The female embryo sac or gametophyte is an oval structure found in the ovary of flowering plants. The ovary in the plant is a structure that encloses female reproductive cells consisting of the nucleus, integument, and female gametophyte. The embryo is said to form when the haploid megaspore nucleus splits. It has two haploid nuclei and six haploid cells with no cell walls.

Embryo Sac Structure

Angiosperms: The standard mature embryonic stem or female Gametophyte usually contains 7 cells, 8-nucleated sac-like structures found in the ovule nucellus. It is located in the megaspore due to mega gametogenesis (multiple mitotic stages). The active Embryo bag may be a sac, haploid, short-lived and heterotrophic structure.

It usually has:

1. Egg Apparatus
2. Antipodals 
3. Central Cell

1. Egg Apparatus

It is a structure with 3 cells and is always present in the middle cell to the end of the micropylar. It has a pair of synergids and an egg yolk. All three cells are fully present as a triangular structure. Its protoplasts are highly differentiated.

• Synergids: These are long cells with a sharp or pointed end at the end of the micropylar. They have a lot of finger-like speculation on the wall in the cytoplasm called Filiform Apparatus. It is a wedge or spheroidal structure composed of cellulose microfibrils. They may decompose quickly after fertilization.

• Egg cell: Egg cells have a special and haploid structure with a polarized protoplast. Shares the wall with antipodals and a cell in the middle of the micropylar end. In comparison, the distal part has a thin wall. It consists of a closed cytoplasm with many plastids near the micropylar end and a large nucleus at the far end. Its nucleus of the female gamete results in a Zygote due to Syngamy (Fertility) through the incorporation of the male gamete.

2.  Antipodals

• Antipodals show significant differences between parts of the embryo sac. They usually decompose just before or immediately after fertilization. 
• In many plants, antipodals persist and exhibit certain structural and cytological features that enhance their potential role in embryo sac nutrition. 
• The antipodals divide the mitotic in the grass to form about 300 cells. During the further division of antipodals, the cell walls of many cells remain incomplete, leading to the formation of multinucleate protoplast or syncytium. 
• Antipodal nuclei may also be polyploid due to endopolyploidy or polytony.
• Antipodal maize cells have many mitochondria, plastids, and multicriterial dictyosomes. The cytoplasm has many small vesicles found in the endoplasmic reticulum or dictyosomes. 
• An interesting feature of some of these cells is the presence of I of papillate wall outgrowth, resulting in the cytoplasm. These outgrowths, limited to the border cells of the nucellus, appear to resemble filiform apparatus. 
• A similar guess on the wall has also been observed in the antipodal cells of the POPPY plant. Histochemical tests reveal that antipodal cells are rich in protein, peroxidase, cytochrome oxidase, ascorbic acid, and sulfhydryl compounds. They show a very low concentration of RNA and polysaccharides.
• There are several cases where antipodal cells act as haustoria. 
• In Argemone Mexicana, the antipodal cells are very large and persistent until they reach the heart-shaped stage of the embryo. They eat 8-10 layers of nucellar cells at the end of chalaza1. 
• The role of antipodals is known. The role of nutrients has been considered in those persistent antipodals. The nature of its cytoplasm suggests that they are highly active cells. 
• They are like the cells of the secretory anther tapetum and the integumentary tapetum by having high DNA content. The presence of wall speculation in the antipodal cells of certain plants indicates that they may be fully associated with the feeding of the embryo sac.

3. The Central Cell

• This is the largest cell in the embryo sac, as well as the mother cell of the endosperm. The proliferation of the embryo sac after the final nuclear breakdown is mainly due to the depletion of the large central vacuole cell. 
• The vacuole in the inner cell is almost a pool of sugar, amino acids, and salts of inanimate matter. The nucleus of the central cell is also called the polar nuclei, is very large, and is characterized by a clear nucleolus.
• They are present either in the central cell, suspended by cytoplasmic strands, or in the cytoplasm near the egg apparatus. In the latter case, the chalaza portion of the embryo sac is encased in a large vacuole. 
• Two polar nuclei combine before or during double fertilization to form a second nucleus. The cytoplasm of the middle cell is rich in plastids, mitochondria, numerous dictyosomes, and ribosomes or small polysomes. This cell appears to be the centre of deep embryo activity in the embryo sac.
• The size of the central cell wall varies. It is very thick in regions where it meets the nucellus. When the central cell touches the egg and synergids, it indicates a common feature of the partial wall. 
• It is thinner towards the chalaza end of the egg apparatus, and finally, in the chalaza region, there is no wall between the plasma layers of the middle cell and those of the egg and synergids. The middle cell is connected to the egg, synergids, and antipodals through plasmodesmata interactions.

Conclusion

After double fertilization, the developed embryo sac metamorphoses into embryonic and endospermic structures in the ovule, and so forth. Angiosperms, like all other plants, reproduce in a generational cycle. A sporophyte generation (a common flowering plant) generates haploid spores, which mature into their plant generation, the gametophyte generation. The pollen grain is a male gametophyte that consists of an exterior coat, two sperm cells, and one tube cell when fully matured. At maturity, the female gametophyte has a seven-cell embryo sac, one of which is the egg. An ovule is a structure on the parent sporophyte plant that houses the female gametophyte.