Oogenesis in Anisocampium sheareri was studied using ultrastructural methods to examine the egg structure influencing fertilization in the core leptosporangiate ferns,which is less studied.Ultrastructural observations indicated that the young egg and the canal cells are closely connected with each other in the cavity of the archegonium.Then,a separation cavity and a temporary wall are first formed above the egg during oogenesis with a pore region persistently connecting the egg and the ventral canal cell.The egg envelope is formed by deposition of amorphous materials in the separation cavity on the outer surface of plasmalemma.The egg envelope is not formed across the pore region;instead,a fertilization pore is formed.In the later stage of oogenesis,the egg nucleus becomes highly irregular and produces extensive evaginations and nuclear holds.The present investigation provided new data on oogenesis of the ferns,which contributes to understanding the cytological mechanism of the sexual reproduction of ferns and their evolution.