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a spore from which develops the female prothallium (in gymnosperms, the primary endosperm nucleus; in angiosperms, the embryo sac). The megaspore arises as a result of the division of the megasporocyte (mother cell). In Pteridophyta the number of mature megaspores in the megasporangium varies. The Isoetes have many megaspores. The Lepidodendron and Sigillaria have eight to 16; the Selaginella, two to four; the water ferns, only one. Megaspores grow into female prothallia either inside (Lepidodendron, Selaginella, Salvinia) or outside (Marsilea, some Selaginella) the megasporangium. The first stages in the development of the prothallium occur inside the megaspore; the membranes of the megaspore then open and the parts of the prothallium that bear the archegonia emerge (in Salvinia they turn green). In seed plants, four megaspores usually form in each nucellus (the central part of the ovule).