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 tunica intima is the inner layer of blood vessel that includes epithelium and elastic fibers. The tunica media is the middle layer of blood vessel that contains primarily smooth muscle fibers. The tunica extema is the extreme outer layer of the blood vessel that contains collagen fibers.
Veins are usually larger in diameter and thinner-walled than arteries.
Veins channel blood to the heart, while arteries channel blood away from the heart. Capillaries, the intermediaries between the arteries and the veins, are generally composed only of endothelium because they are where- most of the diffusion in the circulatory system occurs. The junction at the capillaries marks an anastomosis, or a peripheral union between the blood vessels. The embryonic heart is formed from the splanchnic layer of the mesoderm. When first developed, the heart is composed of two layers.