Megagametogenesis

Megagametogenesis is the development of a megaspore into an embryo sac, which is the gametophyte - though a highly reduced one - stage in the life cycle of vascular plants.

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Eudicots

In eudicot plants, the entire process happens inside the ovule of a plant. The details of the process vary by species, but the process described here is common. This process starts with a single diploid megasporocyte in the nucleus. This megasporocyte undergoes meiosis to form four cells that are haploid. Three cells die and one that is most distant from the micropyle develops into the megaspore. This megaspore becomes larger and the nucleus of it undergoes mitosis three times until there are eight nuclei. These eight nuclei are then arranged into two groups of four. These groups both send a nucleus to the center of the cell which then becomes the polar nuclei. The three cells left at the end of the cell near the micropylar become the egg apparatus with an egg cell in the center and two synergids. A cell wall forms around the other set of nuclei and forms the antipodals. The cells in the center develop into the central cell. This entire structure with its eight nuclei is called the embryo sac.

Monocots

References

Raven,Peter H., Evert, Ray F., Eichhorn, Susan E.(2005). "Biology of Plants, 7th Edition". W. H. Freeman Chapter 19: 442-449.