Fig. 2

Humeral cross sections of Andrias japonicus. A, cross section of humerus of ASAGS-0078 in normal and B, polarized light. C, coss section of humerus of ASAGS-0077 in normal and D, polarized light. E, cross section of humerus of ASAGS-0050 in normal and F, polarized light. G, cross section of humerus of ASAGS-0100 in normal and H, polarized light. I, cross section of humerus of ASAGS-0197 in normal and J, polarized light. K, cross section of humerus of ASAGS-0106 in normal and L, polarized light. M, cross section of humerus of ASAGS-0101 in normal and N, polarized light. The shape of the cross section does not change sizeable. Most important is the change from a small central medullary cavity in small individuals (A-F) towards a central cavity that is accompanied (G-H; K-L) or even obscured by erosion cavities and sometimes by the deposition of (secondary) endosteal bone (I-J, M-N) in large individuals. Please note that the large size of the cavity in ASAGS-0197 (I-J) is a technical artefact. The presence of erosion cavities at the margin or close to the medullary cavity indicates the onset and increase of resorption activities. Secondary deposition of endosteal bone occurs in Andrias japonicus only at the margins of the erosion cavities and no cavity is completely filled. Note the differences in the amount of resorption and redeposition within the group of larger individuals (G-H, K-L versus I-J, M-N). A cyclicity in bone growth is visible in form of an alignment in rows of osteocytes in ASAGS-0197 (I-J) and in ASAGS-0106 (K-L). True LAGs (lines of arrested growth) are only identified in the outer cortex of ASAGS-0100 (G-H) and ASAGS-0106 (K-L) and throughout most of the cortex in ASAGS-0101 (M-N). Cyclicity in bone growth is best visible at the anterodorsal bone side, where the cortex is thickest. Humerus ASAGS-0050 (E-F) shows the nutrient canal as a round cavity/foramen to the right of the somewhat larger central medullary cavity. Humerus ASAGS-0100 (G-H) shows the nutrient canal as a real canal extending from the inner central medullary cavity to the outer cortex. In the other humeral cross sections, the nutrient canal is not visible. Most differences visible here are related to ontogeny. However, a certain intraspecific plasticity is also obvious. For details on age, size and sex of individuals see Table 1. Abbreviations: p-posterior, v-ventral