 In contrast to Weismann’s preformationism, Hertwig pointed to the interactions of cells and to the differences among cells for the source of differentiation. Complexity is not built in from the beginning, but emerges over time, dynamically, and interactively. A cytologist himself, Hertwig saw the intricate structures already part of the unfertilized egg, and the changes that occur with fertilization. The egg is not a completely unstructured blob, but rather a complex of different materials that can respond to influences both within the egg and from the external environment. Cells behave like small organisms, and it is the interactions of these separate organisms that makes the whole. As Hertwig put it: