Transmission electron micrograph of ferritin crystal in the midgut cell of a Membracine Treehopper (Campylenchia latipes). Iron absorbed in the intestinal mucosa of mammals combines with a protein apoferritin to form the substance ferritin, the form in which iron is stored in cells. Ferritin is a large molecule visible in unstained electron micrographs because of its high iron content (23 per cent). Because there appears to be no excretory pathway for excess iron, ferritin may accumulate in cells in considerable quantity. In mammals, iron accumulating in excess of the capacity of the ferritin storage mechanism forms granular aggregates of hemosiderin. Crystallization of ferritin is uncommon in vertebrate tissues, but in some insects, large crystals are formed in cells of the intestinal tract. In the accompanying micrograph of a cell from the midgut of a Homopteran insect, large numbers of ferritin particles are visible in the cytoplasmic matrix and a large crystal of ferritin occupies the center of the figure.