Fig. 2

Development of remex-type flight feathers before hatching. a, b Ventral view of the FFF region at E18 (a) and E19 (b). Scale bars: 1 mm. Arrowheads indicate joints between distal and proximal parts of the elongated feather bud in the FFF region. c–e, Sections of the feather bud under the skin at E 18; (c), a shallow level, (d), an intermediate level and (e), a deep level. Scale bars: 100 μm. Asterisk in (e) indicates the position of the prospective rachis that exhibits an irregular pattern of barb ridges. f Rows of feather buds in the posterior wing, including the FFF region, at E18. Distal to the right and dorsal to the bottom. U: ulna. Asterisks indicate the position of prospective rachis, showing that the position is biased to the dorsal. Scale bar: 100 μm