comes early on in the novel, in chapter one. When the overweight boy is referred to asPiggy, this represents only a part of him—his physical size. 

Ralph did not take the hint so the fat boy was forced to continue. "I don't care what they call me," he said confidentially, "so long as they don't call me what they used to call me at school." Ralph was faintly interested. "What was that?" The fat boy glanced over his shoulder, then leaned toward Ralph. He whispered. "They used to call me 'Piggy.'" Ralph shrieked with laughter. He jumped up. "Piggy! Piggy!" "Ralph—please!"

In chapter 4, entitled "Painted Faces and Long Hair," Jack makes a mask and wears it to become something other than he had been. 

He spilt the water and leapt to his feet, laughing excitedly. Beside the pool his sinewy body held up a mask that drew their eyes and appalled them. He began to dance and his laughter became a bloodthirsty snarling. He capered toward Bill, and the mask was a thing on its own, behind which Jack hid, liberated from shame and self-consciousness. The face of red and white and black swung through the air and jigged toward Bill. Bill started up laughing; then suddenly he fell silent and blundered away through the bushes. Jack rushed toward the twins. 'The rest are making a line. Come on!" "But—" "—we—" "Come on! I'll creep up and stab—" The mask compelled them.

Jack is referred to as the mask and that is an example of synecdoche.