Stage 13, Labrum. All scale bars 100 μm. Sytox staining, a-a''; SEMs; b-d. a: Lateral view. Cheliceres (Ch), pedipalps (P) and the four walking limbs (L1-L4) are more prominent in the prosoma. Because of the growth that has taken place along the ventral curvature, the posterior end of the opisthosoma approaches (white line) the anterior border of the precheliceral lobes (PcL). a': Frontal view. Anterior to the stomodeum (Sto) the prominent labrum (Lb) is evident. Cheliceres (Ch) and pedipalps (P) are more pronounced. a'': Posterior view. The ventral sulcus (VS, white dotted line) extends posteriorly to the eighth opisthosomal segment (O8). b: Precheliceral region. Two distinct fields of precursor neural tissue are evident within the crescent-shaped precheliceral lobes: the medial (ms) and lateral (ls) subdivisions. The cheliceres (Ch) have a proximal base (bs) and a distal fang (f). The pedipalp (P) is four-segmented (1-4) and bears an endite (en) on its first segment. c: Prosoma in postero-lateral view. The white arrowheads show that each walking leg (L1-L4) is subdivided into five podomeres (1-5). There is a point-like depression (black arrows) of what is presumed to be neural precursor tissue at the distal tip of each leg. d: Opisthosomal region. Nine opisthosomal segments (O1-O9) are visible while an additional segment is still connected to the growth zone (GZ). Black arrowheads indicate segmental furrows that mark the boundary between opisthosomal segments. A slit-like invagination (black arrow, primordial respiratory tissue) is evident at the posterior base of the limb bud at opisthosomal segment two (O2). The ventral sulcus (black dotted line) extends posteriorly to the eighth opisthosomal segment (O8).