The thrombocytus (more commonly known as the platelet/thrombocyte) is a minute, non-nucleated, disc-like cytoplasmic body found in the blood plasma of mammals that is derived from a megakaryocyte and functions to promote blood clotting.
The German anatomist Max Johann Sigismund Schultze discovered platelets in 1865 while examining blood under a microscope. In his publication he describes a part of the blood which was, according to him, previously neglected, and notes the presence of clumps of colourless globules which were irregular in shape and varied in size from 0.001 mm to 0.002 mm. He also described them as granular masses. Here is a photomicrograph of a blood smear showing tiny purplish platelets surrounded by the much larger red blood cells under a light microscope: