Megagametogenesis is the process of formation of female gamete or egg from megaspores of plants. It occurs as follows:
The process takes place inside the ovule of a plant. The 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 (2n). 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 antipodal cells. The cells in the center develop into the central cell. This entire structure finally called as embryo sac.
Finally, the embryo sac contains seven cells, out of which one is diploid and the rest are haploid.