The functional megaspore becomes bigger in size.
The mitotic division occurs in the nucleus to make 2 nuclei that go in direction of opposite poles.
Every nucleus at the poles undergoes two mitotic divisions to form 4 nuclei in each pole or a total of 8 nuclei.
Two nuclei from each pole move to the center to produce the polar nuclei.
The other nuclei, three at each pole, get surrounded by a bit of cytoplasm to form cells.
The female gametophyte or the embryo sac thus has 7 cells and eight nuclei.