The dorsal aorta (DA) and the cardinal vein (CV) are the first vascular pair to form during development. Although it is generally accepted that the DA derives from angioblasts, the cellular origin of the mammalian CV is currently unknown. Now, on p. 1120, Rong Wang and colleagues reveal that the mammalian CV is formed, at least in part, from endothelial cells that originate from within the DA. Using markers specific to either aortic or venous-fated endothelial cells, the authors show that the DA contains a mixed population of endothelial cells with either venous or arterial identity, but that the number of venous-fated endothelial cells decreases over time. The authors use time-lapse microscopy to visualise the migration of endothelial cells away from the DA and to the CV, and further show that this process requires ephrin B2/EphB4 signalling. The authors propose a mechanism whereby ephrin B2/EphB4-mediated cell repulsion drives the segregation of venous-fated endothelial cells away from the DA, facilitating their movement to the CV.