The mechanisms that regulate blood vessel assembly at appropriate sites in the organism are poorly understood, yet understanding this regulation is critical to the ability to design therapeutics around vessel production in vivo. Classic embryologic studies have yielded descriptive analyses of vascular pattern formation, and they show that angioblasts and endothelial cells respond to environmental cues to assemble at precise embryologic sites. The present study incorporated a genetic model, the mouse, into these studies to obtain mechanistic information regarding vessel assembly and patterning. The data show that both embryo-derived and stem-cell-derived mouse angioblasts respond to host cues in the avian embryo and pattern properly, and also highlight a critical role for the vascular endothelial cell growth factor signaling pathway in these patterning events.