At what stage does cell wall formation occur during the development of the embryo sac?

After the 8-nucleate stage

Option 4 is correct . After the 8-nucleate stage.

In flowering plants, the development of the female gametophyte or embryo sac begins with the formation of the megaspore mother cell, which undergoes meiosis to produce four megaspores. In the majority of flowering plants, only one of these megaspores is functional, while the other three degenerate. This functional megaspore then undergoes a series of divisions to form the embryo sac.

After the functional megaspore is formed, its nucleus divides mitotically to form two nuclei, which move to the opposite poles of the cell. This stage is known as the 2-nucleate embryo sac. Subsequently, two more sequential mitotic divisions occur, resulting in the formation of the 4-nucleate and 8-nucleate stages of the embryo sac. During these divisions, the nuclei divide without immediate cell wall formation, and as a result, the embryo sac remains as a multinucleate structure.

It is after the 8-nucleate stage that cell wall formation occurs in the embryo sac. Cell walls are laid down around the nuclei, leading to the organization of the typical female gametophyte or embryo sac. The cell walls separate the nuclei and form distinct cells within the structure.

At this point, the embryo sac typically consists of seven cells. Three cells are grouped together at the micropylar end and constitute the egg apparatus, which consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid.

Three cells are located at the chalazal end of the embryo sac and are called the antipodals. The large central cell, as mentioned earlier, contains two polar nuclei. Thus, the mature angiosperm embryo sac, although 8-nucleate, is composed of seven cells.

In summary, cell wall formation in the embryo sac occurs after the 8-nucleate stage. Prior to this stage, the nuclear divisions are free nuclear, without immediate cell wall formation. After the 8-nucleate stage, cell walls are laid down, resulting in the organization of the female gametophyte or embryo sac and the formation of distinct cells within the structure.