Combine gelatin and 1/2 cup of the sugar in top of double boiler. Beat egg yolks slightly. Combine with water and lemon juice; add to gelatin mixture. Place over boiling water and cook, stirring constantly, until gelatin is dissolved and mixture is slightly thickened. Add lemon rind. Chill until mixture is slightly thicker than unbeaten egg white. Beat egg whites until stiff. Gradually add remaining 1/2 cup sugar; beat until very stiff. Slowly fold egg whites into gelatin mixture. Pile into cooled, baked pastry shell. Chill until firm.