SUMMARY1It has been shown by staining techniques and by autoradiography that the cytochemistry of the eggofPteridium aquilinumis strikingly different from that of the somatic cells of the gametophyte. Deoxyribonucleic acid is present in both nucleus and cytoplasm of the mature egg, and in comparison with that of a somatic cell the cytoplasm of the egg contains large amounts of ribonucleic acid and basic proteins.2Examination of ultra‐thin sections of archegonia by means of the electron‐microscope shows that during oogenesis the old organelles descended from those of the spore degenerate. The mature egg contains numerous new organelles and there is evidence that some at least of these are generated from the nucleus.3Relationships are suggested between the ultrastructural and cytochemical peculiarities of the egg, and the possible significance of these peculiarities in relation to the morphological alternation in the life cycle is discus