The Control of Pattern Across the Antero-Posterior Axis of the Chick Limb Bud by a Unique Signalling Region

SYNOPSIS. The proposition that pattern formation across the antero-posterior axis of the developing chick limb is controlled by a special signalling region has prompted many experiments. No result is inconsistent with the basic hypothesis, and much evidence supports the notion of a graded signal from the zone of polarizing activity. We describe the location in space and time, the size, the graded nature of the signal and the distance over which it acts; but despite increasing interest the molecular mechanism remains unknown. A major change in our understanding of the interaction between the polarizing region and the responding limb field is that growth as well as pattern is involved.