Complex sentence: In 1892, Witmer left Germany and returned to the University of Pennsylvania, becoming the Director of The Laboratory of Psychology, effectively filling the shoes left by his old advisor, James McKeen Cattell.
Split into simpler sentences: In 1892, Witmer left Germany and decided to go back to the University of Pennsylvania. On his return, he became the Director of The Laboratory of Psychology, effectively filling the shoes left by his old advisor, James McKeen Cattell.