For mammalian and bird embryos to survive, a complex vascular system must form in the yolk sac surrounding the embryo and connect correctly to the embryonic circulation. On p. 361, le Noble et al. propose that haemodynamic flow plays a central role in arterial-venous patterning and differentiation within the chick yolk sac. The researchers show that during this developmental process, thin vessels from the arterial domain of the sac disconnect from the arterial tree before reconnecting to the venous system, indicating that arterial-venous differentiation is plastic. To test whether haemodynamic flow is involved in this plasticity, le Noble and colleagues altered the flow in the developing vasculature. Their results, which include a complete loss of arterial-venous patterning and differentiation in the absence of perfusion, suggest that haemodynamic flow may be the master regulator of yolk sac vascular development.