By using a new method of silver staining, Kon demonstrated the presence in the cytoplasm of tissue cells of dark granules, which possess a great affinity for silver. This silver cell reaction, as Kon calls the phenomenon, is present in the cells of every organ—secretory glands, viscera, brain, or ductless glands, such as the hypophysis and the thymus. The granules are considered by Kon specific, for they differ from other granules present in tissue cells, such as mitochondria or Altmann's granules, lipoid droplets or granules enclosed within histiocytes after the use of vital staining methods. The monograph is an epitome of extensive and laborious work of fifteen years by Kon himself and a number of his collaborators, some of whom are representative investigators in their special fields of research. The organs of an enormous number of animals (herbivora, mammals, amphibia) have been studied under various conditions, in fresh and fixed