In
seed plants, the
ovule ("small
egg") is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The
integument(s) forming its outer layer(s), the
nucellus (or remnant of the
megasporangium), and female
gametophyte (formed from haploid
megaspore) in its center. The female gametophyte—specifically termed a
megagametophyte—is also called the
embryo sac in
angiosperms. The megagametophyte produces an
egg cell (or several egg cells in some groups) for the purpose of
fertilization. After fertilization, the ovule develops into a
seed.