The search for a thymic hormone

in chronic inflammatory states bur not of ad~a.ptive immune responses. The more primitive types of rays and sharks have a definitive thymus, spleen and lymphocytes and are capable o'f immune responses to hom(~grafts of tissue and to antigenic stimuli. In recent years, h'owever, most experimental studies have been carried ou't in mammals an,d birds. In mammals the organ is usually ,bi-l~bed and lies in the lo,wer cervical region and/or within the inlet of the thorax, while in Jbirds the thymus consists of a series of lobes along the cervical region and extending into the superior mediastinum. The thymus consists of a well-marked cortex ,and medulla, but differs from 'o+her lymphoid organs no~ only in its peried of maximum development in youth, but also in the presence of epithelial reticular cells scattered throughout the organ, but which are more numerous in the medulla, ~hile .in the cortex the large numbers of densely pa,eked lymphoeytes are the more prominent feature. The organ was originally named " .thymus " on a~count o$ its supposed resemblance to the leaf of .~'he thyme plan~. Galen (A.D. 129-199) described the lhyr0us and considered it c~u.l,d serve as a mechanical buffer between the great veasels and the c'hest wall. Sir Astley Cooper (1832) des:cribed cavities and a duct system within the thymus in calves, goats, rabbits and man. This author ,considered the thymus yeas a g]anMore