Stage 18, Prosomal shield. All scale bars 100 μm. Sytox staining, a-a'', e, g; SEMs, b-d, f. a: Lateral view. The dorsal yolky mass is divided into three distinct yolk sacs (YS). a': Frontal view. The dotted white line shows the position of the labrum posterior to the cheliceres, which partially cover it with their bases. a'': Dorsal view. The top of the white (dotted line) trapezoid designates a narrowing region that will eventually become the petiolus, a short length of thin connecting tissue between the prosoma and opisthosoma as shown in Figure 18g and 19b. The base of the trapezoid indicates the broader opisthosoma that continues in advanced stages (e.g. Figure 18g, 19a). The heart primordium (H) is evident in the dorsal midline. b: Frontal view. The anterior brain region is almost covered by the prosomal shield (white arrows). c: Postero-lateral view. The elongated walking legs (L1-L4) show their final segmentation into seven podomeres (black lines). The second opisthosomal segment (O2) shows a prominent opening for the book lung system (BL). d: Tiny egg teeth (ET) appear laterally on the most proximal segment of the pedipalps (P). e: Ventral view of the prosoma, with the legs trimmed. The sternites start to fuse medially from anterior to posterior (white arrows) in a process that will eventually result in a single sternal plate. f: Lateral view of the left opisthosomal region in a late stage 18. The broad openings (primordial spiracles) of the book lung system (BL) and the tracheal (TrO) systems are clearly visible. g: Ventral view of the opisthosoma; the prosoma is cut off (same embryo as in e). The opisthosomal sternites start to fuse medially from posterior to anterior (white arrows). Asp, anlage of anterior spinneret; Ch, chelicere; en, endite; f, fang; Lb, labrum; MSp, anlage of medial spinneret; O3-O5, opisthosomal segments three to five; PS, prosomal shield; PSp, anlage of posterior spinneret; Ter, tergite.