The thymus has been an enigmatic organ for centuries. Some of the ancient Greeks considered it to be the seat of the soul. Galen, on the other hand, thought that it acted as a cushion to protect the great vessels from the bony sternum. Since then, contentions and controversies have involved most of the questions regarding its embryology, anatomy, physiology, and pathology. Indeed," has it yet fallen to the lot of any writer upon the thymus to write the truth and be believed?"(Beard, 1902).

Development and Structure The thymus in mammals arises as paired structures from the endoderm of the third and fourth branchial clefts. Its epithelial character is plainly evident until the end of the second month of intrauterine life in the human. At that time the small thymic cells begin to appear. There has been much controversy concerning the manner in which the embryonic epithelial thymusstructure comes to assume its lymphoid appearance. Some, following Hammar (1908) and Maximow (1909), contended that the small thymic cells were of mesodermal origin, having been derived from mesenchymal cells which had migratedinto the epithelial rudiment of the thymus. Others, following Maurer (1885) and St6hr (1906), maintained that the thymic round cells arose directly from the reticular epitheliumand believed also that they had the potentiality of reversion to their original epithelial form, a view also adopted by Burnet and MacKay (1962). The controversy about theorigin of thymic lymphocytes appears to have been settled by Auerbach (1961), who used elegant tissue-culture techniques to analyse the morphogenetic events-in thymus develop-ment. Thymic rudiments obtained from 12-day mouse embryos were separated into epithelial and mesenchymal components by the use of trypsin. It was shown that the epithelial component of the rudiment was capable of forming the lymphocytes of the thymus, the mesenchyme providing the initial inductive stimulus and serving to furnish the stromal elements of the gland. Neither host migrating cells, nor thymic mesoderm, nor generalized mesenchyme appeared to contribute significantly to the initial lymphoidal cell population of the thymus. In size the thymus increases more or less in step with the general growth untilabout puberty, whenit slowly begins to atrophy. Its phase of most rapid growth therefore occurs in early life.