Polypodiaceae is the most evolutionary group in the ferns.However,the egg development in this group is less studied.The present investigation chooses the fern Phymatosorus hainanensis of Polypodiaceae,to examine the detailed development of the egg.Ultrastructural observations indicate that:(1)The young egg,the ventral canal cell and neck canal cell are closely connected with each other by plasmadesmata.(2)During egg development,the cell wall above the egg become obvious thickened and then a separation cavity,which contains plenty of amorphous materials,is formed beneath the wall.(3)A connection region persistently connects the egg and the ventral canal cell in the central area of the upper surface of the egg.(4)An egg envelope is formed by deposition of amorphous materials in the separation cavity on the outer surface of the plasmalemma.The egg envelope was not formed across the connection region;instead,a fertilization pore was formed.(5)In the later stage of oogenesis,the egg nucleus becomes irregular in outline and produces extensive evaginations.The present investigation provides new data on oogenesis of the ferns,which contributes to understanding of the cytological mechanism of the sexual reproduction of ferns and their evolution.