THE XENOPUS-LAEVIS TAIL-FORMING REGION

A fate map is produced for the Xenopus tail-forming region at the neur ula stage by orthotopic grafting of tissue labelled with fluorescein-d extran amine. It is shown that the axial tissues of the tail are deriv ed from a rectangle 700 mu m wide by 600 mu m long, while the epidermi s of the tail is drawn from a much larger area. The fate map shows tha t much of the final tail is not formed from the tail bud itself, but b y a displacement of trunk axial tissue relative to the proctodaeum. A specification map is also produced by culturing parts of the tail-form ing region in vitro or as grafts on a neutral site on host embryos. Fo r the axial tissues this map is identical to the fate map, showing tha t the tail-forming region is embryologically mosaic. The prospective t ail epidermis can, however, regulate defects. It is shown that previou s claims of regeneration of the Xenopus tail bud are misleading. Remov al of the tail-forming region totally prevents tail development. Remov al of the tail bud leads to a partial tail, formed by the normal proce ss of displacement of trunk tissue relative to the proctodaeum. Even w hen only part of the tail bud is removed the tail is still truncated. This shows that there is no terminal regeneration of the tail at embry onic stages.