Fig. 7.

Model for the cellular basis of jaw size. Specification of the neural plate establishes the basis for a shorter and wider midbrain in the region generating jaw NC precursors in duck embryos (event 1). This difference, represented by distinct Otx2 expression domains, is evident by HH6. The wider midbrain of duck contributes to a larger allocation of NC to the jaw primordia in HH13 embryos (event 2). NC cells of the duck jaw mesenchyme exhibit an increase in proliferation compared with quail. Increased proliferation (event 3) is driven largely by differences in developmental rate (see time scale on the side of the diagrams). Thus, the larger jaw size of duck is the result of at least three distinct developmental events (outcome 4).