            By 1912, however, Einstein had reached an impasse  , realizing that he needed to go beyond the mathematics that he knew and was familiar with.
<coherence>             By 1912, however, Einstein had reached an impasse  , realizing that he needed to go beyond the mathematics that he knew and was familiar with.
 By 1912, Einstein had reached an impasse , realizing that he needed to go beyond the mathematics that he knew and was familiar with.