Describe the development of endosperm after double fertilization in an angiosperm. Why does endosperm development preceeds that of zygote?

Pollen tube penetrates the stylar canal and enters the ovule through micropyle, chalaza or integument followed by discharge of male gametes in the embryo sac. One male gamete fuses with egg nucleus (syngamy) to form a diploid embryo. The other male gamete fuses with diploid secondary nucleus / central cell (triple fusion) to form triploid endosperm. Syngamy and triple fusion are collectively referred to as double fertilization which is a characteristic feature of angiosperms. Endosperm serves as a source of nutrition for developing embryo and hence develops before zygote.