Traditionally, three germ layers are recognizable: ectoderm, endoderm, and mesoderm as the outer, inner, and middle layers, respectively. Coelenterates and sponges possess only two of these layersectoderm and endodermand so are diploblastic. All other metazoans (multicellular animals) have the three germ layers and so are triploblastic.
In recent years, some developmental biologists consider the neural crest tissue of vertebrate embryos to be a fourth germ layer. It has been recognized as a distinctive layer restricted to vertebrates, which are therefore considered as quadroblastic.