NOTOCHORD FORMATION IN AMPHIBIANS - 2 DIRECTIONS AND 2 WAYS

The formation, structure, and rearrangements of the notochord were stu died in Pleurodeles waltlii and Xenopus Laevis embryos. We measured le ngth and cross-sectional area and calculated volume of the notochord a t different stages of development. In both species, the notochord emer ges as a condensation and a columnarization of dorsal mesoderm along t he midsagittal plane in the midgastrula. In P. waltlii embryos, the no tochord segregates from archenteron roof by apical cell surface shrink ing. The wave of shrinkage begins in the middle of the rudiment and si multaneously 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 rudime nt to ectoderm within existing delaminating boundaries from somite mes oderm, Rates of elongation in the different directions are approximate ly 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. (C) 1995 Wiley-Liss, Inc.