embryo sac The female gametophyte (an oval structure in the nucellus of the ovule) of flowering plants, formed by the division of the haploid megaspore nucleus, and the site of fertilization of the egg and development of the embryo. It consists of 6 haploid cells without cell walls (2 synergidae, 3 antipodal cells, and an egg cell) and 2 haploid nuclei (polar nuclei). Sometimes the 2 haploid, polar nuclei fuse to form a single, endosperm moter cell. At fertilization, 1 male nucleus fuses with the egg nucleus to form a zygote which develops into the embryo. The second male nucleus fuses with the primary endosperm nucleus to form the endosperm nucleus. This then divides to form the endosperm. See also DOUBLE FERTILIZATION.

embryo sac 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 oosphere (egg cell), which is associated with two synergid cells to form the egg apparatus, is fertilized by a male nucleus and becomes 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.