Complex sentence: In the late 19th and early 20th centuries, Łódź had been a major Polish industrial center, heavily urbanized and industrialized, and its large working class made in an important stronghold of the Polish socialist movement.
Split into simpler sentences: In the late 19th and early 20th centuries, the heavily urbanized and industrialized Łódź was a major Polish industrial center. Its large working class made it an important stronghold of the Polish socialist movement.