Appendix 1—table 3. Chiral counter-rotations in experiments and in a minimal geometric model.

Left: Experimentally measured chiral cortical flows (± error at 95 % confidence interval) of the ABa and ABp cell at late stages of the spindle skew for different conditions. Flows on both cells are measured on the future left (${\mathrm{v}}_{\mathrm{L}}^{\mathrm{(}\mathrm{a}\mathrm{)}}$, ${\mathrm{v}}_{\mathrm{L}}^{\mathrm{(}\mathrm{p}\mathrm{)}}$) and right side (${\mathrm{v}}_{\mathrm{R}}^{\mathrm{(}\mathrm{a}\mathrm{)}}$, ${\mathrm{v}}_{\mathrm{R}}^{\mathrm{(}\mathrm{p}\mathrm{)}}$). Right: Assuming two rigid, counter-rotating and skewing surfaces, the velocities measured in the lab frame (left) can be mapped via Equations S13–S16 to the skewing properties (${\mathrm{\omega }}_{\mathrm{g}}$, $\mathrm{\delta }$) and surface velocities (${\mathrm{v}}_{\mathrm{a}}$, ${\mathrm{v}}_{\mathrm{p}}$) in the co-rotating frame (± propagated uncertainty). Distance between cytokinetic ring and points of velocity measurements: ${\displaystyle R=4.5\mu m}$.

Measured vel.	wt	ect-2	rga-3	Equations S13–S16	wt	ect-2	rga-3
$v_L^{\left(a\right)}$ [µm/min]	$-4.6\pm 0.8$	$-3.0\pm 0.7$	$-5.9\pm 0.7$	${\omega }_g$ [°/s]	$-0.31\pm 0.14$	$-0.19\pm 0.18$	$-0.38\pm 0.15$
$v_R^{\left(a\right)}$ [µm/min]	$5.0\pm 0.7$	$2.6\pm 1.0$	$5.3\pm 0.9$	$\delta /R$	$1.14\pm 0.13$	$0.80\pm 0.42$	$0.83\pm 0.13$
$v_L^{\left(p\right)}$ [µm/min]	$8.6\pm 0.7$	$5.7\pm 0.7$	$9.8\pm 0.6$	$v_a$ [µm/min]	$4.8\pm 0.8$	$2.8\pm 0.85$	$5.6\pm 0.8$
$v_R^{\left(p\right)}$ [µm/min]	$-2.4\pm 0.5$	$-2.5\pm 1.0$	$-3.2\pm 0.6$	$v_p$ [µm/min]	$5.5\pm 0.6$	$4.1\pm 0.85$	$6.5\pm 0.6$