A typical embryo sac is 8 nucleate and 7 celled. Six of the eight nuclei become surrounded by cell wall and are organised into cells. Two of the nuclei, called polar nuclei remain in the centre of the large central cell. Three cells are grouped together at the micropylar end forming the egg apparatus. Of these, one is the female gamete and two are synergids; the synergids have special cellular thickening called filiform apparatus at the micropylar tip.
Three cells are grouped at the clealazal end; they are called antipodals. The large cell in the centre of the embryo sac is the central cell. Later 2 polar nuclei in the centre cell fused to form a depolid secondary nucleus or endosperm nucleus. Thus the embryo sac of flowering plants is 8 nucleate 7 celled at maturity. This type of embryo sac is called monosporic because it is formed from only one of the 4 megaspores.
