Fig. 5

Deep invagination of the feather bud toward the bone primordium in the FFF region. a–h Sagittal sections of abdominal skin (a–d) and coronal sections of the FFF region (e–h) at E10 (a, e), E11 (b, f), E12 (c, g) and E13 (d, h). Arrowheads in (f) indicate the follicular cavity that starts invagination. i–l. Transverse sections of the FFF region showing the invagination process of the feather bud in the FFF region at E10 (i), E11 (j), E12 (k) and E13 (l). Scale bars: 100 μm. The blue core is the cartilaginous rudiment of the ulna, and the red layer surrounding the ulna cartilage is the ossifying bone collar