Solution:

The mitotic division (reduction division) of the mother megaspore gives rise to female gametophytes. The megaspore divides mitotically thrice to form firstly 2 then 4 and out of these 4 forms 8 nucleate embryo sacs. The process of formation of 7-celled, 8 nucleate nature of female gametophyte is as follows:

After the cell undergoes the first mitotic division, two nuclei are formed.

These two nuclei move to opposite poles, that is towards the micropylar end and the chalazal end, respectively.

The two nuclei divide into 4 and redivide to form 8 nucleate stages.

These 8 nuclei arrange themselves in a manner so that, there are four nuclei each on either end.

At the micropylar end, three out of the four nuclei differentiate into an egg cell and synergids.

At the chalazal end, three out of the four nuclei differentiate as antipodal cells.

The remaining cells, each from either end move towards the center and are known as polar nuclei.

Therefore, on maturation, the female gametophyte looks like a 7-celled structure.