Fig. 3. Active forces in the chick embryo.

(A) Experimental fluorescence images at HH1 and HH3. A and P mark the anterior-posterior direction, and red arrows illustrate the dominant active force F_A distribution from (B). At HH1, F_A is dominant in the posterior embryonic region perpendicular to the AP axis. At HH3, F_A is perpendicular to the PS throughout the AP axis. These F_A configurations are consistent with the corresponding distribution of supracellular actomyosin cables in Fig. 1E. Dimensions are 1300 μm × 1900 μm. (B) and (C) Magnitude and direction of forces at HH1 (top) and HH3 (bottom). Magnitudes are normalized by the maximal spatial force. (B) We compute F_A from the general force balance for highly viscous active flows F_A + F_V = 0 (cf. Eq. 1a), which gives F_A = −(∇[∇ · v] + 2p₁Δv), where the right-hand side can be computed using experimental velocities. We use an averaging filter in space (window size ≈25 μm) and time (window size ≈15 min) on the PIV velocities and compute velocity derivatives using finite differencing. (C) F_A is from Eq. 1a. We note that the model F_A at HH3 arises from the dynamical variables evolved by Eq. 1 over the entire gastrulation period. They closely predict the corresponding inferred experimental forces [(B) bottom], further validating our model.