Figure 4.

Morphologic progression of facial prominences between Classes embryos during development. (A, B) In the early stage of development, the mouse and chick embryos share similar facial morphology. (C, D) With time, phenotypic divergence becomes evident within the frontonasal prominence. The chick frontonasal prominence shows a bulbous appearance whereas the mouse frontonasal prominence has an infranasal depression. (E, F) Later in development, the frontonasal prominence forms an upper beak in chick, and a muzzle in the mouse. (G) The variation in facial morphology increases over time during development. Between Classes (e.g. Mammals vs. Aves) the variation is greater and the phylotypic period is reduced. Between Orders (e.g. duck vs. quail), the phylotypic period is longer and there is less variation in facial morphology. (H) Disruption of Fgf signaling transforms a chick face to resemble that of a mouse.