Summary of in vivo development of axonal arbors from ferret layer 6 pyramidal neurons. Dendrites are omitted for clarity. At P13–15 layer 6 pyramids have not yet sent an axon collateral into the overlying cortical layers but have a single descending axon extending into the white matter. ByP19–20, neurons have a few recurrent axon collaterals that are just beginning to form branches in layer 4. Later in development, the density of axonal arbors continues to increase in layers 4 and 2/3, with very few arbors ever forming in layer 5. Figure modified from Callaway and Lieber (1996). Scale bar, 200 μm.