Additional Supporting Information may be found in the online version of this article.

DVDY_22570_sm_suppinfofig1.mov5943KSupp. Movie S1. Beating animal cap at control stage 45 induced by injection of Activin β B mRNA. Note that all explants show rhythmic movements and that the movement involves the entire explant.
DVDY_22570_sm_suppinfofig2.mov5615KSupp. Movies S2–5. Beating and non-beating explants from Activin-expressing animal cap co-injected with tlMO inhibiting GATA4 (Movie S2), GATA6 (Movie S3), and both GATA4/6 combined (Movie S4). Note a substantial reduction in the number of beating explants in the morphants. Note also that none of the uninjected control explants displays rhythmic beating in their movement (Movie S5).
DVDY_22570_sm_suppinfofig3.mov5798KSupporting Movie S3.
DVDY_22570_sm_suppinfofig4.mov5737KSupporting Movie S4.
DVDY_22570_sm_suppinfofig5.mov5637KSupporting Movie S5.

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