 Early twentieth-century embryology highlighted epigenesis. But a new twist on preformationism soon arose in the form of genetics. This pointed to the nuclear chromosomes as loci for the causes of differentiation. Yet unlike Weismann and Roux, new geneticists did not see the genetic material as divided up into a mosaic to explain ensuing cell and tissue differences. Rather, the inherited nuclear material was the same in every cell, but it acted differently according to an internal program. This interpretation appealed to some scientists for metaphysical reasons since it focused on the material units of heredity and apparently of causation. Epistemologically, it was more difficult to point to evidence that inherited genes explain development.