Drosophila melanogaster fruit flies in the laboratory. She worked with these as subjects of her research for some years before Morgan adopted them as his model organism. Sex determination Although Stevens and Wilson both worked on chromosomal sex determination, many authors have credited Wilson alone for the discovery. Additionally, Thomas Hunt Morgan has been credited with the discovery of sex chromosomes although at the time of these cytological discoveries, he argued against Wilson's and Stevens' interpretations. Morgan's recognition came in part from his work on sex linkage of the white mutant gene of fruit flies and was especially heightened by his Nobel Prize award in 1933. Stevens was not even recognized immediately after her discovery. For example, Morgan and Wilson were invited to speak at a conference to present their theories on sex determination in 1906 but Stevens was not invited to speak. Following Nettie Stevens' death, Thomas Hunt Morgan wrote an extensive obituary for the journal Science. In that article, Morgan said she had "a share in a discovery of importance.” He continued in the obituary to describe in some detail the implications of this work. However, Morgan also claimed that she had confirmed McClung's hypothesis about sex chromosomes when she actually refuted his main claim that the larger (X) chromosome determined sex. Morgan recognized Wilson's parallel but less complete and convincing studies as producing a "joint discovery" with Stevens. But Wilson said in a later footnote that she made the discovery. Also, Morgan claimed Stevens seemed to "appear at times wanting in that sort of inspiration that utilizes the plain fact of discovery for wider vision.” Apparently, Morgan was forgetting the fact that she was often excluded from scientific dialogue, for example she was not invited to speak at the meetings where he and Wilson expounded the theory of sex chromosomes. In his textbook, The Mechanism of Genetics, published in 1915, he did not credit either Stevens or Wilson with the discovery of sex chromosomes. He described sex linkage of the white'' gene in the chapter immediately before the one in which he described Stevens' results without mentioning her name, implying that his own laboratory's sex linkage analysis was the basis on which one should understand sex determination. In an earlier letter of recommendation he wrote, "Of the graduate students that I have had during the last twelve years I have had no-one that was as capable and