Megaspores are produced from the megaspore mother cells after:

A hypodermal cell of the nucellus enlarges to form an archesporia cell which transformed into a megaspore mother cell. Each diploid megaspore mother cell undergoes meiosis to form linear tetrad of megaspores. Three megaspores degenerate and remaining functional one enlarges at the chalazal end. Related Theory The functional megaspore undergoes three subsequent mitotic divisions and forms the eight nuclei. The eight nuclei arrange themselves into three groups. Three nuclei migrate at the micropylar end and forms the egg apparatus, consisting of two synergids and a round egg cell. The other three nuclei arrange themselves at the chalazal end and form the antipodal cells. The remaining two nuclei come together as polar nuclei in the centre of the embryo sac. All the cells of the embryo sac are haploid.