The vertebrate body plan is thought to be derived during the early Cambrian from a worm-like chordate ancestor. While all three germ layers were clearly involved in this innovation, the role of the endoderm remains elusive. According to the hourglass model, the optimal window for investigating the evolution of vertebrate endoderm-derived structures during cephalochordate development is from the Spemann's organizer stage to the opening of the mouth (Stages 1–7, described herein). Regulatory gene expression, examined during these stages, illustrate that the cephalochordate endoderm is patterned into 12 organ primordia. Early vertebrates inherited at least a portion of 6 of these primordia, while the remainder were lost. Of those that were preserved, we demonstrate that the vertebrate symmetric mouth was built on a vestige of the anterior pre-oral pit, that the pre-existing pharyngeal pouch in this chordate ancestor laid the foundation for the new neural crest cell (NCC)-derived vertebrate-type pharyngeal arches, that the thyroid evolved from the posterior endostyle primordim, that the pancreas was derived from the Pdx1-expressing diverticulum primordium, and the small and large intestines originated with the Cdx1-expressing hindgut rudiments. This investigation uncovers the evolutionary foundations of vertebrate endoderm-derived structures, and demonstrates that the number of organ primordia were reduced during evolution.

脊椎动物的身体计划被认为是在寒武纪早期从蠕虫状的r酸盐祖先获得的。尽管所有三个细菌层都明确参与了这项创新，但内胚层的作用仍然难以捉摸。根据沙漏模型，调查头孢酸盐发育过程中脊椎动物内胚层衍生结构演变的最佳窗口是从Spemann的组织者阶段到张口（阶段1-7，在此描述）。在这些阶段中检查的调节基因表达表明，头孢菌内胚层被图案化为12个器官原基。早期的脊椎动物至少继承了这些原基中6个的一部分，而其余的则丢失了。在那些被保存的表达Pdx1的憩室原基，小肠和大肠起源于表达Cdx1的后肠残基。这项研究揭示了脊椎动物内胚层衍生结构的进化基础，并证明了器官原基的数量在进化过程中减少了。