Notochord formation in amphibians: Two directions and two ways

The formation, structure, and rearrangements of the notochord were studied in Pleurodeles waltlii and Xenopus laevis embryos. We measured length and cross‐sectional area and calculated volume of the notochord at different stages of development. In both species, the notochord emerges as a condensation and a columnarization of dorsal mesoderm along the midsagittal plane in the midgastrula. In P. waltlii embryos, the notochord segregates from archenteron roof by apical cell surface shrinking. The wave of shrinkage begins in the middle of the rudiment and simultaneously extends cranially and caudally. In X. laevis embryos, the notochord elongates in two directions also, but the mode of notochord formation is not identical in anterior and posterior parts of embryo. While anterior development looks similar to that found in P. waltlii, posterior development features closer binding of the notochord rudiment to ectoderm within existing delaminating boundaries from somite mesoderm. Rates of elongation in the different directions are approximately the same in the two species studied: It is 45–47% for the anterior and 53–55% for the posterior direction compared to total length of the notochord. © 1995 Wiley‐Liss, Inc.