1. Critical size analysis of the lymphoid cell populations of the developing chick bursa of Fabricius and thymus indicates that characteristic shifts in cell size distribution, modal cell volume and mean cell volume occur during morphogenesis of these organs.
2. Comparison of bursa and thymus shows that while there are clear differences between the lymphoid cells of these two structures, the pattern of morphogenetic change is similar.
3. 19-nortestosterone injected in low doses can partially inhibit bursal development without affecting thymus morphogenesis. Depending on the age of the embryo at time of injection, however, doses sufficient to produce complete bursal inhibition may also retard thymic morphogenesis.
4. The size differences found between bursal and thymic lymphocytes in the chicken parallel the size differences found between lymph node and thymic lymphocytes in the mouse.