We are grateful to Pui-Leng Ip, Ken Birnbaum, and the personnel of GenCore at NYU for their expert assistance with the FACS, ScreenTape, and high-throughput sequencing. We thank Nicole Kaplan for sharing the image of Mesp > tagRFP, MyoD905 > GFP, and Hand-r > tagBFP labelled Ciona late tailbud. Research in the laboratory of L.C. is supported by R01 awards HL108643 and GM096032 from the NIH/NHLBI and NIH/NIGMS respectively; and by grant 15CVD01 from the Leducq Foundation. C.R. has been supported by a long-term fellowship ALTF 1608-2014 from EMBO.