Properties of the primary organization field in the embryo of Xenopus laevis. II. Positional information for axial organization in embryos with two head organizers.

The results are reported of a series of experiments, the exact geometry of which has been presented in a previous paper. Late blastulae and early stage-10 gastrulae are supplied with a second head organizer region at varying angular distances, in the marginal zone, from the presumptive site of their own organizer. The configuration of positional information existing in the mesodermal mantle of the late gastrula or earliest neurula, as a final result of such operations, was recorded by observing the pattern of axial organ differentiation obtained by tailbud stages (26-28). The operational differences between various current theories as to the nature of embryonic differentiation fields are briefly discussed, as a framework within which to consider the results of experiments such as those reported here. It is suggested that in the future, and using the present results as a basis, experiments may be possible that are more critical in distinguishing between the various theoretical suppositions involved. Evidence is presented that the final configuration of positional information, achieved as a result of the implantation of a second head organizer at or before the onset of host gastrulation, becomes stable some time before it is irreversibly expressed in terms of a pattern of cell commitment in the mesodermal/endodermal mantle. It is insensitive both to relative ages of host and graft at the time of operation, over the range employed and, probably, to the ambient temperature of development between operation and the time of cell differentiation, being dependent only on the angular distance originally existing between graft and presumptive host organizer sites. In the discussion, a model is given for the visualization of positional information in partially double fields, produced in a two-dimensional sheet of cells where the normal endpoint of field formation is a bilateral symmetry of differentiation zones.