Abstract Hieronymus Fabricius (1621) reported the existence of a sac in the podex of the hen which he believed served as a semen receptable. (Apparently he didn’t notice its presence in cocks). Named the Bursa of Fabricius in his honour, the function of this organ was unknown until the discovery of its role in antibody production by a remarkable piece of scientific serendipity (Glick, 1956). The study of immunogenesis in the chicken has received added stimulus from the recent discovery of a dissociation in immuniological function of the bursa and the thymus. Whereas the bursa appears to control the development of circulating antibody, the thymus is responsible for the development of immune reactions mediated by cellular elements, (Warner et al., 1962; Aspinall et al., 1963; Arvy, 1963; Jankovic and Isvaneski, 1963). In mammals the rejection of dispersed cellular homografts is believed to be largely controlled by circulating antibody (Harris et al.,…