Complex sentence: He entered the Ecole des Beaux-Arts in Paris in the atelier of Marcel Gimond then, at the Académie de la Grande Chaumiere, he was a student of Ossip Zadkine.
Split into simpler sentences: He entered the Ecole des Beaux-Arts in Paris in the atelier of Marcel Gimond. Then at the Académie de la Grande Chaumiere, he was a student of Ossip Zadkine.