Developmental progress of feather skin and scale skin. Left side of chicken embryo: feather skin development from E7 to E9. Right side of chicken embryo: scale skin development from E9 to E11. The development stages for feather and scale skins are matched; top to down represents the developmental trend from undifferentiated skins to differentiated skins. During the process, skin gradually loses developmental plasticity in exchange for gaining regional specificity (right panel).