A typical embryo sac is ellipsoidal in structure with tapering ends and it is made up of cellulosic cell wall. Embryo sac is present at the centre of the nucellus of the ovule. It is seven celled and eight nucleated structure which is composed of three celled egg apparatus, three antipodals and a bi nucleated central cell or secondary nucleus.
The egg apparatus is found at the micropylar end of the embryo sac. It consists of two synergids and one egg cell. The synergid partly encloses the egg and each synergid is associated with the filiform apparatus at the micropylar end. The filiform apparatus is a mass of finger like projections of the walls of the synergids which consist of tightly packed micro fibrils enclosed in the non fibrillar sheath. One synergids degenerate before the entry of the pollen tube into the embryo sac. Whereas the other synergids degenerate after the discharge of male gametes from the pollen tube. In certain plants synergids functions as haustoria.
The antipodals are three in number present at the chalazal end. Antipodals degenerates after the fertilization. Antipodals possess a definite cell wall. In some embryo sac antipodals are completely absent. The cytoplasm of the antipodals cells is rich in mitochondria, plastids and dictyosomes.
Central cell is the largest cell of the embryo sac and is considered as the mother cell of the endosperm. Large central vacuole is present in the central cell which acts as the reservoir of amino acids, inorganic salts and sugars which is used up during the fertilization and endosperm formation. Two polar nuclei fuse to form the secondary nucleus; after fertilization the secondary nucleus forms the endosperm.
