Question
Tue February 28, 2012 By:

Are there four germ layers as well?

Expert Reply
Wed February 29, 2012
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 layers—ectoderm and endoderm—and 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.
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