Endosperm is formed when the two sperm nuclei inside a pollen grain reach the inside of an ovule or embryo sac. One sperm nucleus fertilizes the egg, forming a zygote, while the other sperm nucleus usually fuses with the two polar nuclei at the center of the embryo sac. This makes the first endosperm cell, which develops into the endosperm. Because it is formed by a separate fertilization, the endosperm is an organism distinct from the embryo (though placed next to it in the seed).