Evolutionary developmental biology of the tetrapod limb.

New insights into the origin of the tetrapod limb, and its early development and patterning, are emerging from a variety of fields. A wide diversity of approaches was reported at the BSDB Spring Symposium on 'The Evolution of Developmental Mechanisms' (Edinburgh, 1994); here I review the contributions these various approaches have made to understanding the evolutionary developmental biology of the tetrapod limb. The fields covered included palaeontology, descriptive embryology, experimental embryological analysis of interactions within developing limbs plus description and manipulation of homeobox gene expression in early limb buds. Concepts are equally varied, sometimes conflicting, sometimes overlapping. Some concern the limb 'archetype' (can the palaeontologists and morphologists still define this with precision? how far is there a limb developmental bauplan?); others are based on identification of epigenetic factors (eg secondary inductions), as generating pattern; while yet others assume a direct gene-morphology relationship. But all the contributors ask the same compelling question: can we explain both the similarity (homology) and variety of tetrapod limbs (and the fins of the Crossopterygians) in terms of developmental mechanisms?