Stage 16, Inversion III. All scale bars 100 μm. Sytox staining, a-a'', c; SEMs, b, d, e. a: Lateral view. The white line indicates the increased distance from the precheliceral lobes (PcL) to the opisthosomal tail compared to previous stages (compare with Figure 10a and 11a). The prosomal tergites start to extend dorsally (white arrows). a': Frontal view. The white dotted line indicates the more posterior position of the mouth opening in relation to the lateral subdivision of the brain (compare with Figures 10a' and 11a'). a'': Posterior view. The white dotted line shows the progress of inversion (see the lower diagram in Figure 11d which schematically illustrates inversion). b: Detail of the head region. The medial subdivision (ms) is growing anteriorly (black arrows), partially covering the anterior furrow (AF). The lateral furrow (LF) is totally covered by tissue from the lateral subdivision (ls). The mouth opening is covered by the medially enlarged tip of the labrum (Lb). Anlagen of the segmental sternites (Ste, white dotted line) are evident medial to the pedipalps (P) and walking legs (L1). c: Ventral view showing all prosomal appendages and the extent of the widening of the ventral sulcus (VS). d: Detail of left anterior opisthosoma. At the posterior base of the limb bud on opisthosomal segment two (O2), three pulmonary furrows (black arrows) and a lateral opening of the pulmonary sac (PuS) are evident. At the latero-posterior insertion of the limb bud on opisthosomal segment three (O3), the opening of the tubular trachea (TrO) is visible. The globular limb bud on opisthosomal segment four (O4) will differentiate into the anterior spinneret (ASp), whereas the dorso-ventrally elongated limb bud on opisthosomal segment five (O5) will differentiate into the posterior (PSp) and medial (MSp) spinnerets. e: Detail of the posterior opisthosomal region. Eight opisthosomal segments (O4-O11) are clearly evident here. On the dorsal surface, the primordial tergite plates (Ter) have further expanded (compare with the later stage in Figure 14c). Between the eleventh opisthosomal segment (O11) and the growth zone (GZ), small bilateral lobes probably represent the twelfth opisthosomal segment (O12?). Ch, chelicere; Lab, labium; Te, telson.