The change in spatial pattern of retinal cell production that occurs after midlarvae stages in Xenopus is shown to occur because of a greater proliferative response to thyroxine in the ventral than in the dorsal margin of the retina. The asymmetry of retinal cell production is not caused by a dorsoventral difference in the duration of the mitotic cycle. It is concluded that the asymmetry is due to differences in the number of proliferative cells in different parts of the retinal margin.