 In the beginning of the twentieth century, at first Thomas Hunt Morgan resisted the Mendelian-chromosome theory of inheritance that saw inherited units of heredity carried on chromosomes as determinants of developed characters. If all the cells contain the same chromosomes, then how can their inheritance explain anything, he asked. Instead, he insisted that “We have two factors determining characters: heredity and the modification during development” (Morgan 1910a: 477). Morgan wrote to a friend that “my field is experimental embryology” and not the genetics with which he became associated (Morgan 1908). Like his Woods Hole colleagues at the Marine Biological Laboratory, Morgan did not see how inherited chromosomes could explain development of form from non-form. He had rejected Weismann’s interpretations and continued to reject the idea of inherited determinants.