Movie 1.

ECs sprouting from the PCV form the first posterior circulatory loop. Maximum intensity projections of confocal z-stacks of Tg(fli1a:nEGFP)^y7; Tg(-0.8flt1:RFP)^hu5333 double transgenic embryos, where all EC nuclei are labelled in green, and all arterial ECs are labelled in red. Time-lapse imaging reveals endothelial cells sprouting from PCV and reverse migration of ECs expressing flt1:RFP to form the posterior circulatory loop. Scale bar: 50 μm.