part of part of unilaminar epithelium extraembryonic tissue extraembryonic endoderm forms the yolk sac (Gilbert) 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). EHDAA2:0000473 EHDAA:105 UBERON:0008945 The origin of extraembryonic endoderm is a highly controversial issue EMAPA:16052 entire extraembryonic component