Figure 15

Stage 19, Heart. All scale bars 200 μm. Sytox staining, a-a'', d, e; SEMs, b, c. a: Lateral view. BL indicates an internal mass of cells that will become book lung tissue. The brain region has sunk into the prosoma (compare with earlier stages, e.g. Figure 18a) and is partially covered by the prosomal shield (PS). a': Frontal view. The dotted line indicates the advancing edge of the prosomal shield that will eventually cover the brain region. The grey asterisks (on the left half of the body only) show that the brain has differentiated into interconnected lobes. a'': Dorsal view. As described in the legend for Figure 14, the top of the trapezoid (white dotted lines) spans the narrowing region that will become the petiolus while the base of the trapezoid shows the continuing breadth of the opisthosoma. The tubular heart (H) is developing in the dorsal midline and is the main identifying feature for this stage. b: Lateral view. A suture between the prosoma and opisthosoma is visible (white arrow heads). Some embryonic cuticle was torn off, exposing the opening of the book lung system (black arrow). The posterior-most segments of the opisthosoma are further compressed and together form the anal tubercle (AT). c: Frontal view. The brain region is almost completely covered by the prosomal shield (PS) and the labrum (Lb) lies ventral to the cheliceres (Ch). d: Ventral view of the prosoma. The legs are trimmed to show the medially fused sternites (indicated by white arrows) of all prosomal segments. e: Ventral view of an opisthosoma separated from the prosoma (same embryo as d). White arrows indicate the progress of the ventral closure of opisthosomal sternites. The tubular heart (H) is seen in cross section as a circular structure. BL, book lung system; en, endite; f, fang; L1-L4, walking legs one to four; O4, O5, opisthosomal segments four and five; P, pedipalp; Tr, tracheal system.