First Sentence

"In order to generalize the relativistic kinematics of rigid bodies in rectilinear uniform motion to whatever kind of motion, following Minkowski's ideas we obtain the following statement: To say that a body remains relativistically rigid means: it deforms continuously by arbitrary motion so that each of its infinitesimal elements Lorentz contract (relative to its rest length) all the time in accordance with the instantaneous velocity of each of its elements, as observed by an observer at rest."