A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges (eumetazoans and agnotozoans) produce two or three primary tissue layers (sometimes called 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 between these two layers (appropriately called the mesoderm) making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.
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... Because of its great importance, the neural crest is sometimes considered a fourth germ layer ... It is, however, derived from the ectoderm ...
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