A Path to Pattern.

The field of developmental biology is not the same one that I entered in 1975. At that time, it seemed that most of its practitioners used various kinds of microscopes to watch animals as they matured, described morphological details with impressive temporal and spatial resolution, and recorded responses to physical and genetic insults. The number of genes whose mutant phenotypes offered insights into developmental mechanisms was small, the expression and functionalities of these genes were unknown, and because the extent of evolutionary conservation between different animals or even different organs in the same animal was also unknown, the vocabularies that were used to describe development were unique to each system. The distance between the descriptors and inferred molecular mechanisms was vast; it was a descriptive discipline. Today genome sequences are available for the animals that developmental biologists study, saturation genetic screens are possible, transgenesis offers powerful ways to modify genomes, and the proteins that direct and implement developmental processes can be imaged in real time. These advances have transformed the field into one that merges with cell biology, physiology, neurobiology, and immunology, and they have transformed our understanding of development. In this essay, I offer my perspectives and my sense of some principles that have emerged.