These cellular fragments are platelets. What is the purpose of platelets?
In response to blood vessel trauma, they contain several different chemicals to help in the formation of a platelet plug and assist in blood coagulation (clot formation). Platelets originate from Megakaryocytes in the bone marrow.
In non-mammalian species, such as birds and reptiles, they contain a nucleus and are referred to as thrombocytes. Two are present in the image below of avian blood. The precursor for thrombocytes have yet to be identified in the bone marrow. As mentioned previously, avian erythrocytes are also nucleated.