Complex sentence: America's first biomedical institute, The Rockefeller Institute for Medical Research, was founded in 1901 with Welch, nicknamed "dean of American medicine", as its scientific director, who appointed his former Hopkins student Simon Flexner as director of pathology and bacteriology laboratories.
Split into simpler sentences: America's first biomedical institute, The Rockefeller Institute for Medical Research, was founded in 1901 with Welch, by then the "dean of American medicine", as its first scientific director. Welch appointed his former Hopkins student Simon Flexner as its first director of pathology and bacteriology laboratories.