A series of Xenopus egg batches has been exposed to doses of u.v. (2537A) light on the vegetal hemisphere at precleavage stages, calculated to result in a range of minimal axial deficiency syndromes in the developing larvae. At the time of onset of gastrulation in synchronously fertilized but non-irradiated batch members, each experimental group was regularly scanned so that small subsamples of embryos could be set aside as showing particular, progressive degrees of delay in onset of the visible gastrulation movements. Such sampling was found to have preselected embryos showing generally progressive degrees of pattern impairment at larval stages, and this observation was extended by histological examination of the anterior axial anatomy. Such examination was also made of the least abnormal-looking members of a series of larvae resulting from excision of the presumptive head endo-mesoderm, traditionally called 'the organizer', from stage-10 gastrulae (Cooke, 1975). The results support the notion that production of the most anterior endo-mesodermal pattern parts (and of their inductive capacities in giving rise to the brain pattern) occurs only in material whose timing, in the onset of gastrulation activity, is close to the normal onset time after fertilization. Either an early failure of the egg to generate a location with the 'position value' corresponding with this extreme of the pattern, or the much later excision of the region from a physiologically normal gastrula, results in a system of pattern formation permanently truncated at its apical (head and dorsal) end. There is no evidence for any dynamic, in the system ascribing position value, that will cause regulative restoration of this cellular state (the most extreme 'activation' for development) in response to its absence after precleavage stages. An earlier statement (Cooke, 1975) that this could occur was based upon inadequate analysis of larvae with an often misleading external anatomy. The present results are discussed as supporting the overall view of the early Xenopus patterning system that has been developed in the previous two papers of the series.