With a neat diagram,explain the $7$-celled,$8$-nucleate nature of the female gametophyte.

(N/A) The female gametophyte (embryo sac) develops from a single functional megaspore. This megaspore undergoes three successive mitotic divisions to form an $8$-nucleate embryo sac.
The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other moves towards the chalazal end. Then,these nuclei divide at their respective ends and redivide to form an $8$-nucleate stage. As a result,there are four nuclei at each end,i.e.,at the micropylar and the chalazal end in the embryo sac.
At the micropylar end,out of the four nuclei,only three differentiate into two synergids and one egg cell. Together,they are known as the egg apparatus. Similarly,at the chalazal end,three out of four nuclei differentiate as antipodal cells. The remaining two nuclei (one from the micropylar and one from the chalazal end) move towards the centre and are known as the polar nuclei,which are situated in a large central cell.
Hence,at maturity,the female gametophyte appears as a $7$-celled structure,though it has $8$ nuclei.