Beat egg yolks at medium speed of an electric mixer until thick and lemon colored. Add sweetened condensed milk, powdered sugar, and Key lime juice, stirring well. Spoon filling into crust. Beat egg whites and cream of tartar at high speed of an electric mixer 1 minute. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form and sugar dissolves (2 to 4 minutes). Spread meringue over filling, sealing to edge of pastry. Bake at 350 one 9-inch pie. ----