Complex sentence: Witmer was asked by George Fullerton to become Cattell's assistant and, Witmer accepted this great offer, eventually, He decided to resign his teaching position at the Rugby Academy and attend the graduate school at the University of Pennsylvania as a full-time student.
Split into simpler sentences: Witmer was asked by George Fullerton to become Cattell's assistant and Witmer accepted this offer. Eventually, he decided to resign his teaching position at the Rugby Academy and attend the graduate school at the University of Pennsylvania as a full-time student.