As machines and systems become more complex, conventional feedforward methods of design and construction become inadequate. Future engineering of complex systems that must remain robust and autonomous without human intervention require methods and materials for intrinsic self-construction and self-repair that make use of environmental feedback. In a first step towards such methods we demonstrate how a simple multicellular organism with Braitenberg-like behavior can assemble itself by replication from a single cell. Each cell contains the same small set of chemical reactors that produce and consume chemicals as a function of their local chemical environment, and a small set of modular competences. These sets, and the conditions under which they are activated are encoded in a single gene-like specification replicated into each cell. Overall, this system is roughly analogous to the transciption / translation mechanisms of biological cells. We show how the interplay between the gradually increasing complexity of the environment produced by the organization of the successively differentiating cells gives rise to the serial unfolding of the final functional organism that is able to detect and follow a trace of food, while retaining the ability to repair itself after significant damage.