When assembling his Mass in B minor, Bach drew from many of his older works.  Here is one great example.  If you, like us, have always been mesmerized by the haunting sound of the "Qui tollis" movement from the Mass in B minor, you absolutely must hear the opening chorus to the cantata Schauet doch und sehet, which is the full version of the material, and which concludes in a twirling, twisting fugue.  See the performance of Schauet doch und sehet (BWV 46) by the Netherlands Bach Society, conducted here by Hans-Christoph Rademann Compare with the "Qui tollis" movement of the Mass in B minor, Netherlands Bach Society, led by Jos van Veldhoven