The mature egg of Pteridium aquilinum is surrounded by an acetolysis-resistant membrane. Cytochemical tests indicate that this consists of lipid, possibly in a polymerized form. There is no evidence for the presence of polysaccharide, protein, lignin, suberin, or sporopollenin. The membrane is believed to be formed from materials secreted by the egg itself, and to be analogous to the inner portion of the megaspore membrane of a gymnosperm which has a comparable origin and staining properties.