It's pretty obvious to me that this whole "imitative form" is gonna be rather difficult in comparison to our previous assignments. That being said, I will analyze Fugue No. 21 in Bb Major from the first volume of Bach's The Well Tempered Klavier. With three voices and 3 melodies, (subject and 2 counter subjects), Tim Smith compares the Fugual form to that of a Tumbling Block quilt. "Stitching" the 3 melodies together, the 3 voices create "swatches" of colors. Seeing a visual representation of this analysis allows one to really visualize this musical quilt.
The first four measures are expository. The next voice enters in the next 4 bars playing the subject, while the top voice plays the first countersubject. The following 4 measures brings in the 3rd voice, while the middle voice goes to the first countersubject and the top plays the final countersubject. This constant switching of subject and countersubject between the voices gives the song a "tumbling feeling."
In measures 17 through 21 we hear brief repetitions of sequenced subjects. This one measure repeated melody can be found back in it's original form in measure 4. Underneath this we see 3 measures of inverted subject, helping serve as a transition to measure 22.
We then go back to the interwoven fabric of three voices, now in the relative minor key of g minor, then 4 bars later in the key of c minor. This is a developmental section. Measures 30-32 are the inversion of measures 19-21 and again serve as a transitional/modulatory section. Measure 37 brings us to Eb major, and we land finally in Bb major in measure 41. The tumbling subjects have brought us full circle.