Invagination of the vegetal plate in the single-layered blastula forms the blastopore, which will become the anus.

Further differentiation yields a gastrula with three embryonic tissue layers.

• Endoderm. Infolding forms the archenteron, which later develops into the digestive tube with a second opening that becomes a mouth.

• Mesoderm. Mesenchyme cells migrate into the blastocoel, and will later form the skeleton.

• Ectoderm.