Caption

Frog red blood cells. Light micrograph of red blood cells (erythrocytes, purple) from a frog. These cells contain haemoglobin, a pigment that allows these cells to carry oxygen around the body. Non-mammalian red blood cells (such as those of a frog) are oval in shape and contain a cell nucleus (red). Human red blood cells are disc-shaped and do not have a cell nucleus. Frogs are cold-blooded, meaning they rely on external heat to warm their blood, as opposed to mammals, who can generate their own internal heat supply. Magnification: x3500 when printed 10 centimetres wide.