Megaspore

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Plant ovules with megasporocytes before meiosis: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right

Megaspores, also called macrospores, are a type of spores that is present in heterosporous plants. These types of plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female (egg-producing) gametophyte, which is fertilized by sperm produced by the male gametophyte developing from the microspore. Plants which are known to do this include those in the following genera:

Selaginella: the spikemosses, included within the lycopod, or clubmoss group
Marsileaceae: the water-clovers, Marsilea, Pilularia, and Regnellidium
Azolla: in the Azollaceae, within the ferns
Salvinia: in the Salviniaceae, within the ferns
gymnosperms and flowering plants

In gymnosperms and flowering plants, the megaspore is produced inside the nucellus or embryo sac of the ovule. A diploid precursor cell, the megasporocyte or megaspore mother cell, undergoes meiosis to produce initially four haploid cells. Three of those cells typically degenerate resulting in one functional megaspore per ovule. The megaspore then undergoes megagametogenesis to give rise to the female gametophyte.