A: Transverse section through an embryonic day (E) 2.1 quail embryo equivalent to Hamburger and Hamilton (HH) stage 14. At this stage, the intestinal primordium is open and comprised of the splanchnic mesoderm (red; SpM), endoderm (yellow; En) and an intervening endothelial plexus (green; EP). B: At E6, the intestine is completely closed and composed of a mesothelium (orange; Mes), a two layered endothelial plexus (green; EP), a visceral smooth muscle layer (red; viSM), and endoderm (yellow; En). C: In the adult intestine, villi are lined with a mucosal epithelium (yellow; Mu) and contain a lamina propria (LP) composed of capillaries, a lymphatic lacteal, and connective tissue. A four-layered muscularis externa (ME) surrounds the lamina propria. A serosal membrane (Se) lines the coelomic surface.