Introduction:
Angiosperms are flowering plants that bear seeds within an enclosing fruit. They are the most diverse group of land plants and have evolved a unique reproductive structure called the embryo sac. The embryo sac is a female gametophyte that develops within the ovule of a flower and is responsible for producing the female gametes necessary for sexual reproduction.

Maturity of the Embryo Sac:
At maturity, the embryo sac of angiosperms is 8-nucleate, which means it contains eight nuclei. However, it is only 7-celled, which means it contains seven separate cells. This discrepancy is due to the fact that one of the nuclei is duplicated during the development of the embryo sac, resulting in two nuclei that are identical in genetic content.

The Structure of the Embryo Sac:
The embryo sac of angiosperms is composed of three main parts: the egg apparatus, the central cell, and the antipodal cells.

The Egg Apparatus:
The egg apparatus is located at one end of the embryo sac and consists of three cells: the egg cell and two synergids. The egg cell is responsible for producing the female gamete that will eventually fuse with the male gamete to form a zygote.

The Central Cell:
The central cell is located at the other end of the embryo sac and contains two nuclei that have fused together. This cell is responsible for producing the endosperm, which is a nutrient-rich tissue that provides nourishment to the developing embryo.

The Antipodal Cells:
The antipodal cells are located between the egg apparatus and the central cell and are responsible for supporting the development of the embryo sac.

Conclusion:
In conclusion, the angiosperm embryo sac, at maturity, is 8-nucleate but only 7-celled due to the duplication of one of the nuclei during its development. The structure of the embryo sac is composed of the egg apparatus, the central cell, and the antipodal cells, each with its own important role in the sexual reproduction of angiosperms.