Developing feathers
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Cross section of the rachis of a growing contour feather of a Muscovy Duck (Cairina moschata) (HE): detail
Pigmentation (brown) and keratinization (pink) has started in the rami and barbules. In between, cells at the tips of the barb septa and axial plates degenerate and this process will go on further inside, creating open spaces between barbs and barbules. See also next picture.