination of the BLASTULA or BLASTOCYST. Archenteron gives rise to the alimentary canal of the DIGESTIVE SYSTEM.

The developmental stage that follows BLASTULA or BLASTOCYST. It is characterized by the morphogenetic cell movements including invagination, ingression, and involution. Gastrulation begins with the formation of the PRIMITIVE STREAK, and ends with the formation of three GERM LAYERS, the body plan of the mature organism.

Synonyms: Archenteron Archenterons Guts, Primitive Gastrocoele Gastrula Blastopore Gastrulas Blastopores Gastrocoeles Gut, Primitive Primitive Guts Primitive Gut