The positional coding system in the early eye rudiment of Xenopus laevis, and its modification after grafting operations.

In a series of Xenopus embryos, 60-90 degrees sectors at various positions distributed around the eye rudiment were replaced with sectors grafted from the opposite position in the rudiment on the same side of the head of a donor. The majority of the operations were carried out before stage 28 (Nieuwkoop & Faber, 1956), and many before stage 26. The patterns of retinotectal connectivity which then developed were assayed electrophysiologically soon after metamorphosis. The visuotectal maps were frequently compound, giving evidence that many parts of the rudiment had already been equipped with distinct tissue positional codes by the time operations were performed (i.e. before neurogenesis). Although graft-derived sectors of retina connected to tectal sectors that were more nearly appropriate for their original positions in the rudiment than for their translocated ones, the 'handedness' of these ectopic components of the compound maps tended to bear a mirror-image relation to the major map, rather than the point-symmetrical one to be expected from a complete autonomy of mapping functions in grafted tissue. The results are discussed in relation to possible modes of organization of the developing eye, considered as a pattern-forming system.