Original: Upon returning to Italy, Toscanini returned to his chair in the cello section, and participated as cellist in the world premiere of Verdi's Otello (La Scala, Milan, 1887) under the composer's supervision.
Simplified: When he got back to Italy Toscanini returned to his chair in the cello section, and took part as cellist in the first performance of Verdi's Otello (La Scala, 1887).