The effect of scale changes on repetitive patterns: Influence of 6-aminonicotinamide on feather germ number

There are many examples of patterned developing systems which are size invariant: if the total size of the system is reduced, then the pattern responds by changing its scale in such a way that the number of pattern elements remains constant. This phenomenon is one of the bases which underaly the formulation of the concept of positional information, one of the great unifying ideas in developmental biology. However, there are less common examples of patterns which are size dependent. In these, alterations of overall size lead to a reduction in pattern elements. Such size-dependent patterns are therefore of theoretical interest. Here we describe how the number of feather germs along the wing bud of the developing chick embryo responds to shortening of the limb, and consider the implications of these observations.