Timing mechanisms in early embryonic development.

Embryological development takes place in four dimensions and requires the existence of time measuring processes within the embryo. Evidence is accumulating that suggests that the emergence of many events during early embryonic development is controlled by timing mechanisms or developmental clocks. The purpose of this work is to review recent studies on developmental timing with speculations about underlying possible mechanisms. It is an attractive idea that the development of an embryo is timed by a single clock set in motion at fertilization, but this seems not feasible. The clock mechanism which determines the time of initiation of cellular differentiation may be independent of that for the timing of morphogenesis. The clock mechanism for cellular differentiation may be closely associated with the cycles of DNA replication, while the clock which counts the time to onset of early morphogenetic events is found in the cytoplasm. These ideas can provide a framework which may help to organize existing observations and to stimulate new experimental approaches to the problem.