Cut the grape-fruit into halves, crosswise, and scoop out the pulp, rejecting the white inner skin as well as the seeds. Clean the shells; cut the edges with a sharp knife into scallops and throw them into cold water. Set the pulp on the ice. At serving time put a teaspoon of cracked ice in the bottom of each shell; fill with the pulp, mixed thoroughly with powdered sugar and a little sherry, if desired; and place a maraschino cherry or bit of bright-colored jelly in the centre of each. Lay on paper doilies or surround with bits of asparagus fern.