Article: The Period of Division from, A.D., 1138 – A.D., 1314, which included nearly 200 years of feudal fragmentation and which stemmed from Bolesław III's division of Poland among his sons, was the genesis of the social structure which saw the economic elevation of the great landowning feudal nobles (możni/Magnates, both ecclesiastical and lay) from the rycerstwo they originated from. The prior social structure was one of Polish tribes united into the historic Polish nation under a state ruled by the Piast dynasty, this dynasty appearing circa 850 A.D.

Now answer this question: What stemmed the period of division?
Bolesław III's division of Poland among his sons