In the past, there has been some controversy as to the composition and function of the pancreas of Aselli (lymph nodelike or equivalent to the avian bursa of Fabricius [35] ). The aim of this study was therefore to clarify this matter with up-to-date methodology. In general, the composition of the pancreas of Aselli was very similar to that of a lymph node. Beneath the capsule were marginal sinuses of variable width, containing disseminated lymphocytes (mostly T and B cells in equal proportions), macrophages and neutrophils, the latter either disseminated or as small accumulations. In four adult shrews, the marginal sinuses exhibited focal to extensive fibrosis. Beneath the sinuses lay a cortex containing exclusively secondary follicles ( Fig. 3A-C) , with the exception of one adult male where both primary and secondary follicles were present (Fig. 4) . Germinal centres generally exhibited a high cellular turnover, with variable but high numbers of PCNA-positive, proliferating cells (often more than 50% of cells; Fig. 3E ) and often numerous apoptotic cells (Fig. 3F ). T cell zones formed a paracortex located immediately beneath the follicles (Fig. 3D ) and were generally similar in size and cell density in animals of all age groups. The organ's centre (medulla) contained loosely arranged sinuses with only low numbers of macrophages. The remainder of the medulla was made up almost entirely of plasma cells (Fig. 3G, H) .