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Abstract
Intense illumination causes centripetal movement of pigment granules into the region of the rhabdom within retinular cells so that the rhabdomeres are "coated" with light-absorbing pigment.4 Moderate illumination alone causes other changes,6 but little pigment migration. Colchicine moves the pigment to the light position where it stays after hours in the dark. Retinular cells have previously undescribed 240 Å diameter microtubules which are oriented radially. Because of the action of colchicine, we postulate that these microtubules control pigment position. There is no acute change in the electron microscopic appearance of the colchicine-treated microtubules in this preparation. Colchicine does not interfere with the electrophysiologic recording of nerve fiber activity.