Complex sentence: Rock decided to transfer into the field of dentistry and, after an 1849 apprenticeship with a Dr. Harbert, a white dentist, opened a dental practice in Philadelphia in January, 1850.
Split into simpler sentences: Rock decided to transfer into the field of dentistry. After an 1849 apprenticeship with a Dr. Harbert, a white dentist, Rock opened a dental practice in Philadelphia in January, 1850.