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Illustration of the formation of the rachis-dominated modified pennaceous feather of basal birds. (A,B) formation and initial growth of the feather bud; (C) barb ridge formation; (D) barbs fuse to form a rachis; (E) as feather continues to form, apoptosis ceases, vane becomes undifferentiated; (F,G) illustrate the conversion of a three-dimensional structure into the two-dimensional feather.
