Complex sentence: He became professor of paediatrics at the Univesity of Wisconsin Medical School, which he had joined in 1958, and in 1966 he moved to the chair of the University of Washington, Seattle, where he spent the remainder of his career.
Split into simpler sentences: He became professor of paediatrics at the Univesity of Wisconsin Medical School, which he had joined in 1958. In 1966, he moved to the chair of the University of Washington, Seattle, where he spent the remainder of his career.