A quantitative assessment of the density of hyperbolic markings in the fracture surfaces of polymethyl methacrylate has been made showing that the density is proportional to the fracture toughness. In general the fracture toughness at any instance is a random function, but there is a definite tendency for it to increase with velocity of fracture propagation. It is suggested that the craze material ahead of the true crack tip may be assumed to have constant strength and Dugdale's model for elasto—plastic materials applied to organic glasses as well.