mesothelium

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

• noun The layer of flat cells of mesodermal origin that lines the embryonic body cavity and gives rise to the squamous cells of the peritoneum, pericardium, and pleura.

from The Century Dictionary.

• noun The epithelium lining the entire primitive cœlom or body-cavity of the embryo; the cœlarium.

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

• noun (Biol.) Epithelial mesoderm; a layer of cuboidal epithelium cells, formed from a portion of the mesoderm during the differetiation of the germ layers. It constitutes the boundary of the cœlum.

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

• noun biology A membrane of flat cells that lines the body cavity of embryos and forms the squamous cells of the peritoneum, pericardium, and pleura

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

• noun epithelium originating in the embryonic mesoderm; lines the primordial body cavity