A mature archegonium of Cerotopteris thalictroides (L.) Brongn. consists of a short neck with three to five (typically four) tiers of cells curved backward away from the notch, an enlarged venter surrounded by a layer of sterite jacket cells and an axial row of a binucleate neck canal cell, a ventral canal cell and an egg. The archegonium is derived from the archegonial inital together with its surrounding prothallial cells rather than just from a single Initial as commonly thought. Furthermore it was observed that the archegonial initial also gives rise to the prothallial tissue which is not related to archegonium.