Msgn1 and Spt are essential for tail somite formation. (A-D,E-H) Live zebrafish embryos typical of their genotypic classes. (A′-D′′ ′,E′-H′′ ′ ′) In situ hybridisation for ntl and cb1045 (xirp2a), myoD (myod1), tbx24 and mespaa expression in uninjected wild-type (wt) siblings and in the genotypes indicated. spt-/- mutants were derived from a heterozygous spt+/- cross. msgn1-/- mutants, spt+/-;msgn1-/- mutants and spt-/-;msgn1-/- double mutants were derived from a double heterozygous msgn1+/-;spt+/- cross. Indicated is the number of embryos (n) observed with the phenotype shown in each panel, and when embryos were derived from mutant crosses the obtained n corresponded to the expected frequencies for each genotype. Asterisk indicates that wt and msgn1 mutants have an undistinguishable phenotype at the 14-somite stage.