Pollen tube grows through the style and reaches the micropylar end. At the micropylar end, there are two synergids around the egg. The synergids have filiform apparatus that guides the pollen tube towards the egg. Thus the pollen tube enters embryo sac through one of the synergids.
Related Theory
The fully developed embryo sac at consists of three antipodal cells, two synergids, one egg cell and one secondary nucleus. The pollen tube pierces through the nucellus and penetrates the wall of the embryo sac. The tip of the pollen tube disintegrates and the two male gametes are set free in the embryo sac. Of the two male gametes, one fuses with the egg cell and the other with the secondary nucleus on the polar nuclei. The fertilised egg becomes zygote and the secondary nucleus fuse to forms primary endosperm nucleus, which is the result of triple fusion.