ROLE OF THE THYMUS 395 



thoracic duct and lymph nodes (Schooley and Kelly, 1958). It 

 could be argued from these results that the lymph nodes and spleen 

 are, therefore, important destinations for thymus lymphocytes. 

 On the other hand, these effects of thymectomy might be 

 related to the production by the thymus of a lymphocytosis- 

 stimulating factor (Metcalf, 1958) which may to some extent 

 control lymphopoiesis in lymphoid organs. 



Transfused thymus cells have been traced to the red pulp of the 

 spleen, particularly perifollicularly, at the site where plasma cell 

 proliferation is known to take place during antibody formation 

 (Fichtelius, i960). This led to the suggestion that the thymus and 

 the spleen may together constitute a large type of central lymph 

 node, the thymus producing lymphocytes and the spleen forming 

 antibodies with their aid. By using appropriate chromosome 

 markers, one might be able to determine whether thymus lym- 

 phocytes do indeed settle and multiply in the spleen of neonatally 

 thymectomized mice subsequently grafted with thymus tissue. 



Experiments on the transfer of immunological capacity might 

 help to test the theory that immunologically competent cells are 

 removed by neonatal thymectomy. If such cells are absent one 

 might expect that the capacity for an immunological response 

 could not be transferred to appropriate hosts by means of lym- 

 phoid cells obtained from tissues of neonatally thymectomized 

 donors, and that such lymphoid cells would not be capable of 

 initiating a graft-versus-host syndrome in neonatal hosts, Fj 

 hybrid hosts or lethally irradiated adult mice. If, on the other 

 hand, transfer of immunological capacity proved to be possible 

 one would have to envisage the possibihty that cells capable of 

 immunological reaction are present in neonatally thymectomized 

 mice but require some non-cellular factor from the thymus in 

 order to acquire full immunological competence. Experiments 

 along these lines would seem to be rewarding. 



The question of a non-cellular factor from the thymus has 

 interested leukaemia students and endocrinologists for many years 



