Supplementary Movie 4

Requirement of outward movement in air sac-like structure. Cell division probability was set at p=0.8×10⁻⁵ until t=24×10⁴, and p=0.8×10⁻⁶ afterwards. The degree of apical constriction was set as k^a=5 initially, increased by 0.0012 at each time step to k^a=293 until t=24×10⁴, and reduced by 0.0035 until k^a reached to 0.5. The cell numbers increased from 40 (t=0) to 153 (t=34×10⁴).