Bird red blood cells. Coloured scanning electron micrograph (SEM) of red blood cells (erythrocytes, red) from avian 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. Human red blood cells are disc-shaped and do not have a cell nucleus. Magnification: x600 when printed 10 centimetres wide.