The New Student's Reference Work/Embryo-Sac

Em′bryo-Sac (in plants). In the ovule of angiosperms there is developed what appears to be a large cavity, but is really a single large spore which is not discharged (see Megaspore). This spore germinates and develops within itself a very simple gametophyte consisting of seven cells (see Gametophyte). This spore containing its gametophyte, before it was known to be a spore,
was called the embryo-sac, because the embryo was observed to develop within it. At one end of the sac three of the seven cells are grouped together, the group being called the egg-apparatus, since the central cell is the egg, which is to produce the embryo, and the other two aid it in the process of fertilization. At the opposite end of the sac is another group of three cells known as the antipodal cells. Usually they disappear very soon after they are formed. In the central region of the sac a very large cell appears, called the definitive nucleus or endosperm nucleus. It is this cell which produces the endosperm or food-cells for the use of the embryo. In many seeds, as corn and wheat, it is the endosperm which supplies the food-material used by man.