endoderm

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

• noun The innermost of the three primary germ layers of an animal embryo, developing into the gastrointestinal tract, the lungs, and associated structures.

from The Century Dictionary.

• noun In zoology, the completed inner layer of cells in all metazoan animals, formed by the cells of the hypoblast or endoblast, and representing, under whatever modification, the lining of the enteron: opposed to ectoderm.

from the GNU version of the Collaborative International Dictionary of English.

• noun The inner layer of the skin or integument of an animal.
• noun The innermost layer of the blastoderm and the structures derived from it; the hypoblast; the entoblast. See Illust. of ectoderm.

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

• noun One of the three tissue layers in the embryo of a metazoan animal. Through development, it will produce the digestive system of the adult.

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

• noun the inner germ layer that develops into the lining of the digestive and respiratory systems