After elementary school, Paul attended the time-honored secondary school Maria-Magdalenen-Gymnasium in Breslau, where he met Albert Neisser, who later became a professional colleague. As a schoolboy (inspired by his cousin Karl Weigert who owned one of the first microtomes), he became fascinated by the process of staining microscopic tissue substances. He retained that interest during his subsequent medical studies at the universities of Wroclaw, Strasbourg, Freiburg im Breisgau and Leipzig. After obtaining his doctorate in 1882, he worked at the Charité in Berlin as an assistant medical director under Theodor Frerichs, the founder of experimental clinical medicine, focusing on histology, hematology and color chemistry (dyes).
