  In a recent letter, Kawasaki et al. (PRL 99, 215701 (2007)) study glass
formation in a 2-dimensional (2D) model of polydisperse repulsive disks. They
give numerical evidence for a direct relation between slowing down of the
relaxation, extension of some medium-range order, and development of dynamic
heterogeneities. We disagree with the authors' interpretation on two points
which we believe to be potentially important for understanding the glass
transition.
