Review of the American Ballet Theatre spring season at the Metropolitan Opera House, the New York City Ballet in Balanchine's Liebeslieder Walzer, and the May 13 gala performance in celebration of the Metropolitan Opera's centennial featuring Margot Fonteyn, Alicia Alonso, Antoinette Sibley, Marcia Haydée, Natalia Makarova, Lynn Seymour, and Liz Jeppeson.