Limited axial structures in nodeless chick blastoderms.

The author has been using post-nodal segments of early chick blastoderms as an experimental system to study early morphogenesis and differentiation. The segment is produced by a transverse cut at about 0-4 mm from the primitive pit and is known for its very limited potencies in chorio-allantoic transplantation or in culture, in contrast to anterior parts containing Hensen's node (Hunt, 1931; Willier & Rawles, 1931; Rudnick, 1938; Waddington, 1933; Spratt, 1952; Butros, 1962). One of its characteristic features is the appearance of a concavity in the middle of the anterior border in the shape of a ' V (Waddington, 1935; Rudnick, 1938), but it is not established whether this is due to regression movements of the primitive streak or to retraction of lateral parts (Jacobson, 1938). Experiments attempting to clarify this point are reported in this paper. Another feature noted upon explanting the post-nodal segment is the accumulation of mesoderm in the midline, forming columns that seem to supplement the primitive streak gradually. This mesoderm is not sufficiently organized to form somites and notochord. It was hoped to obtain better mesodermal organization and induction of neural tubes with the increase of lateral mesoderm effected by leaving an anterior extension to the post-nodal piece in the form of lateral 'arms'. True to expectation, as the experiments reported here will indicate, columns of mesoderm both somitic and lateral accumulated in the midline and as a result neural folds or tubes developed in these explants.