in biology, middle layer of tissue formed in the gastrula stage of the developing embryo. At the end of the blastula stage, cells of the embryo are arranged in the form of a hollow ball. Continued cell movement results in an invagination of the bottom region of the embryo, producing a form that resembles a double-layered cup. A third layer, the mesoderm, is formed between the other two by growth of cells derived from a marginal zone. The mesoderm is the germ layer that forms many muscles, the circulatory and excretory systems, and the dermis, skeleton, and other supportive and connective tissue. It also gives rise to the notochord
, a supporting structure between the neural canal and the primitive gut. In many animals, including vertebrates, the mesoderm surrounds a cavity known as the coelom
, the space that contains the viscera. See embryo
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