Fig. 2. The four fibro-lamellar bone types observed in the long bones of the king penguin chick. Undemineralized diaphyseal thin-sections, observed under ultraviolet light, bone periphery at the top. Two periosteal labels are shown: (1) alizarin injection 3 weeks after hatching; (2) fluorescein injection at 4 weeks. The outer (top) border of the two labels and the sides of the field delineate a tissue `patch', the thickness of which was measured to yield growth rates. (A) longitudinal bone; the asterisks indicate labelling of the primary osteon filling; (B) radial bone; (C) reticular bone; (D) laminar bone. Scale bar, 200 µm.