Megaspore represents the first stage of the female gametophyte. It develops to form the female gametophyte. The female gametophyte is also called the embryo sac. The megaspore enlarges and its nucleus divides mitotically to form two nuclei. A vacuole appears between these two nuclei and pushes the nuclei towards the opposite poles of the cell. Both nuclei divide mitosis division to form eight nuclei, four at the micropylar end and four at the chalazal end. This is followed by the cellular organization of the embryo sac.
one nucleus from each pole moves towards the center of the embryo sac and fuse to form a polar nucleus or secondary nucleus. The remaining three nuclei at the micropylar end organize into egg apparatus. The egg apparatus consists of two lateral synergids and a medium-large egg cell or female gamete. Similarly, three nuclei of the chalazal end form the antipodal cells. These cells help to provide nutrition for the embryo sac. A mature female gametophyte before fertilization has seven nuclei (3 in egg apparatus, 3 in antipodal cell, and a diploid polar nucleus). The egg apparatus and antipodal cells are haploid in nature. The formation of female gametophytes is followed by fertilization of the egg cell and the secondary nucleus.