Beat egg-yolks and place in top of a double boiler with sugar, grated rind and juice, stirring until sugar melts (about 8 minutes). Remove from heat and keep beating until the mixture is luke-warm. Dissolve gelatin in 1/4 cup of cold water and melt over low heat. Stir into lemon/egg mixture.
Whip cream and fold in. Beat the egg-whites until stiff and fold into the mixture.
Pour into a deep bowl and refrigerate for 2 hours or until firm.