SANJAY NIGAM, a novelist, once told a reporter that “structuring a novel is a set of decisions about what to pursue and not to pursue.” He might equally well have been discussing his latest research project. For Dr Nigam is also a physician and, in collaboration with Robert Stuart and Kevin Bush, two of his colleagues at the University of California, San Diego, he has begun to reveal the paths pursued and not pursued by developing cells as they turn themselves into bits of organs.