In animal development, the first tissues to be formed include such major components as muscle, nerve cord, notochord and the eye. In the vertebrates, all of these tissues are formed by embryonic induction, a process by which some of the cells within a mass of tissue are caused to change their direction of differentiation as a result of close proximity to cells of another kind. The induced cells typically form a solid coherent mass with a distinct border between them and the remaining uninduced cells. This clean separation between induced and uninduced cells is much sharper than can readily be explained as a result of the induction process. We describe here the culture of amphibian cell and tissue recombinations in solid gels containing cytochalasin in which cell division and cell movement is inhibited during response to induction. This has revealed an effect in which the ability of a cell to respond to induction by differentiating as muscle is enhanced by, or even dependent on, other neighbouring cells differentiating in the same way at the same time. This seems to be a newly described process in animal development, termed the community effect. It helps to explain the formation of blocks of tissue from sheets of cells, and could be of widespread occurrence and significance in morphogenesis resulting from embryonic induction.