Movie S1.

Movie of HH stage 10–11 (E1.5–E2) chick hearts preonset and postonset of beating. Prebeating hearts exhibit “shivering” behavior of uncoordinated cardiomyocyte contractions. Postbeating hearts exhibit fully coordinated beats. Movie plays at 3× real time. Micropipette in view has an outer diameter of ∼50 μm.