This paper concerns the apparent rapprochement, now underway, between genetics and developmental biology.¹ For there to be a rapprochement, of course, there must have been long-standing disagreements. The disagreements between embryology and genetics bear all the stigmata of a deep discordance between research traditions built on conflicting assumptions and practices. They have been discussed by many historians of science and shown to rest on conceptual and institutional differences, on differences in the experimental and field practices of geneticists and embryologists and on the distinctive behaviors of the biological materials they traditionally employed.' Suffice to say that the disagreements were based in part on the absolute inability of geneticists to show how genes could account for the Bauplan of an organism and on their failure to give any weight to such phenomena as cytoplasmic gradients in the egg, polarities in the egg and the early embryo, cell death in organogenesis, and so on.³ These complaints against genetics were just and remain so, although that fact does not reveal whether they are biologically or philosophically important.