**FIGURE 6. ** Local estimation of the number-weighted mean volume of a type 2 pneumocyte. In order to use the planar rotator, the section is isotropic (see figs 2⇑ and 3⇑). Isotropic, uniform, random and parallel test lines (drawn with full lines), a distance (d) apart, are applied to the sampled cell. The lattice of the test lines must be placed so its axis (arrows) passes through the sampled nucleus. For each test line crossing the cell height (h), linear intercepts from the arrow line to the cell border are measured on both sides of the arrow line. The number-weighted cell volume is estimated by squaring all intercepts at both sides of the arrow line and summing over all test lines. If a test line intersects the cell membrane twice, the length of this test line is calculated by adding up the squared lengths with alternating signs, with the intersection most distant from the arrow line being positive.