Gastrulation is a dramatic restructuring of the animal embryo during the gastrula phase. Gastrulation varies in different phyla; the following description concerns the gastrulation of triploblast s, or animals with three embryonic germ layers.

At the beginning of gastrulation, the embryo is hollow, with an animal pole and a vegetal pole. The cells of the vegetal pole begin to divide and bud inwards, forming a hollow called the archenteron (literally, primitive gut) on the outside surface of the gastrula. Some of the cells of the vegetal pole detach and become mesenchyme cells. The mesenchyme cells divide rapidly, migrate to different parts of the blastocoel, and form filopodia, strands that help to pull the tip of the archenteron towards the animal pole. Once the archenteron reaches the animal pole, a perforation forms, and the archenteron becomes a digestive tract passing all the way through the embryo.

The three embryonic germ layers have now formed. The endoderm, consisting of the archenteron, will develop into the digestive tract. The ectoderm, consisting of the cells on the outside of the gastrula that played little part in gastrulation, will develop into the skin and the central nervous system. The mesoderm, consisting of the mesenchyme cells that have proliferated in the blastocoel, will become all the other internal organs.

The next stage is called the neurula .