An extraembryonic epithelium that is formed by the delaminated cells of the primitive endoderm (hypoblast) that goes on to form the yolk sac. In mammals, as in avians, these cells do not produce any part of the newborn organism (i.e. they are extraembryonic). [ https://orcid.org/0000-0002-6601-2165 https://www.worldcat.org/search?q=bn%3A9780878932504 http://www.ana.ed.ac.uk/database/humat/notes/extraemb/mesoder.htm ]