Due to its measurable geometry, its evolutionary adaptability, its functional significance, and its easily visualized embryogenesis, the vertebrate skull has long been the subject of intensive research on size and shape. This feature of developmental systems has allowed the vertebrate craniofacial complex to be both highly conserved in its basic anatomical organization and extraordinarily diversified in its size and shape. Individual morphological units within the craniofacial complex can become modified rapidly over time, yet still, maintain connections and keep relationships that are required for meeting structural and functional demands. By focusing on the molecular and cellular regulation of species-specific pattern in the craniofacial complex, the authors hope their work has helped pinpoint precisely where and when changes to developmental programs can affect the course of morphological evolution. Their experiments in quail and duck embryos reveal that neural crest mesenchyme (NCM) plays a special role in generating species-specific pattern in the craniofacial complex.