Comment on "Correlation between Dynamic Heterogeneity and Medium-Range   Order in Two-Dimensional Glass-Forming Liquids"
  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. 
Fran\c{c}ois Sausset (LPTMC), Gilles Tarjus (LPTMC)