Early loss-of-function phenotype after MO-mediated interference with Prdm1 activity. (A-E) Ventral view, anterior to the left; tail region is shown. (F-I,N-Q,U-X) Lateral view, dorsal to the right. (J-M) Animal view, dorsal to the bottom. (R,S) Dorsal view, anterior to the top; somitic region is shown. (F,J;G,K;H,L;I,M) Different views of same embryo. (A,B,D,F,H,J,L,N,P,R,U,W) Untreated controls (ctrl). (C,E) Embryos microinjected with 1 ng of MOprdm1 (MO). (G,I,K,M,O,Q,S,V,X) Embryos microinjected with 2 ng of MOprdm1. (A) Wild-type tail. (B,C) Interference with Prdm1 function partially suppressed duplicated fin fold phenotype in ogo mutant embryos. (D,E) Similar experiment in din mutant embryos did not show this effect. (F-I) bmp4 expression was reduced after injection of MOprdm1 in ogo but not din mutants. (J-M) Anterior neural ectoderm was expanded mediolaterally in ogo but not din mutants injected with MOprdm1 (white arrows) compared with untreated siblings (white open arrows). (N,O) Animal-vegetally elongated morphology of morphant embryos compared with control siblings at the end of gastrulation; dimensions measured are indicated, embryonic length (yellow), dorsal mesoderm extension (green), animal-vegetal axis (red), mediolateral axis (blue). (P,Q) Animal-vegetal elongation of morphant embryonic axis persisted throughout early segmentation. (R,S) Forming somites were mediolaterally expanded. (T) Schematic overview of photoactivation of caged lineage tracer dye, dorsal at the embryonic shield and ventral at the blastoderm margin. (U,V) Tracking of dorsally photoactivated lineage tracer (between green arrows) at the end of gastrulation. (W,X) Tracking movements of ventrally photoactivated lineage tracer (between green arrows) at the end of gastrulation. Scale bars: 150 µm in B,D; 200 µm in G,U; 100 µm in R. av, animal-vegetal axis; bm, blastoderm margin; d, dorsal; dm, dorsal mesoderm extension; el, embryonic length; es, embryonic shield; ml, mediolateral axis; s5, fifth somite; v, ventral; vm, ventralmost mesoderm; ye, yolk extension.