Fig. 1.

Vertebrate axial organization. (a) Diagram of a sagittal section through a Xenopus gastrula, showing the involution of the dorsal mesoderm (d.m., dark red) at the dorsal lip. The neural plate (n.p., blue) overlies the dorsal mesoderm. bl blastocoel, v.l.m. ventrolateral mesoderm (orange), e endoderm (yellow). (b) Sagittal (left panel) and coronal (right panel) diagrams of a tailbud-stage Xenopus embryo showing the elongated anterior-to-posterior axis and organization of tissues within. The neural tube is located dorsally and will form the entire central nervous system (c.n.s.). The dorsal mesoderm gives rise to the notochord and somites, ventrolateral mesoderm (v.l.m.) will form the kidneys, body wall muscles and vascular system. The endoderm forms the gut and its derivative organs. The cement gland (c.g.), a larval amphibian anchoring structure, is shown at the anterior end. After Hausen and Riebesell (1991)