Complex sentence: Occitan literature started in the 11th and 12th centuries in several centres it thence gradually spread out, first over the greater portion, though not the whole of southern France, and then into Catalonia, Galicia, Castile and into what is now the north of Italy.
Split into simpler sentences: Occitan literature started in the 11th and 12th centuries in several centres. It thence gradually spread out, first over the greater portion, though not the whole of southern France, and then into Catalonia, Galicia, Castile and into what is now the north of Italy.