germ layer

A germ layer, primary germinal layer, or germinal layer is a primary layer of cells that forms during animal embryonic development. Germ layers form during gastrulation, when the early embryo is formed of two or three layers of cells.

Sponges do not have a gastrulation stage and possess no true germ layers. They do though have two layers of cells separated by a gel-like mesohyl. Some aquatic invertebrates such as cnidarians, and comb jellies, develop from only two germ layers, an ectoderm and an endoderm, and are known as diploblasts. But most animals have a bilateral symmetry, and develop from three germ layers, an ectoderm, endoderm, and a middle layer of mesoderm, and are triploblastic.