There are two types of cells in the blood. The erythrocytes are the red blood cells, which carry hemoglobin that binds oxygen to transport to the tissues. The leukocytes are the white blood cells that destroy foreign bodies through phagocytosis and are involved in the immune response. Hemopoietic tissues produce the blood cells. In embryos, hemopoietic tissue is found distributed throughout the body.
Blood vessels are the first indicator of the formation of the circulatory system.
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.