Expression of Ct-Pax3/7. Stages are marked in the lower left corner of each panel. Anterior is to the left for all panels. All panels are ventral views except A, B, I and P. Panels A, B and P are lateral views with ventral down; panel I is a ventrolateral view. Specimens shown in D, F, H, L and O were stained with Hoechst. The mouth is indicated where present (asterisk). (A) Ct-Pax3/7 expression in lateral ectodermal cells (arrow) during early Stage 3. (B,C) By late Stage 3, expression expands into lateral bands (arrows). (C,D) DIC (C) and epi-fluoresence (D) images of the same animal. Arrowhead in D indicates nuclei of the presumptive telotroch. (E,F) DIC (E) and epi-fluorescence (F) of the same animal. By Stage 4, expression extends from the stomodeum to the telotroch (arrows). (G,H) DIC (G) and epi-fluorescence (H) of the same animal. Longitudinal bands of expression (arrows) persist as the first larval segments appear. (I) Expression in surface ectoderm (arrow). (J) During Stage 6, expression diminishes in anterior segments (arrows). (K) Expression becomes limited to the posterior segments and PGZ (arrows). (L) Enlarged view of same animal in K showing expression adjacent to the ganglia (large arrows). Small arrow points to ganglion of the ventral nerve cord in a more anterior segment. (M) At Stage 8, expression is restricted to two patches of ventrolateral ectoderm in the PGZ (arrows). (N,O) Posterior region of a juvenile. DIC (N) and epi-fluorescence (O) images of the same specimen showing expression in and posterior to the nascent segment (large arrows). Dotted line marks posterior boundary of the nascent segment. (P) Posterior end of a juvenile showing expression on the ventral side of the body (arrow). juv: juvenile; tt: telotroch.