In the first level middleground the prolongations of the bass line are very limited, only occuring between the initial I and V.

One common way in which the bass can generate longer spans of a piece of music is shown in the example below. The leap from I to V is turned into a rising arpeggiation - Schenker calls this a third-divider because it divides the fifth into two thirds.

The first example creates a first inversion tonic chord which does not give much scope for extended prolongation but in the second example the E becomes a scale step creating the opportunity for a section in E major. This is particularly common in the minor where III is the relative major.