Phylogenetic studies on the immune response. 3. Autoradiographic studies on the lymphoid system of the Australian echidna Tachyglossus aculeatus.

In the Australian echidna , a close relative of the extinct ancestors of higher mammals, the lymphoid organs were investigated by means of autoradiographic studies after the injection of [H]thymidine and I-labelled flagellar antigen of . The histology of organs from echidnas injected with colloidal carbon was studied in parallel. The results lead to the conclusion that each lymph nodule in the echidna represents a single lymphoid follicle comparable to a cortical follicle in lymph nodes of higher mammals. Studies on lymph nodules of echidnas injected with [H]thymidine revealed the presence of highly active germinal centres, usually one per nodule. Antigen became localized first around the entire nodule; later on it was found within the germinal centre or in its peripheral parts. Often a germinal centre was eccentrically located, in which case the labelled antigen formed the typical germinal centre cap known to be characteristic of secondary follicles in the rat lymph node. A significant uptake of antigen was also seen in the appendix, the Peyer's patches of the gut and in the Hassall's corpuscles of the thymus. Our findings for the echidna lymph nodule are discussed with respect to their possible significance in the evolutionary development of the multi-follicular lymph node of higher mammals.