germ layer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of three cellular layers, the ectoderm, endoderm, or mesoderm, into which most animal embryos differentiate and from which the organs and tissues of the body develop through further differentiation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun embryology A collection of cells assembled during embryogenesis that eventually develop into all tissues and organs via organogenesis in animals.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (embryology) any of the 3 layers of cells differentiated in embryos following gastrulation