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| colspan="2" |Section through the embryo
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|Latin
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|- class="hiddenStructure"
|[[List of subjects in Gray's Anatomy:6#Gray.27s_page_.23|Gray's]]
|colspan="2"|[subject #6 ]
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|System
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|- class="hiddenStructure"
|Precursor
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|MeSH
|colspan="2"|[A16.254.425.660]
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|Dorlands/Elsevier
|colspan="2"|[/]
|}
The germ layermesoderm forms in the embryos of animals more complex than cnidarians, making them triploblastic. Mesoderm forms during gastrulation when some of the cells migrating inward to form the endoderm form an additional layer between the endoderm and the ectoderm.
This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the formation of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.
Not all triploblastic animals have a coelom, like the simplest animals with organs that form from three tissue layers: flatworms. Three different configurations of mesoderm in relation to ectoderm form a method of categorizing animals.
Acoelomates
*no coelom
*tissues and organs packed between gut and body wall
Pseudocoelomates
*false coelom
*unlined or partially lined body cavity between gut and body wall
Coelomates
*proper coelom
*lined cavity between gut and body wall
Derivatives
General
Note: Not all triploblasts produce all of the items listed.
