Figure 3.

Differential BM migration can be quantified via a simple force balance. (A) The free body diagram of forces at each edge of the blastoderm margin. Red arrows indicate a constant driving tangential force, F_Tan. Blue arrows indicate the force generated by CCT. The vectors F_Tan and F_CCT form the angle θ. (See also Fig. S4 and Movies S2 and S3.) (B) The projection of F_CCT (blue arrows) adds to F_Tan on the right edge and counteracts F_Tan on the left edge, creating a larger net force at the right edge of the blastoderm margin (purple arrows). (C) Experimental data shown in blue are Δv and Δcosθ calculated at 30 min intervals between 30% and 80% epiboly in 16 embryos. A seven-point moving average is shown in green to indicate the overall trend. A linear fit of Δv versus Δcosθ is shown in red (R² – 0.12), which yields a slope of 23 ± 6 nm s⁻¹ (p-value < 0.001 equivalent to 0 nm s⁻¹) and intercept of 0.30 ± 1.56 nm s⁻¹, with uncertainties determined based on the least-squares fit of the raw data (blue) to a line (red). E2(+): E2 medium including 1 mM Ca²⁺.