Complex sentence: In April 1946 MacMillan was a founder member, and quickly made progress, being cast by de Valois' principal choreographer, Frederick Ashton, in a leading role in a new ballet, "Valses nobles et sentimentales", in October 1946.
Split into simpler sentences: In April 1946 MacMillan was a founder member, and quickly made progress. He was cast by Frederick Ashton, de Valois' principal choreographer, in a leading role in a new ballet, "Valses nobles et sentimentales", in October 1946.