<P> A germ layer is a primary layer of cells that form during embryogenesis . The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans, (animals more complex than the sponge) produce two or three primary germ layers . Animals with radial symmetry, like cnidarians, produce two germ layers (the ectoderm and endoderm) making them diploblastic . Animals with bilateral symmetry produce a third layer (the mesoderm), between these two layers . making them triploblastic . Germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis . </P> <P> Caspar Friedrich Wolff observed organization of the early embryo in leaf - like layers . In 1817, Heinz Christian Pander discovered three primordial germ layers while studying chick embryos . Between 1850 and 1855, Robert Remak had further refined the germ cell layer concept, and introduced into English were the terms "mesoderm" by Huxley in 1871 and "ectoderm" and "endoderm" by Lankester in 1873 . </P> <P> Among animals, sponges show the simplest organization, having a single germ layer . Although they have differentiated cells (e.g. collar cells), they lack true tissue coordination . Diploblastic animals, Cnidaria and Ctenophora, show an increase in complexity, having two germ layers, the endoderm and ectoderm . Diploblastic animals are organized into recognisable tissues . All higher animals (from flatworms to humans) are triploblastic, possessing a mesoderm in addition to the germ layers found in Diploblasts . Triploblastic animals develop recognizable organs . </P> <P> Fertilization leads to the formation of a zygote . During the next stage, cleavage, mitotic cell divisions transform the zygote into a hollow ball of cells, a blastula . This early embryonic form undergoes gastrulation, forming a gastrula with either two or three layers (the germ layers). In all vertebrates, these progenitor cells differentiate into all adult tissues and organs . </P>

Formation of three germ layers in human embryo