Why embryo sac in flower is called 7 celled and 8 nucleate?

The female gametophyte (embryo sac) develops from a single functional megaspore. This megaspore undergoes three successive mitotic divisions to form eight nucleate embryo sacs. The first mitotic division in the megaspore forms two nuclei. One nucleus moves towards the micropylar end while the other nucleus moves towards the chalazal end. Then, these nuclei divide at their respective ends and re-divide to form eight nucleate stages. As a result, there are four nuclei each at both the ends 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 differentiates as antipodal cells. The remaining two cells (of the micropylar and 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 nucleate. read less

It is called so because during the process of mega prognosis only one cell remain out of the four and other three degenerate then the fourth cell grows and it's nucleus divides by mitotic division into 8 nucleus after that the nucleus develops cytoplasm and cell membrane around them but two nuclei out of 8 as the others have gone to two opposite poles in group of three the remaining two does not grow different cell membranes around and hence remain in the di-karyon phase for some time they share one cell hence it is said 8 nucleated and 7 celled. read less