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Fig. 1 snail1b knockdown induced morphological defects at gastrulation and segmentation stages. (A) Two different morpholino antisense oligonucleotides were designed against adjacent sequences in the 5' region of the snail1b cDNA. (B,D) Embryos at mid-gastrulation and (C,E) at the 18-somite stage. Morpholino injection (3 ng/embryo) produced defects in extension movements at early embryonic stages that led to the shortening of the body axis at 30 hpf (compare H with I). (F,G,J) This phenotype was rescued by co-injection of snail1b mRNA. (K,L,N,O,P,R) snail1a but not snail1b was prominently expressed in the involuting and posterior mesoderm. (M,Q) Lateral views highlight the expression of snail1a in the posterior half of the embryo and snail1b in the anterior. The dotted lines indicate the position of the margin and the adaxial cells (P) and that of the prechordal plate (R). ac, adaxial cells; ame, anterior axial mesendoderm; cm, condensing mesenchyme; pm, paraxial mesoderm; tbm, tail bud mesenchyme.