THE bursa of Fabricius of birds has an essential role as a central lymphoid organ for the differentiation of B lymphocytes1,2. In addition, the bursa harbours immunocompetent B cells3 which are capable of local antibody production. Haemolytic plaque-forming cells have been observed in the bursa after introducing sheep red blood cells (SRBC) into the bursal duct4. Furthermore, epithelial cells of the bursal follicles have been shown to transport ferritin and India ink into the intercellular spaces of the underlying lymphoid tissue when the material has been introduced into the bursal lumen5. The bursal lumen opens through the bursal duct to the most caudal of the three cloacal chambers, the proctodeum, which is of ectodermal origin6,7. The proctodeum leads to the outside through the anus and is closed externally by a strong sphincter muscle. The vent is surrounded on the outside by upper and lower anal lips. At the other end, the proctodeum opens into urodeum leading to the coprodeum and these form the cranial parts of the cloaca (Fig. 1a).