How is the timing of gene expression controlled in developing embryo?
How can gene expression timing affect the formation of a developing embryo?

We should know that genetic imprinting is passed down. The formation of an embryo is not. Some takes on form when provoked, it is much like when oocytes are stimulated with a prod duplication 
Happens automatically. The embryo is affected by the proximity of cells. Clusters, spread out, far apart. All make the cells behave automatically. But it's the first ignition of development that sets off growth. The cells have been through many phases and timing is not controlled by either proteins or genes.