Limited axial structures in nodeless chick blastoderms

ABSTRACT Definitive streak to head-fold blastoderms were given two lateral cuts parallel to the streak and a third transverse cut at 0·35 mm posterior to the pit, thus removing a rectangular piece around the node 0·8 × 0·35 mm in area. Upon explantation many of these U-blastoderms developed somites and neural tubes, but notochords were absent. Two types of controls were used: posterior segments of blastoderms produced by transection at 0·35 mm posterior to the pit, and lateral regions of blastoderms explanted from outside the central 0·8 mm zone. These controls did not develop true somites or neural tubes but did form ectodermal invaginations and mesodermal aggregations in the midline. The results are interpreted as manifestations of an increase in the amount of mesoderm and of the space available for morphogenetic movements.