Towards a theory of development.

A theory of development would effectively enable one to compute the adult organism from the genetic information in the egg. The problem may be approached by viewing the egg as containing a program for development, and considering the logical nature of the program by treating cells as automata and ignoring the details of molecular mechanisms. It is suggested that development is essentially a simple process, the cells having a limited repertoire of overt activities and interacting with each other by means of simple signals, and that general principles may be discerned. The complexity lies in the specification of the internal state which may be described in terms of a gene-switching network. Pattern formation is a central feature in development; it is the process whereby states are assigned to the cells according to their position. such that the appropriate type of cytodifferentiation is selected from the repertoire. The morphogenesis of the chick limb is briefly discussed. Genetic networks that account for such features as memory, competence and interpretation of positional information are given. The question of how these component parts are organized into a complete control system for development is posed as a problem for future study.