DEBATABLE ISSUES

This paper reports a discussion between Antonio Garcia-Bellido and Lew is Wolpert about a number of questions raised by Alain Ghysen. The que stions follow, in reverse order, the subjects dealt with in this issue : first the principles tare there unifying principles of development?) , then questions dealing with evolution (why are patterns conserved?) and with the homeotic genes (what is their function?), then the cell b iology of development (who is controlling actual morphogenesis?), and the generation and evolution of patterns (what makes development so re producible and how does it change from one species to another?) and fi nally about the genetics of cell determination and specification (how does a cell measure its position?). Obviously the discussion did not p rovide any firm answers to any of these questions. Perhaps more import antly, it provides a vivid picture of two contrasting ways of thinking about developmental problems.