A phase early in the development of the embryo during which the morphology of the embryo is reorganized to form the three embryonic germ layers: ectoderm, mesoderm, and endoderm. Each layer gives rise to specific tissues and organs in the developing embryo.

The lateral "cheek" region of the head that lies behind the frontal sutures on each side of the head

Blastoderm cells on one side of the egg begin to enlarge and multiply. This region, known as the germ band (or ventral plate), is where the embryo's body will develop.