Abstract
The normal ontogeny of the bursa of Fabricius is necessary for the successful genesis of antibody-mediated immunity, while the avian thymus appears to control cell-mediated immunity. Regression-regeneration experiments suggest that the thymic-dependent system is better buffered than the bursal-dependent system. A mammalian equivalent of the bursa may exist. Clarification of the mechanism of bursa action is necessary to understand the control of immunoglobulin or antibody production.