Formation of the mature embryo sac
The megaspore mother cell (MMC) undergoes meiosis and forms a linear tetrad of 4 haploid megaspores.
The process of meiotic formation of haploid megaspores from diploid megaspore mother cell is called megasporogenesis.
Development of Female Gametophyte
The functional megaspore is the first cell of the female gametophyte.
The cell enlarges and undergoes three free nuclear mitotic divisions. The first division produces two nucleated embryo sac. These two nuclei divide twice forming four nucleate and then eight nucleate structure.
One nucleus from each side moves to the middle of the embryo sac. They are called polar nuclei.
The remaining three nuclei form cells at the two ends, three celled egg apparatus at the micropylar end and three antipodal cells at the chalazal end.
The middle binucleate structure organises itself into a central cell.
The female gametophyte in angiosperms is called the embryo sac. It is a 7-celled and 8-nucleated structure.