The embryological origins of the gene theory

The original support for the gene theory of inheritance came largely from the studies of E. B. Wilson, Theodor Boveri, and Thomas Hunt Morgan. Each of these scientists began his career as an embryologist. In this paper, the emergence of the gene theory out of embryology will be seen in the context of these researchers' attempts to solve the problem of which cellular compartmentthe nucleus or the cytoplasmdirected development. Crucial to this transition from embryology to genetics was the discovery of the sex chromosome, a nuclear structure believed to direct sexual development. We shall see that the constant questioning and retesting of the chromosomal theory of sex determination inadvertently formed the basis for Morgan's proof that the genetic factors were physically located on the individual chromosomes. Finally, the research into the chromosomal models of inheritance displays many examples of how the adherence of scientists to older ideas causes them to interpret new data so as not to conflict with previously held assumptions. This conservative tendency is seen the case of McClung, who insisted on the environmental determination of sex even though he had discovered the mechanism for its intrinsic determination, and especially in T. H. Morgan's ten-year refusal to espouse the Mendelian genetics which he would later champion. Morgan's refusal to accept the Sutton-Boveri synthesis of Mendelism and cytology becomes a chief concern in this essay. His arguments against this view are seen to arise from his previous embryological experiences, which convinced him that chemical reactions in the cytoplasm were responsible for development, rather than morphological changes within the nucleus. This view contrasts with other analyses, which relate Morgan's refusal to his "empirical" attitude, his dislike for theorizing, or to his "romantic" temperament. Wilson's rapid acceptance of the Sutton-Boveri hypothesis is seen to result from his prior conviction that the nucleus controlled development. The reasons for these differences between Morgan and Wilson are traced back to two of Wilson's embryological beliefs which were not