Nobody seemed to be very interested in connecting up development with the genes. There was a complete gap between these. And when people began to think about this and began to think that perhaps genes... how, in what sense could genes be considered the units of development, it became clear that that was a non-question. The units of development are cells. And our job is to ask how do genes get hold of the cells, if I can put it in that way. Because that's the way the genes must go through to the developmental process, and indeed to all the complex phenotypes of organisms. They have to get hold of the construction machinery of these... of the organs in order to express their effects. However, the gradients was very interesting, because what impressed Francis is that he made a quick calculation about the sizes of these things. He didn't know the sizes of imaginal discs, so he quickly looked them up, and he made a calculation which is really what Francis is very good at, as to whether diffusion of an average sized small molecule could account for the results because that would... could only operate at a certain scale. That is, it would be very hard... it would take too long to establish something by diffusion if, let us say, it was 10 feet wide. And you wouldn't want to do this if it was just, let us say, 100Å. But this was in the scale of sizes that... that could be done by diffusion. And so Francis got very interested in asking whether you could account for positional information that Lewis was discussing in terms of a process of diffusion. So one had a very simple model then which was that one had a... a source — that is some cells that created this soon became a diffusible morphogen — and you had a sink at the other end and this molecule distributed itself at a... in the form of a gradient across this. And then the idea was that the cells would read this gradient and decide what it is they were going to be. And so you had a lot of discussions as to whether... of course you then had to have set a threshold in each of the groups of cells and of course you can ask how accurate you can do this.
