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.