Until the middle of the present century, the morphology and function of the thymus were primarily of interest to those working in the fields of pathologic anatomy, endo crinology, and pediatrics. However, during recent decades careful and refined histo logic studies of the organ have expanded our knowledge. It now seems certain that the thymus plays a central role in the immune system, and some of the substances pro duced by this organ are considered together under the collective term of "...