Indeed they bore a closer resemblance to the frottola.

They differed in that they were solos with instrumental accompaniment instead of being part songs unaccompanied.
But this difference is not so important as it appears.

The part song method was at the basis of all these old lute songs.

This is well proved by the fact that before the end of the century the device of turning part songs into solo pieces with lute accompaniment had become quite familiar.

It was so common that we are driven to something more substantial than a mere suspicion that Casella and Minuccio employed a similar method and that the domination of polyphonic thought in music had spread from the regions occupied by the church compositions of Dufay and his contemporaries downward into the secular fancies of people whose daily thought was influenced by the authority of the church.
Furthermore this method of turning part songs into solos survived until the era of the full fledged madrigal dramas of Vecchi in the latter part of the sixteenth century, and at what may be called the golden era of the frottola was generally and successfully applied to that species of composition.