These are large, oval, flat, nucleated cells (mammalian erythrocytes are non-nucleated). Occasionally erytheroid structures without a nucleus are observed but these are normal avian erythrocytes from which the nucleus has been lost. The nucleus is centrally located and tends to be elongated in shape with a length of approximately 12 microns and a width of approximately 7 microns. The red colouring is caused by the presence of