Complex sentence: An early alternative to the incremental growth of planetesimals was the settling of dust at the mid-plane until sufficient densities were reached for a gravitational instability resulting in fragmentation of the disk and the gravitational collapse into 1-10 km planetesimals.
Split into simpler sentences: An early alternative to the incremental growth of planetesimals was the gravitational instability. In this model the settling of dust at the mid-plane would reach sufficient densities to result in fragmentation if the disk and gravitational collapse into 1-10 km planetesimals.