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The formation of megaspores inside the ovules of seed plants. A diploid
cell in the ovule, called a megasporocyte or a megaspore mother cell,
undergoes meiosis and gives rise to four haploid megaspores. In most
plants, only one of the megaspores then goes on to develop into a
megagametophyte within the ovule, while the other three disintegrate. In
the ovules of angiosperms, megasporogenesis takes place within a
structure called a nucellus, and it is the megaspore farthest from the
micropyle of the ovary that survives.