Abstract: The mature egg of ferns possesses a layer of egg membrane in the periphery. However, the fine trastructure and the formation of the egg membrane in the egg of ferns are still unclear. The present paper described the formation and the ultrastructure of the egg membrane in the egg of the fern Ceratopteris thalictroides using transmission electron microscopy (TEM). The results show that the egg membrane begins to form at the stage of the maturing egg. The egg membrane in the upper side of the egg is prominent. It is formed by attaching of osmiophilic sheets of endoplasmic reticulum to the inner surface of the plasmalemma . When mature the egg membrane in the upper side of the egg is much thick in the central region and it becomes thinner gradually towards the margin. Some osmiophilic materials are filled in the spaces between the sheets of endoplasmic reticulum . On the contrary, the egg membrane in the lower side of the egg is thin . It is composed of two
osmiophilic membranes which are associated with each other closely. The formation and the ultrastructure of the egg membrane are described for the first time . Some functions of the egg membrane are also discussed .

Key words: Fern