As the name suggests, double fertilisation involves two types of fertilisation-syngamy and triple fusion. The release of two male gametes into the cytoplasm of one of the synergids causes its degeneration.
One male gamete fuses with the egg cell to form a diploid zygote (which later forms embryo). This process is called syngamy or generative fertilisation. The other male gamete fuses with the two polar nuclei in the central cell by a process called triple fusion, which results in the formation of a triploid.
Primary Endosperm Nucleus (PEN) and the central cell becomes the Primary Endosperm Cell (PEC). This PEC divides repeatedly to form triploid endosperm tissue which further undergoes successive free nuclear divisions to give rise to free nuclear endosperm (free-nuclei).
This stage is followed by the wall formation around the cellular endosperm. Thus, the events of double fertilisation are immediately followed by post-fertilisation events involving development of endosperm.