Comparison of the humerus histology of two Pistosaurus longaevus individuals. Match the juvenile SIPB R 46 (A, C) and the adult SMNS 84825 (B, D). A-B, outer cortex in normal transmitted light. Radially and longitudinally vascularized FLB is interrupted by several thick parallel-fibred annuli (marked by arrows). Vascular spaces are filled in by more highly organized bone tissue to a greater degree in the older individual (B) than in the juvenile (A). C, D, inner cortex in polarized light. Embryonic bone is preserved in both individuals and extends to the hatching or birth line (marked by arrow). The rest of the cortex consists of strongly vascularized FLB with distinctly radial primary osteons. Although the medullary region is lined by circumferential layers of endosteal bone in both specimens, the cortex of the juvenile (C) is more highly vascularized than that of the older one (D). E-G, detail (box in C) of the FLB of the juvenile (SIPB R 46) showing the framework of woven-fibered bone, which is only incompletely filled in by lamellar bone. The primary osteons are thus still incompletely formed. The same view is seen in normal-transmitted light in E, in polarized light in F, and with the gypsum plate in G. Abbreviations: emb, embryonic bone, fb, fibrous or woven-fibered bone, lb, lamellar bone, po, primary osteon.