Blood platelets.

Blood platelets are small colourless bodies found in varying numbers ln blood. Osler {1874) was the first to recognise them as formed elements or blood. Their existence has beenrepeatedly affirmed and denied; yet, as Professor Tait -cf. Tait and Burke (1926) - recently stated, "no author who has ventured to extend his survey beyond the group of mammals has doubted either their existence or their close functional similarity to certain distinctive and readily observed cells common to the blood of all non-mammalian vertebrates and of inuumerable invertebrates as well". Their origin remains obscure. [...