The process of formation of haploid megaspores from the diploid megaspore mother cell (MMC)...
Megasporogenesis
The process of formation of haploid megaspores from the diploid megaspore mother cell (MMC) is called megasporogenesis.
In the micropylar region, the nucellus contains cells with dense cytoplasm and prominent nucleus which is differentiated into a single megaspore mother cell (MMC).
Megaspore mother cell is diploid and undergoes meiosis.
MMC first divides transversely into two cells called dyad.
These two cells again divide transversely, as a result a linear row of four haploid cells is produced which is called megaspore tetrad or linear tetrad.
Out of these four megaspores, only one remains functional while the other three degenerates.
The one functional megaspore develops into the female gametophyte or embryo sac. This is called monosporic development.
Megagametogenesis
The formation of female gametophyte (embryo sac) is called megagametogenesis.
Megaspore is the first cell of the female gametophyte.
The megaspore increases in size and its nucleus divides mitotically into two nuclei which move apart to opposite poles. Thus, a 2-nucleate embryo sac is formed.
The two daughter nuclei undergo another mitotic division giving rise to the 4- nucleate stage.
The third mitotic division gives rise to 8-nucleate 7-celled embryo sac.
The central cell contains 2 nuclei known as polar nuclei.
The three nuclei at the micropylar region form the egg apparatus.
In the egg apparatus, the middle cell is the largest and is called oosphere/egg/ovum, while other two naked cells adjoining the egg cell are called synergids.
The three nuclei at the chalazal end are surrounded by cytoplasm and cellular wall. These are called antipodal cells.
The above-mentioned method of female gametophyte formation is known as normal 8-nucleate type, because 8 nuclei contribute in the formation of gametophyte. It is very common among angiosperms.