Figure 2. Animal group-specific neurogenesis in the pallium and the potential evolutionary history of pallial bioarchitecture during the evolution. (A and B) Comparison of the pallial neurogenesis between mammals and birds. The gray, green and magenta circles indicate the neural progenitor, DL and UL subtypes, respectively. The arrows indicate the neurogenetic process. (A) Spatially unbiased progression of neurogenesis constructs homogeneous layers of neuron subtypes across the mammalian neocortex. (B) The spatially biased neurogenesis constructs the mediolaterally separated subtype arrangement in the avian pallium. (C) Hypothetical evolutionary scenario of the emergence of the neocortical layer architecture. Neocortical layer-specific neuron subtypes originated from the common ancestor of the mammals and birds (purple filled star). The layered arrangement of neuron subtypes newly emerged (white star) in the mammalian lineage before the branch leading to the monotremes. Illustrations in (A and B) are reproduced with permission from reference 22.