Soft X-ray photographs show both the outlines of the soft parts of the regenerating limb and the bones. This enables one to make quantitative studies on bone regression and the growth of the regenerate, since good reference points are available. Bone regression of about 0.5 mm was observed to occur within about 20 days following amputation. Growth curves of regenerates cut at different levels confirm Spallanzani's old observation that the total time for regeneration is fairly constant for different levels of amputation – regenerates from proximal levels regenerate faster. However, the intrinsic growth rate curves are remarkably similar for different levels of amputation; the difference in final length arises because the short period of high intrinsic growth rate continues for slightly longer in regenerates from proximal levels.
In terms of the progress zone theory of limb development, three phases of regeneration are recognized: the formation of the blastema and progress zone; the laying down of the skeletal rudiments by the rapidly proliferating progress zone; and the subsequent slower growth of the rudiments. A quantitative model is put forward and it is shown that it can provide a good description of the observed growth curves. An important assumption is that the various skeletal rudiments are the same size when they leave the progress zone and the final differences in size reflect differences in the final growth phase. The progress zone model provides new quantitative insights into the relationship between the growth of the limb as a whole and the behaviour of individual cells.