Since the interest on the thymus as an organ essential for the development of immunological capacity was renewed more than ten years ago, the search for a better understanding of the mecha­ nisms underlying its functions has not come to an end. Although the first observations suggesting a link between the thymus and immunocompetence related to certain clinical disorders in man such as thymoma or Swiss type of agammaglobulinemia, the bulk of evidence demonstrating the role of this lymphoepithelial organ in immune reactivity originates from experimental work as ini­ tiated by Dr. MILLER and Dr. Goon's group. The full significance of the thymic system is now beginning to unfold. The basic question to be answered by the many investigators in the field is no longer whether the thymus does in fact play the role of an immunologically important organ but rather how it acts. In addition we need to know in what period(s) of ontogenesis is a proper development of the thymus a prerequisit for the buildup of an efficient peripheral lymphoreticular system throughout the organism. Perinatal thymectomy proved to be a valuable experimental approach to gain a better insight into thymic functions. At first it looked as if this measure, in most of the species tested, would largely depress or abolish all immune reactions of the animal and lead to an inescapable wasting syndrome within a few months.

1. Introduction.- 2. Phylogenetic and Ontogenetic Considerations.- 2.1. Phylogeny of Acquired Immunity.- 2.2. Ontogenetic Development of the Lymphoreticular Tissue and of Acquired Immunity.- 3. Growth Characteristics and Cellular Kinetics of the Thymus.- 3.1. Growth Characteristics of the Thymus.- 3.2. Proliferation of Thymic Lymphoid Cells.- 3.3. Intrathymic Fate of Newly Produced Lymphoid Cells.- 4. The Effects of Thymectomy on Antibody Formation.- 4.1. Neonatally Thymectomized Animals.- 4.2. Animals Thymectomized in Adult Life.- 4.3. Thymectomy and Acquired Immunological Tolerance.- 5. Additional Examples of Acquired Immunity in Thymectomized Animals.- 5.1. Hypersensitivity Reactions.- 5.2. Transplantation Immunity.- 5.3. Graft-versus-host Reactions.- 6. Thymectomy and Bursectomy in Birds.- 7. Post-thymectomy Wasting.- 8. Morphological and Functional Lymphoid Reconstitution of Thymectomized Animals.- 8.1. Thymus Implants.- 8.2. Injection of Lymphoid Cells.- 8.3. Diffusion Chamber Implants Containing Lymphoid Cells.- 8.4. Summary.- 9. The Role of the Thymus in Immunologic Deficiency Disorders in Man.- 10. Conclusions.- Literature.