Embryo sac is also called female gametophyte which is haploid. Inside the embryo sac, a total of eight nuclei and seven cells are present. Out of them, there is one egg cell, two synergids, two polar nuclei and three antipodal cells.
Angiosperms have a unique event of double fertilization. The pollen tube after entering through one of the two synergids, releases the two male gametes into the synergid's cytoplasm. The process of syngamy is completed when one of the male gametes move towards the egg cell and the nucleus of the egg cell fuses with the male gamete. The process of syngamy results in the formation of a zygote which is a diploid cell. The other male gamete moves towards the central cell, where two polar nuclei are located and fuse with them. This results in the production of a primary endosperm nucleus (PEN)which is triploid. The term triple fusion is used as it involves the fusion of three haploid nuclei. This phenomenon is termed double fertilization because two types of fusion - syngamy and triple fusion take place inside an embryo sac. All other cells like synergids and antipodals will be haploid structure of embryo sac after fertilization that will degenerate. After triple fusion, the central cell becomes the primary endosperm cell (PEC) and it is further developed into the endosperm when the zygote develops into the embryo.