The aggregation, arrangement and bonding together of a set of components to form the spindle, the array of microtubules and associated molecules that serves to move duplicated chromosomes apart, in the absence of centrosomes. Formation is initiated by the nucleation of microtubules (MTs) in the vicinity of condensed chromatin. MTs then attach to and congress around the chromatin due to activity of microtubule motors. A bipolar spindle is formed by focusing of the terminal ends of the MT array into spindle poles by molecular motors and cross-linking proteins.