Put crackers in a plastic bag and tie top; crush with a rolling pin or jar to make fine crumbs. Melt margarine in the bottom of a 8- or 9-inch round cake pan. Spread margarine evenly over bottom of pan. Sprinkle crumbs evenly on bottom only; press gently. Chill in refrigerator. Soak gelatin in cold water. Heat over boiling water to dissolve the gelatin. Combine yogurt and first measure of sweetener; beat at moderate speed with rotary beaters, adding dissolved gelatin gradually. Chill until it is the consistency of unbeaten egg whites. Meanwhile, place iced water, lemon rind, egg white, and dry milk powder in a large bowl. Beat with rotary beaters until soft peaks form. Add lemon juice, vanilla, and second measure of sweetener. Beat at high speed until stiff. Fold into partially set yogurt; blend very well. Spoon mixture on top of crumbs in pan. Chill 4 hours or longer until set. To unmold, loosen around edge of mold with thin spatula right down to bottom of pan. Place larger plate upside down on top of mold. Turn plate and pan over; cover top of pan for a few seconds with a hot cloth that has been run under hot water and then rung out. Remove cloth and lift pan from mold. When ready to serve, garnish with cherries or strawberries and cut into six equal slices. 6 Servings 1 Serving: 1/6 Cake Nutritive values per serving: 13 gm. Carbohydrates; 8 gm. Protein; 2 gm. Fat; 104 calories; 0.3 gm. Fiber; 124 mg. Sodium; 6 mg. Cholesterol Food Exchange per serving: 1 Low-Fat Milk Exchange Low-sodium diets: This recipe is suitable.