Complex sentence: In the centre is an octagonal panel of the coronation of the Virgin, and round it medallions of the Four Evangeliststhe spaces between them being filled up by reclining figures of the Four Sibyls.
Split into simpler sentences: In the centre is an octagonal panel of the "Coronation of the Virgin", and surrounding it, are medallions of the Four Evangelists. The spaces between them being filled up by reclining figures of the Four Sibyls.