Optic nerve terminals segregate by functional class into distinct layers in the lateral geniculate nucleus, the thalamic relay nucleus of the visual system. In the rhesus monkey, the number of geniculate layers changes abruptly from six posteriorly (central vision) to four anteriorly (peripheral vision). The plane of transition between these patterns passes through small laminar gaps corresponding to the perceptual blind spot caused by the exit of the optic nerve from the eyeball. However, this plane of transition has no apparent functional link to the blind spot. A thermodynamic model of geniculate morphogenesis supports the hypothesis that the blind spot traps the transition in its stereotypic position by introducing a singularity in an otherwise smooth gradient in forces guiding the development of geniculate morphogenesis. This relation suggests that small-scale anomalies may be important in the determination of large-scale patterns in biological structure.