The outermost layer of tissue in a developing animal embryo which will eventually become the skin and/or other outer surface of the organism, the outermost parts of the nervous system, and various other outer and external organs, structures, and systems (depends on the organism). Occurs in all animals except for sponges and placozoans.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.