Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Germ layers are the primary layers of cells that form during embryonic development. These layers differentiate into various tissues and organs in animals.
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The three primary germ layers are ectoderm, mesoderm, and endoderm.
Ectoderm gives rise to the nervous system and skin.
Mesoderm forms muscles, bones, and the circulatory system.
Endoderm develops into the lining of the digestive and respiratory systems.
Germ layer formation is a key process in early embryogenesis called gastrulation.
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Related terms
Ectoderm: The outermost germ layer that forms structures such as skin and the nervous system.
Mesoderm: The middle germ layer that gives rise to muscles, bones, and the circulatory system.
Endoderm: The innermost germ layer that develops into internal linings of organs like the digestive tract.