whole plant megagametophyte A whole plant in the gametophyte development stage (PO:0028003) that produces only plant egg cells (PO:0020094). Megagametophytes in bryophytes only produce egg cells, but do not develop from megaspores, because there is no heterospory in bryophytes. In some pteridophytes, megagametophytes develop from megaspores. In angiosperms, Gnetum, and Weltwischia, the megagametophyte is greatly reduced. The megagametophyte in angiosperms is an embryo sac.