The embryo sac

In most flowering plants only one of the four megaspores is functional; the other three degenerate. Forming the embryo sac from a single megaspore is called monosporic development.

The functional megaspore's nucleus divides by mitosis into two nuclei that move to opposite poles (2-nucleate embryo sac). Two more mitotic divisions give the 4-nucleate and then the 8-nucleate stage.

These three divisions are strictly free-nuclear: the nuclei divide without walls forming right after. Walls are laid down only after the 8-nucleate stage, organising the mature embryo sac.

Six nuclei are walled off into cells. At the micropylar end, three cells form the egg apparatus: two synergids and one egg cell. At the chalazal end are three antipodal cells.

The remaining two nuclei, the polar nuclei, lie in the large central cell just below the egg apparatus.

Synergids carry special thickenings at their micropylar tip, the filiform apparatus, which guides the pollen tube into a synergid.

So the typical mature angiosperm embryo sac is 8-nucleate but 7-celled: 3 antipodals + 2 synergids + 1 egg + 1 central cell with 2 polar nuclei.