Feathers were long thought to be an outgrowth of "reptilian" scales. Somehow the scales grew longer and divided into barbs and barbules. Work in the last 10 years, however, suggests that the development of feathers occurs by the interaction of specialized follicles and a series of specialized genes that control the onset and termination of growth. Four sequential stages of feather evolution have been identified, each stage a developmental modification of the previous stage, and each found in living birds (Figure 10.9). These stages are:
1. Formation of a hollow cylinder (the shaft).
2. Loosely associated, unconnected, unhooked barbs (downy feathers).
3. Hooked barbs on a symmetrical vane (contour feathers, such as wrap around the body).
4. Hooked barbs on an asymmetrical vane (flight feathers).
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