La théorie des feuillets: implications embryologiques et évolutives.

Abstract

The germ-layer theory states that all Metazoa develop from embryos composed of two or three concentric cell layers. The embryos of the diploblastic animals comprise two germ layers (ectoderm and endoderm). Those of the triploblastic animals have an additional germ layer (mesoderm). The germ-layer theory is useful for interpreting embryonic development. But it also has evolutionary implications. According to the promoters of the theory, the corresponding germ layers of all Metazoa must be considered homologous. In evolutionary terms, this means that the diploblastic animals have a common ancestor whose soma comprised two cell layers. Similarly, all triploblastic animals are supposed to derive from a three-layer organism. Thus, the germ-layer theory is recapitulative in nature. It implies that early embryos of living animals are similar to their two- or three-layer ancestors. The embryological aspect of the germ-layer theory incites little controversy. its evolutionary aspect is much more difficult to evaluate. If the germ layers are homologous, this could mean that homologous genes direct the formation of each germ layer in all animals, and are active in all cells of a given layer. These genes presumably derive from those that were active in the corresponding germ layers of archaic Metazoa. This conjecture proves to be at least partially true, since homologous genes are expressed in mesodermal cells of early insect and vertebrate embryos.