While evidence for the role of gap functions, particularly during development, has been mounting, it has remained largely correlative, linking structure with presumed functions. With the recent advent of functional antibodies raised to the junctional protein, however, it has become possible to study the role of gap junctions more directly. There is now considerable evidence indicating that they play a vital role in tissue pattern formation and differentiation by allowing direct cell‐to‐cell transfer of developmental signals or morphogens.