The female gametophyte that develops from the megaspores of heterosporous plants. Among heterosporous species of the lycophyte plants, for example, the sporophyte plant produces megaspores stocked with food. These spores grow into megagametophytes that produce eggs. After fertilization of the eggs, the embryos grow into new sporophytes inside the megagametophyte and are nourished by the food within it. In the seed plants, the megagametophyte generation is very small and lives its entire existence within the ovules of the sporophyte plant, where some of its cells become eggs, while others become food reserves for the resultant embryos. See more at alternation of generations, gametophyte.