Complex sentence: Having passed his master's exams, Danilevsky was preparing to defend his thesis on the flora of the Black Sea area of European Russia when in 1849 he was arrested for his membership of the Petrashevsky Circle.
Split into simpler sentences: Having passed his master's exams, Danilevsky was preparing to defend his thesis on the flora of the Black Sea area of European Russia. In 1849, he was arrested there for his membership of the Petrashevsky Circle.