Stage 14, Inversion I. All scale bars 100 μm. Sytox staining, a-a'', d; SEMs, b, c, e, f. a: Lateral view. The distance between the posterior opisthosoma and the anterior border of the precheliceral lobes (PcL) has increased (indicated by white line, compare with Figure 9a). a': Frontal view. Between the precheliceral lobes (PcL) the stomodeum (Sto) has moved posteriorly. The white dotted line shows the more anterior position of the mouth opening in relation to the lateral subdivision (indicated by white arrows) of the brain. a'': Posterior view. The white dotted line shows the progress of inversion (see the upper diagram in Figure 11d which schematically illustrates inversion). b: Head in ventro-lateral view. Anterior to the medial subdivision (ms), the anterior furrow (AF) has formed. The anterior furrow has also been termed semi-lunar or cerebral groove in other arachnids [e.g. [33, 45]]. The lateral subdivision (ls) migrates (black arrows) in the direction of the lateral furrow (LF), partly covering it. The mouth opening is surrounded anteriorly by the labrum (Lb) and posteriorly by the labium (Lab). c: Lateral view of right prosomal region. The most proximal limb segments (coxa, Cx and trochanter/femur, Tro/Fe) of the pedipalp and each prosomal limb are widened in anterior-posterior direction (white stars). d: Lateral view of right prosomal region. The ectodermal tissue medial to the prosomal limbs shows a grid-like formation of black spots (white arrows), presumably primordial neural tissue. e: Opisthosomal region. The black line indicates the relative progress of the ventral sulcus (VS). Nine separate opisthosomal segments are present. The black dotted line indicates an additional segment anterior to the growth zone (GZ). The black asterisks designate lobes of anlagen that will eventually become tergites on the dorsal surface of the body. f: Detail of right limb buds of opisthosomal segments two and three (for orientation see dashed-line box in e). At the lateral base of the limb buds of opisthosomal segment two (O2), the opening of the pulmonary sac (white arrow) can be seen. Medially adjacent to it are two slit-like openings (black arrows) to the developing book lungs. Ch, chelicere; L1-L4, walking legs one to four; P, pedipalp.