  We study the long-time dynamics of hexagonal directional-solidification
patterns in bulk samples of a transparent eutectic alloy using an optical
method which permits real-time observation of the growth front. A slow
dilatation of the patterns due to a slight curvature of the isotherms drives
the system into a permanent regime, close to the threshold for the rod
splitting instability. Thus an apparently minor instrumental imperfection
suffices to maintain the system near a marginal stability point. This answers
the long-standing question of spacing selection in bulk eutectic growth.
