The work presented, in this and the subsequent papers of a series, was begun in order to re-examine the properties of the amphibian primary embryonic field, in the light of current theories concerning the nature of individuation fields in developing animal systems. A detailed description is given of the basic operation whose results are described in this and the subsequent paper. This involves the transplantation, into a late blastula or stage-10 gastrula host, of a supernumerary stage-10 organizer region. The consequences of such operations during the following 4–6 h, up to the late gastrula stage, are also described.
Evidence is presented that, from a time some 2·5 h before the organizer site first becomes externally visible, its presumptive region is immune from interference by the proximity of another, implanted organizer, even one which is itself 2·5 h older. That is to say, the final site of development of host organizer activity is not altered by the presence of such an implant.
Pairs of early organizers at comparable stages of activity appear to set up competing fields of cellular orientation and immigration, which show a fairly sharp boundary at their interface. This is most obvious for pairs of organizers fairly close together, since the cell polarization and stretching is most pronounced in the region near to the apex of the field, i.e. the initial site of cell immigration. Independent initial fields of immigration due to two organizers can reliably be distinguished in cases where they are as little as 30° of angular distance apart in the marginal zone of the host.
These results are to be considered in relation to those of Paper II, for the same series of operations, where the final patterns of cell differentiation are studied, and to those of Paper III, where evidence is given for the development of autonomous polarity in the region of the organizer.
