Complex sentence: Residencies elsewhere then became formalized and institutionalized for the principal specialties in the early 20th century, but even in mid-century, residency was not seen as necessary for general practice and only a minority of primary care physicians participated.
Split into simpler sentences: Residencies elsewhere then became formalized and institutionalized for the principal specialties in the early 20th century. But even in mid-century, residency was not seen as necessary for general practice and only a minority of primary care physicians participated.