Figure 6. A–E in Sculpture and vascularization of dermal bones, and the implications for the physiology of basal tetrapods

Figure 6. A–E, schematic drawings of formation of new sculptural ridges at the bone periphery during growth. A, the vascular opening is located directly in front of a sculptural ridge that bifurcates symmetrically. B–C, the vascular opening is located lateral to the sculptural ridge, and the bifurcation is asymmetric. D, bifurcation of ridges from contralateral sides fuse roughly at the midline of the furrow, and close the furrow. E, in an asymmetrical bifurcation of a sculptural ridge, the 'axilla ridge' of the corresponding vascular opening may be low or suppressed, giving the impression that the new intercalated ridge has an abrupt origin within the furrow. F–H, development of polygonal sculpture from the radial pattern. F, first, the radial ridges broaden slightly at discrete points in the vicinity of a vascular opening, or may be slightly bent in this direction. These are the points of intersection (nodal points) of the radial ridge with the future dividing wall. G, the dividing walls have grown slowly in height between these points proximal to the vascular openings; through this compartmentalization, sculptural cells have developed that have a rather quadrangular shape at first. H, during appositional growth of the bone, the cells attain a polygonal (ideally hexagonal) outline via a sidewards shift (oblique growth) of the nodal points (arrows). For abbreviations, see text.