In the previous chapter we considered the forces leading to subdivision of the fertilized egg and the large-scale morphological consequences of successive cleavages. We followed the processes leading to the generation of the blastula, an important developmental step. In our treatment, however, the blastula was represented as a mass of identical cells. In reality, at the blastula stage not all cells of the embryo are identical: the “totipotent” zygote (with the potential to give rise to any cell type) first generates cells that are “pluripotent”– capable of giving rise to only a limited range of cell types. These cells, in turn, diversify into cells with progressively limited potency, ultimately generating all the (generally unipotent) specialized cells of the body. At the same time, in the life of each dividing cell, regardless of its level of developmental potency, there are transitions from state to state as the cell performs various functions of the cell cycle.

It is the purpose of this chapter to provide a framework for understanding how genetically identical cells can change their physical states in reliable and stable ways in time and space. We will also explore how these states, in turn, can provide a physical basis for a cell's performance of different tasks at different phases of its life cycle and for its descendants' performance of different specialized functions in different parts of the developing and adult organism.

The transition from wider to narrower developmental potency is referred to as determination.