[A comparative analysis of notochord formation in amphibian embryos]

We studied the origin, structure, and development of the notochord in Pleurodeles waltlii (Urodela) and Xenopus laevis (Anura) embryos. The notochord rudiment is formed in both species at the early gastrula stage as a cluster of polarized chorda-mesoderm cells located along the sagittal plane of the embryo. In Pl. waltlii the notochord rudiment is separated from the gastrocoele roof as a result of contraction of apical cell surfaces. The contraction wave spreads forward and backward along craniocaudal axis, i.e., segregation of the notochord rudiment progresses in two directions simultaneously. Similar process takes place in X. laevis embryos; however, propagation of the contraction wave in the anterior part of the body somewhat differs from that in the posterior part. While the "anterior" contraction wave resembles that in Pl. waltlii embryos, progression of the wave in the posterior part of the body is distinguished by a closer association of the notochord rudiment with ectoderm and the presence of its delamination boundaries with the somite mesoderm.