Bach Two Part Invention--No. 4 in d minor

The title pretty much gives it away that there are only two voices in this piece. it is, however, a pretty sweet invention.
the right hand starts off alone in the first two measures, playing straight 16th notes. the left hand comes in in measure 3 and perfectly imitates what the right hand had previously played. by measure 8, the left hand acts as the accompaniment, rather than simply repeating short, primary motives as it had before. it begins to repeat these motives again around measure 12, providing a rather contrapuntal feel. at times, the right hand plays a static note and the left hand has the moving line, and vice versa. again, this just shows how certain expository lines are repeated over and over again.
as far as structural divisions go, i would say this piece has 3 large-ish sections: one starting at the beginning and ending around measure 18; measure 18 until 38, and measure 38 until the end of the piece. the first section is expository. the last two sections feel slightly like the first, yet developmental, like they are playing with the initial motive, but adding on a few new ideas. I wouldn't call each of these sections periods, per se, but maybe phrases will do.

peace out