Based on recent experimental studies of complementary gradients of receptor density (R) on the retinal surface and ligand density (L) on the tectal surface, and mapping of the high point on the receptor gradient to the low point on the ligand and vice versa, the servomechanism model was constructed involving a mechanism for the retinal axon to reach its target automatically sensing a difference between the signal strength (R.L) and the standard value (S). Computer simulations based on the model demonstrated desired two-dimensional topographic mapping of the retinal axons on the tectum, and explained three strange behaviors of the retinal axons that had been observed in stripe assays for retinal axons using stripes composed of tectal membrane fragments: repulsive behaviors of the retinal axons by the ligand substances, uncertainty of the nasal axons whether or not they show regional selectivity between substances of anterior and posterior tecta, and abrupt transition of growth of the axons originating at continuously varied retinal positions on the stripes having graded ligand density. Finally we suggested what is to be improved in stripe assays with the artificial gradient of the tectal membrane fragments.