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After entering one of the synergids, the pollen tube releases two male gametes into the cytoplasm of the synergid. One of the male gametes moves towards the egg cell and fuses with its nucleus, completing syngamy and forming a diploid zygote. Simultaneously, the other male gamete moves towards the two polar nuclei in the central cell and fuses with them, resulting in the formation of a triploid primary endosperm nucleus (PEN). This fusion involving three haploid nuclei is referred to as triple fusion. Endosperm development occurs before embryo development. This sequence is observed because the primary endosperm cell undergoes repeated divisions, giving rise to a triploid endosperm tissue. The cells of this tissue accumulate reserve food materials. In the most common type of endosperm development, the primary endosperm nucleus undergoes successive nuclear divisions, resulting in the formation of free nuclei. This stage of endosperm development is known as free-nuclear endosperm.