Blood is a fluid tissue containing cellular elements that are derived from mesoderm.
Blood islands first form in the yolk and then become contiguous to form a network of vessels. Endothelium lines the blood vessels, and the rest is composed of muscular fibers, collagen and elastic fibers. The embryonic heart is formed from the splanchnic layer of the mesoderm. Afferent branchial arteries lead into the gills from the aortic arches. Blood then flows through the gills through collector loops, and the oxygenated blood travels into the efferent branchial arteries, which continue into the dorsal aorta.