Figure 3

Some air sacs (AS) with dense and opaque contents in a book lung more advanced than those in Figure 2A,B. LM, ventral view, semi-thin section. Centruroides gracilis, newborn first instar. The portions of air sacs evident here are more prominent than in Figure 2A,B, and some air sacs (AS) are wider and more dense than the others because of the granular contents (see below). The air sacs are formed between double rows of cells. Some widening (W) of the air sac entrance is evident at the atrial origin of two air sacs. The asterisks indicate cells that are not yet aligned into rows or separated by primordial air sacs. Probably as a result of the proliferation and inward migration of cells, the atrium (At) no longer has an epithelial layer with basement membrane (compare with earlier stages in Fig. 2A,B). Scale, 20 μm.