Complex sentence: This organum, thought to be written for Christmas festivities, would have retained the same purpose as the original gradual--the cantus firmus, or tenor, "holds" the original chant, while the other parts develop complex melismas on the vowels.
Split into simpler sentences: This organum, thought to be written for Christmas Day, would have retained the same purpose as the original gradual. The cantus firmus, or tenor, "holds" the original chant, while the other parts develop complex melismas on the vowels.