Confocal micrograph of arteries (red) and veins (blue) in the head of a 3 day old zebrafish embryo. Dorsal view with the embryo facing into the bottom left corner and the eye on the vertical plane. Zebrafish are a powerful tool for studying development of a fully functioning vascular system, as this forms in a similar way in other animals including mammals. This zebrafish has been genetically engineered so that the arteries and veins express different fluorescent proteins. As zebrafish embryos are transparent, these arteries and veins can then be visualised as the circulatory system develops. Horizontal width of image is 708.49 micrometres.