Chimera analysis in which double homozygous Mesp1^tm3Ysa and Mesp2^tm3(cre)Ysa mutant embryos were aggregated with wild-type embryos show that embryos develop normally, however double knock-out cells are scarcely seen in the anterior-cephalic and heart mesoderm, but they do contribute to the formation of somites, notochord, and gut, and the atrial appendage is exclusively composed of the mutant cells (J:62984).