Examples of tiling and self-avoidance in vertebrates and invertebrates. A, A simplified diagram showing the tiling of vertebrate retinal ganglion neurons, based on results in . B, Axonal tiling contributes to the organized columnar projection pattern of R7 and R8 photoreceptor neurons and L1 lamina neurons in the medulla of the Drosophila visual system. While L1 neurons arborize at both M1 and M5 sub-layers, R7 and R8 axons terminate at M6 and M3 sub-layers, respectively. Genetic dissection of neuronal circuit formation in the fly visual system has contributed significantly to our understanding of neuronal positioning, axon guidance and neuronal target selection (e.g. [53–57]). C, A schematic diagram showing the non-overlapping coverage of the receptive field by sister branches from a Pv mechanosensory neuron in leech, based on results in . D, A simplified diagram showing self-avoidance in a Drosophila class IV da neuron, where sister branches tend to avoid each other.