The control of growth and the development of pattern across the anteroposterior axis of the chick limb bud.

Two grafts of zone of polarizing activity (ZPA) were made to host limb buds. The grafts define the width of the shared responding field lying between them. They cause a change in the growth pattern of the bud so that there is an increase in the width of the tissue between the grafts. Concurrently they redefine (respecify) cell states in the responding tissue so as to cause formation of a mirror-image reduplicate hand between them. The number and type of digits formed depends on the initial distance between the grafts. The results suggest that the initial presumptive hand field is very small (approximately 300 micrometers), that it is not a classical morphallactic system, and that it is able to regulate its growth pattern. A point-source diffusion model is presented.