<div><p>All embryos are shown as optical sections, and oriented in a similar manner as in <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040291#pbio-0040291-g002" target="_blank">Figure 2</a> with anterior to the top and left of each panel and dorsal in the top right of each panel, unless otherwise specified.</p>
<p>(A) <i>Hh</i> expression in the apical tip of a day 3 embryo, and (B) at day 5 at the same location.</p>
<p>(C) Endodermal expression of <i>nk2–2</i> (also called <i>nkx2.2</i>) in the late gastrula, with a sharp delineation between presumptive proboscis mesoderm and definitive endoderm, and (D) at day 4, with expression continuing in the endoderm but down-regulated in the pouches of the forming gill slits (black arrowhead).</p>
<p>(E) Expression of <i>msx</i> in the late gastrula, and (F) at day 3. Expression occurs exclusively in the ectoderm of the metasome and is down-regulated in the telotroch, the cilated band, as marked (white arrowhead).</p></div
