A large cell that develops in the ovule of flowering plants. It is equivalent to the female gametophyte of lower plants, although it is very much reduced. Typically, it contains eight nuclei formed by division of the megaspore mother cell (see illustration). The haploid oosphere (egg cell), which is associated with two synergid cells to form the egg apparatus, is fertilized by a male nucleus and becomes the diploid zygote, which subsequently divides to form the embryo. The two polar nuclei fuse with a second male nucleus to form a triploid nucleus that gives rise to the endosperm. The three remaining nuclei form the antipodal cells.

Embryo sac