Figure 1:

Conditionally specified endomesoderm (or mesendoderm) shown in magenta across a selection of metazoan embryos as it resolves into endodermal lineages (yellow) and mesodermal lineages (red). In nematode embryos (A), the EMS cell gives rise to the MS and E cells (Ai), which specify mesoderm and endoderm respectively. In developing hemichordates (B), a conditionally specified endomesoderm later resolves into endoderm and mesoderm after gastrulation (Bi). Tunicate embryos at the 16-cell stage (C) exhibit high levels of nuclear β-catenin in the NNE lineage before it resolves into endoderm and neural-notochord (i.e. ectoderm-mesoderm) lineages (Ci). In fish embryos (D), during epiboly, ingressing hypoblast cells give rise to either endoderm or mesoderm (Di), suggesting a conditionally specified fate earlier. Mouse embryo epiblast (E) also exhibits cells with a potential to form endoderm or mesoderm (Ei), fates that become committed as cells pass through the primitive streak.