During the first half of the 19th c., the duty of directing Italian opera was divided between the maestro al cembalo (sometimes known as the maestro concertatore) and the direttore d'orchestra, with greater responsibility falling on the former. The consolidation of these two roles into one led to the emergence of the modern conductor of Italian opera. The careers of Angelo Mariani and Emanuele Muzio illustrate this transition.
"The Emergence of the Modern Conductor,"
Performance Practice Review:
1, Article 5.
Available at: http://scholarship.claremont.edu/ppr/vol4/iss1/5