Stage 9, Prosomal limb buds. All scale bars 200 μm. Sytox staining, a-a''; SEMs, b, c. a: Lateral view of embryo. The dorsal field (DF) in earlier figures is probably now a mass of extra- and intra-cellular yolk (Y). Also evident are the precheliceral area (Pc), cheliceres (Ch), pedipalps (P), walking legs (L1-L4), opisthosomal segments (O1-O3) and the growth zone (GZ). a': Frontal view. Between the developing appendages the ventral sulcus (VS) is visible as a narrow length of midline tissue with a low cell density compared with the bilateral appendage regions. The white arrows show that the precheliceral region (Pc) extends anteriorly from the anterior base of the cheliceres. a'': Posterior view. The dotted lines indicate the progress of opisthosomal segment formation anterior to the growth zone (GZ). b: Detail of the prosomal region in fronto-ventral view. The white dotted lines indicate the lateral margins of the precheliceral region (Pc). c: Detail of the opisthosomal region. Limb buds are barely evident at this stage on opisthosomal segments 1-3 (O1-O3). The white dotted lines indicate the progress of the formation of additional opisthosomal segments.