Harmony. The canonic imitation of the melody pushes the harmony up a perfect fourth: it was centered on G; now it’s on C, with C minor implied during the passage and C major (a sort of Picardy third) stated at the end. The harmony is in now in synch with the original melody (both within C-G), but in tension with the canonic voice (F-C). Looking back to the beginning of the work, the harmony has ascended two perfect fourths, from D to G and from G to C.