The top of the diagram illustrates the terminally differentiated sperm and egg (oöcyte). These gametes fuse to form the fertilized egg or zygote and begin preimplantation reprogramming (emphasized by the bracket at the left). Following the steps counter-clockwise around the figure sequential cell divisions are illustrated with corresponding differentiation from totipotent through pluripotent to differentiated. The right half of the figure represents gametogenic reprogramming (emphasized by the bracket at the right), the epigenetic marking of the primordial germ cells that will become the sperm or eggs of this new individual at sexual maturity and setting up another cycle.
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