ments of the animals under these conditions; one is the symmetrical structure of the animal, and the second is the photochemical action of light. We will consider the two separately. In regard to the photochemical action of light, we know to-day that a great many chemical reactions of organic bodies are accelerated by light. Especially is this true of oxidations. The mass of facts presented to us here concerning this relatively young branch of physical chemistry is already so great that we have the right to assume that the determining action of light upon animals and plants is in its last analysis due to the fact that the rate of certain chemical reactions in the cells of the retina or of other photosensitive regions of the organisms is modified by light; with increasing intensity of light the rate of certain chemical reactions, for instance, oxidation, increases according to a definite law, namely, the law of Bunsen and Eoscoe.
The second factor is the symmetrical structure of the animal. As expressed in the gross anatomy of the animal this is shown by the well-known fact that the right and left halves of the body are symmetrical. But it is my belief that such a symmetry exists in a chemical sense as well as in an anatomical sense—by which I mean that symmetrical regions of the body are chemically identical and have the same metabolism, while non-symmetrical regions of the body are chemically different, and in general have a quantitatively or qualitatively different metabolism. In order to illustrate this difference it is only necessary to point out that the two retinas, which are certainly symmetrical, have an identical metabolism, while a region of the skin which is not symmetrical with the retina has a different metabolism. The individual points on the retina are also chemically unlike. The observations upon visual purple, the differences in the color-sensitiveness of the fovea centralis, and the peripheral parts of the retina indicate that the points of symmetry of the two retinas are chemically like, but the nonsymmetrical points chemically unlike.
Now if an unequal amount of light falls upon the two retinas, the photo-chemical reactions in the retina which receives more light will also be more accelerated than in the other. The same thing naturally holds true for every other pair of symmetrical photosensitive surface elements. For it should be mentioned just here that photochemical substances are not only found in the eyes, but also in other places on the outer surface of many animals. In planarians, as my experiments and those of Parker have shown, not only the eyes, but also other places on the skin, are photosensitive. But if more light falls upon one retina than upon the other, the chemical reactions, for instance, the or-
- Luther, "Die Aufgaben der Photochemie," Leipzig, 1905. C. Neuberg, Biochem. Zeitschr., Bd. 13, S. 305, 1908. Loeb, "Vorlesungen über die Dynamik der Lebenserscheinungen," Leipzig, 1906. In addition, the work of Ciamician, as also Wolfgang Ostwald (Biochem. Zeitschr., 1907).