Comparing expression patterns of orthologous genes between insects and vertebrates, we have recently proposed that the ventral nerve cord in insects may correspond to the dorsal nerve cord in vertebrates. Here we show that the early development of the insect and vertebrate brain anlagen is indeed very similar. Insect and vertebrate brains express similar sets of genes in comparable areas with similar functions in the adult. In addition, early axogenesis establishes surprisingly similar patterns of axonal connectivity in both groups. We therefore propose that insect and vertebrate brains are built according to a common ground plan, and that specific areas of the insect and vertebrate brains be considered as homologous, meaning that these areas already existed, with their specific functions, in their common ancestor.