Evolutionary Developmental Biology, Volume 141 focuses on recent research in evolutionary developmental biology, the science studying how changes in development cause the variations that natural selection operate on. Several new hypotheses and models are presented in this volume, and these concern how homology may be properly delineated, how neural crest and placode cells emerged and how they formed the skull and jaw, and how plasticity and developmental symbiosis enable normal development to be regulated by environmental factors. - New models for homology - New hypotheses for the generation of chordates - New models for the roles of plasticity and symbionts in normal development