In large bowl, beat egg yolks and sugar until thick. Beat in the butter, lemon juice and lemon rind. Beat in the flour. Add milk and mix well. In a separate bowl, beat the egg whites until stiff peaks form. Fold the egg whites in the batter. Pour batter into the pie shell and bake at 350F degrees for 55-60 minutes, until the pie is set and browned on top. Cool completely before cutting.