At all levels of the neural system, progressive differentiation of specific elements and structures coupled with progressive commitment of those elements to functional systems appear to be the governing principles of brain development.
Link to Publication: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989000/
Stiles J and Jernigan TL. The basics of brain development. Neuropsychol Rev 2010; 20:327–348.