WHEN a drop of strong silver nitrate is placed on a thin layer of 5–10 per cent, gelatine containing about 11 per cent, of potassium bichromate, remarkable phenomena are observed. The gelatine under the drop is coloured red-brown by the abundant precipitation of silver chromate. The nitrate spreads gradually by diffusion into the gelatine, the rusty brown area of precipitation enlarges, it forms at its periphery a dull whitish seam, and further outwards in the gelatine a system of numerous concentric rings is developed, spreading like rings on the surface of a quiet pool. These are the well-known Liesegang's rings or zones, and the central idea of Prof. Küster's investigation is that these throw light on zoned structure in cells and tissues. He has made numerous experiments with the diffusion zones formed in colloidal media in vitro, and he seeks to utilise the phenomena observed in the interpretation of organic structures—such as cross-striping in leaves, annular and other markings in cells and vessels, the layers in starch-grains, the markings on diatoms, the lines on butterflies' wings, on shells, on feathers, on porcupines' quills, and what not.