Expression of Gbx during ontogenesis of Wirenia argentea. Anterior (a)-posterior (p), dorsal (d)-ventral (v), and left (l)-right (r) axes indicate the orientation. Anterior faces up in all micrographs. Asterisks mark the mouth opening. (a,b) Freshly hatched larvae (0–1 dph) express Gbx in a dorso-ventral gradient with the highest expression in the ventral epidermal cells close to the pseudo-blastopore (pb). The early embryo possesses flattened and large test-cells (tc) that do not express Gbx. (c–e) Early larvae (6–7 dph) express Gbx ventral laterally in epidermal cells (2) that line the peri-imaginal space (pis). This expression domain extends towards the lateral portions of the mouth and also includes subepidermal cells (2). Two bilateral groups of Gbx-expressing cells (1) are located ventrally at the posterior pole of the outgrowing trunk. These expression domains might be associated with the developing pedal nerve cords. Note that the trunk is less retracted in (e) compared to (d) and therefore Gbx-expression domain “1” is clearly distinguishable from other expression domains. The developing cerebral ganglia (cg) show faint Gbx-expression. (f–g) Mid-stage larvae (10–11 dph) express Gbx in the developing pedal nerve cords. Gbx is still expressed in epidermal cells (2) which line the peri-imaginal space and in subepidermal cells close to the latter cells (2). Some individuals show Gbx-expression adjacent to the developing foregut (fg) (arrowheads). Gbx is also expressed in the region of the developing cerebral ganglia. (h,i) Late larvae (14–15 dph) express Gbx in the developing pedal nerve cords. Gbx is expressed ventro-laterally in epidermal cells (2) adjacent to the peri-imaginal space and in subepidermal cells close to the latter cells (2). This expression domain extends towards the lateral portions of the mouth. Gbx-expression is also visible adjacent to the developing foregut (arrowheads). Abbreviations: pc, pedal nerve cords. Scale bars: 50 µm.