In many embryos, the removal of cells whose descendants would normally have formed entire parts of the body pattern is followed by apparently normal morphogenesis, which implies an ordered readjustment of the activities of the remaining cells before their potentialities become restricted. Special cell lineages cannot underly the generation and regulation of pattern in such embryos. It is proposed instead that there must be some regulatory communication system in the developing embryo that normally ensures an appropriate spatial pattern of differentiation but which is also able to adjust to the removal, addition or transposition of material at a sufficiently early stage. Precise models for such a mechanism have recently been suggested, and have been tested experimentally. I have performed surgical manipulations at pre-gastrula embryonic stages in two distinctly related amphibian types, Xenopus and Ambystoma, and report here an assessment of the regulation achieved in terms of pattern proportions. The results are problematical for most current theories of pattern control.