I t has been known for many years that the visual cells and retinal pigment of most fishes, amphibians and birds undergo marked changes of position that are correlated with light and darkness (Arey, '15; '16a; '16b). In the frog, for example, these movements are definite. The pigment granules shift position strikingly within the cytoplasm of fixed cells, whereas the rods and cones actually lengthen and shorten in a specific region known as the myoid.
By experimenting on excised eyes some attempts have been made to discover the degree to which these responses are independent of body influences. The first was in the early years of the investigation of photomechanical movements. Hamburger (1889) reported that the cones and retinal pigment of the excised eye of a frog change to the positions characteristic of light or darkness when a corresponding change is made in the condition of illumination. Many years later, when this question was first reopened, there was only partial confirmation of Hamburger's claim. Dittler ('07), observing frogs' retinas isolated in Ringer's solution, found a shortening of cones in the light but failed to obtain the opposite response in the dark. Fujita ('11) and Kumagai ('17) likewise concluded from single experiments that the retinal pigment of the frog's eye expands incompletely when Contribution no. 402 from the Anatomical Laboratory, Northwestern University. 487