Figure 2

Proliferation and migration of precursor book lung cells from the invaginated epithelium (E) of the atrium (At). Light micrographs (LM), ventral views, semi-thin sections. Centruroides gracilis. A. Newborn first instar. The air sacs (AS) in this book lung are at an early stage and barely discernible. Little or no widening is evident at the atrial origin of the air sacs. The cuticular wall (Cu) of the atrium is absent or very thin at the sites where air sacs are forming. The epithelial cells of the atrial wall are in a distinct layer where lamellae are not being formed; a basement membrane (BM) is present at the basal surface of these cells. The cells in the epithelial layer toward the left in the photo are more numerous as though proliferating near the site where air sacs are forming. On either side of the region of developing air sacs, the basement membrane is absent or disrupted and the precursor cells appear to be migrating inward (asterisks). B. Embryo book lung with some air sacs more advanced than those in Figure 2A. Inside the wall of the atrium, epithelial cells (E) of the hypodermis form a layer with a basement membrane (BM) at their basal surface. The cuticular wall (Cu) of the atrium is much thinner at the site where air sacs are forming. The primordial air sacs (AS) separate aligned precursor cells into double rows. Some cells (asterisks), not yet aligned into rows, appear to be dispersing inward from the atrial epithelium, and at these sites the basement membrane is disrupted or absent. Some widening (W) of the air sac entrance is evident at the atrial origin of two air sacs. In the lateral region (right) of the book lung, the developing air sacs are barely evident among cells not yet aligned. Scales, 20 μm.