closed space within the ectoplacental cone, formed by the fusion of the parts of the amniotic fold that separate it from the amniotic cavity[MP]. a developmental cavity that exists in some mammals and is derived by division of the proamniotic space; it is further removed from the embryo than the amniotic cavity in some mammals [ http://www.medilexicon.com/medicaldictionary.php?s=epamniotic+cavity ]