Figure 1. Key Figure. The coordination of early embryonic cell cycle changes with development.

As illustrated schematically, the cell cycle acquires gap phases in two distinct steps as embryogenesis progresses. Separate introduction of the gap phases apparently follows from the fact that the programs controlling G1 and G2 are different, and each requires distinct regulatory modifications. The central bar is a linear representation of the progression of cell cycle phases through the successive cycles of early development. The associated labels and images illustrate the coordination of the cell cycle timeline with developmental events as described in detail in box 1 and summarized briefly in the text.