The mechanisms of Darwinian theory applied at the cellular level can explain the embryogenesis of an organism. On the one hand, DNA is not the bearer or carrier of a program composed of rigid instructions, in which the adult organism is « written », in advance. It is a generator of diversity that functions in a probabilistic fashion and thus enables cells to change states without being guided by signals. On the other hand, the environment is not only that which (...) is external to the organism. It exists within the embryo inasmuch as it constitutes the micro-environment of the cell. It determines the concentration of metabolites to which the cell has access and which enable it to multiply. In a colony of cells there are numerous variations in the composition of this microenvironment. Cells exchange the products of their metabolism. They use what their neighbours provide them. By means of their probabilistic functioning, they adapt to changing life conditions by activating the necessary proteins, whence the cellular differentiation generating the tissue of which an adult being is composed. In this theory, the embryo is not the passive object subject to the cumulative determinisms of the genome and the environment. These are integrated into a single mechanism. The cell is the fundamental level which constructs the living being by an active process of Darwinian selection. By bringing together phylogeness and ontogenesis in one explanatory model, cellular Darwinism allows us to get rid of the old concepts of purposiveness and formal cause which genetics has rehabilitated in the new clothing of the « generic program ». (shrink)