An archegonium (pl: archegonia), from the ancient Greek αρχη (beginning) an γονος (offspring), is a multicellular structure or organ of the gametophyte phase of certain plants producing and containing the ovum or female gamete. The archegonium has a long neck and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the horned liverworts they are embedded.