Cut avocados lengthwise into halves; remove the stone and skin. This is done easily by first running a blunt silver knife around the inner skin. Place halves cut side down and slice a thin oval from the outside of each. Sprinkle the ovals with lemon juice and press them gently around the sides of a 6-cup ring mold. Sieve the rest of the avocado or whirl in a blender. Soften the gelatin in ¼ cup lime juice and dissolve over hot water. Beat the cheese until soft and smooth (or whirl in blender). Remove the tough stems of the watercress only if necessary. Combine and beat all ingredients. Turn into the mold and chill until firm. If you are in a hurry, the process may be speeded by putting the ring in the freezing compartment of the refrigerator for a while. The center of the ring may be filled with chicken, shrimp, or crabmeat salad.