Germ Layers and the Germ-Layer Theory Revisited

Abstract

Remak's legacy remains. ``Ectoderm forms the outside, endoderm forms the inside, and mesoderm forms what's in between'' is the shorthand caricature of germ layers so often given to undergraduates. These are old ideas. We have known that embryos of animals such as the chick are built from three germ layers for 180 years, and that all vertebrates are built on a three-layered plan for 169 years. Huxley announced 148 years ago that two of the germ layers in vertebrate embryos'ectoderm and endoderm'are homologous to the two layers of adult coelenterates. Phyla have been classified as mono-, diplo-, or triploblastic, i.e., as having one, two, or three germ layers, for 124 years. Just as old is the germ-layer theory that homologous structures in different animals (must) arise from corresponding germ layers. Germ layers are part of the foundation of our understanding of animal organization.