PREPARE IN ADVANCE: Have clams shelled, save shells. Discard shallow halves, scrub the deep halves. Drop the shells into boiling water, boil for 2-3 minutes, then drain. Rinse and dry. TO COOK AND SERVE: Bring the Sake to boil over high heat. Sprinkle MSG, then drop in the clams, stir gently, and cover. Cook over moderately high heat for 3-4 minutes, then remove the clams and place in the shell. Garnish each clam with a slice of lemon. Serve at room temperature.