This is the innermost tissue layer in a developing animal embryo which will eventually become the digestive tract, respiratory tract, and various other things depending on the organism. Occurs in all animals except for sponges and placozoans. Also called the entoderm, endoblast, entoblast or hypoblast.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.