Abstract Every cell in a developing embryo has the same genotype. The genes, identified as G1, G2.....Gn, are programmed to operate sequentially, G1 acting at time (T1), G2 at T2, and Gn at Tn. These genes will transcribe their message, however, only if they are in the immediate vicinity of the appropriate stimulus. These stimuli, S1, S2 .....Sn, are specific for the genes with the same subscript. If a gene is not “turned on” at the appropriate time, it loses its opportunity, and the ontire system shifts to the next time phase. This theory is discussed, in terms of both embryological data and as a working model for the design of future experiments.