Aster centering stiffness decreases after centration. (A) Time-lapse of a centering aster subjected to two consecutive external magnetic forces. Top: Low-magnification views of the embryo and magnet tip. Bottom: Position of aster centers (nuclei) and beads aggregate. Orange line: the centration path (fertilization from the top); orange arrows: the direction of the magnetic forces. Bars, 20 µm. (B) Time-lapse projection of aster deviation from the centration path upon the first force application. (C) Kymograph of aster deviation from the cell center from the second force application. Black and white arrowheads: the initial centered position and the maximum deviation (plateau) during force application, respectively. Bars, 5 µm. (D) Quantification of paired aster centering spring constants (connected by gray dashed lines [n = 10]). The “single-pull” category corresponds to force measurement performed only once after aster centration (n = 15). (E) Quantification of paired aster drags. (F) Average displacement/force plotted as a function of time. Error bars are SEM. Dashed lines: Voigt model fit (see Materials and methods). Results were compared using a two-tailed Mann–Whitney test. ns, P > 0.05; **, P < 0.01.