In the mature ovule, within the nucellus towards the micropylar end is a large cell called embryo sac. It is also called female gametophyte. It contains cytoplasm, two free nuclei and six small cells. Two free nuclei fuse to form secondary nucleus. Out of these three cells constitute egg apparatus (two synergids and one oosphere or egg cell). The other three cells lie at chalazal end of embryo sac and are called antipodal cells. They are small and enclosed in thin cell walls.

In egg apparatus egg fuses with male gamete to form zygote or oospore which later gives rise to embryo. Synergids help in fertilization. Secondary nucleus fuses with the second male gamete to form primary endosperm nucleus which later on forms endosperm.

The three antipodal cells degenerate.

Structure of typical embryo sac.