Stage 15, Inversion II. All scale bars 100 μm. Sytox staining, a-a''; SEMs, b, c. a: Lateral view. The white line indicates the increased distance from the precheliceral lobes (PcL) to the opisthosomal tail compared with previous stages (see Figure 10a). a': Frontal view. The white dotted line indicates the mouth opening between the two lateral subdivisions of the developing brain (compare with Figure 10a'). The two labral lobes have completely fused and the labrum (Lb) is now an unpaired structure. a'': Posterior view. The white dotted line shows the progress of inversion (middle diagram in d). b: Opisthosomal region. Separated opisthosomal segments four to nine (O4-9) are visible. The tenth (O10) and the future eleventh segments (black dotted line) are located together with the growth zone (GZ) in a tail-like portion of the germ band that protrudes from the mass of yolk. Small bulges of tergite anlagen (Ter) are evident on the dorsal surface (compare with the more differentiated tergite anlagen in Figure 14c). c: Detail of the right third and fourth walking legs (L3, L4) and the limb buds of opisthosomal segments two and three (O2, O3). At the posterior base of the limb bud of O2 the opening of the pulmonary sac (white arrow) is seen, and adjacent to it medially are two slit-like openings (black arrows) to the book lungs. The podomeres of the fourth walking leg (L4) are numbered (1-5) from base to tip. d: Schematic illustration of the steps of inversion corresponding to stages 14 (compare with Figure 10 a''), 15 (compare with Figure 11 a''), and 16 (compare with Figure 12 a''). Posterior view, dorsal is at the top of the diagrams. The germ band (brown areas) has divided, and the ventral sulcus (VS) is increasing in width. The bilateral regions of the germ band are migrating dorsally (black arrows), enclosing the yolk area (Y) and eventually meeting in the dorsal midline (stage 17, dorsal closure). By stage 15, the dorsal edges of both halves of the germ band lie in a line when viewed from posterior. AF, anterior furrow; Ch, chelicere; P, pedipalp; X, damaged area, cuticle torn.