We have studied the low speed fracture regime for different glassy materials with variable but controlled length scales of heterogeneity in a carefully mastered surrounding atmosphere. By using optical and atomic force microscopy (AFM) techniques we tracked in real-time the crack tip propagation at the nanometer scale, on a wide velocity range (10(-3) to 10(-10) m s(-1) and below). The influence of the heterogeneities on this velocity is presented and discussed. Our experiments revealed also-for the first time-that the crack advance proceeds from nucleation, growth and coalescence of nanometric damage cavities inside the, amorphous phase, which generate large velocity fluctuation. Implications of the existence of such a nano ductile fracture mode in glass are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.