Fig. S2.

Spatial expression of delta, hesC, and tbrain at early gastrula stage in Et. Delta is observed in a few mesodermal cells at this stage and begins to be expressed in a scattered pattern in the ectoderm. HesC is expressed in the nonskeletogenic mesoderm and endoderm, is absent from the ecto-endodermal boundary, and is expressed in a diffuse, nonspecific pattern in the ectoderm. Tbrain expression is seen throughout the mesoderm and here can be seen in the ingressing spicule precursor cells at the tip of the archenteron.