 At first the factory metaphor simply identified the cell as the place in which chemical reactions occur, but over time researchers began to focus on the different machines within the factory. The focus on separate machines in the factory was promoted in the late nineteenth and early twentieth century when, using improved microscopes and applying stains to enhance contrast, investigators began to identify structures within cells and theorize about their functional significance. Researchers were particularly successful in characterizing the nuclear events in cell division and fertilization. Flemming (1879, 1882) described in detail how the threads that he called chromatin (due to their absorption of dye), later named chromosomes, divided longitudinally, with the two halves moving apart so that one of each would end up in each daughter cell  (Figure 3).  Soon after researchers such as Weismann and Correns pointed to links between chromosome transmission and heredity, but it was Boveri (1902) and Sutton (1903) who provided the compelling evidence that Mendel’s factors (what would soon be called genes) are in or on chromosomes. Darden and Maull (1977) analyze the linking of genes to chromosomes as a major example of what they termed interfield theories, theories that do not try to reduce one account to another but integrate the findings of different fields in a theory that bridges them.