The roles of node regression and elongation of the area pellucida in the formation of somites in avian embryos.

Experiments have been carried out on explanted chick embryos to test certain widely accepted concepts about the role of Hensen's node in somite formation. The relationship between elongation of the area pellucida and regression of Hensen's node has also been investigated. We conclude from these experiments that: (a) The timing of somite formation is not controlled by the regression of Hensen's node, nor by the shearing of the mesoderm into right and left halves. (b) Somite size and shape are probably controlled by local conditions in the chick embryo. (c) Elongation and regression are two different events. (d) The position of the probably depends on mechanical tensions in the area pellucida. (e) The notochord is not required for the stability of somites in vivo.