Reptilian red blood cells. Coloured scanning electron micrograph (SEM) of whole and fractured red blood cells (erythrocytes, brown) from reptilian tissue. These cells contain haemoglobin, a pigment that allows them to carry oxygen around the body. Non-mammalian red blood cells (such as these) are oval (discoid) in shape and contain a cell nucleus (pink). Human red blood cells are disc-shaped and do not have a cell nucleus. Fibrin (blue, used in blood clotting) can be seen between the cells. Magnification: x2000 when printed 10 centimetres wide.