Complex sentence: Meyer-Sée left The Sackville Gallery in August 1912 to start the Marlborough Gallery at 34 Duke Street, London, where in April 1913 he organised an exhibition by Gino Severini, one of the Futurists who had taken part in the 1912 exhibition.
Split into simpler sentences: Meyer-Sée left The Sackville Gallery in August 1912 to start the Marlborough Gallery at 34 Duke Street, London. In April 1913 he organised an exhibition there by Gino Severini, one of the Futurists who had taken part in the 1912 exhibition.