394 J- F- A. p. MILLER 



life and are particularly susceptible to functional elimination 

 whilst in their immature state. But how can the existence of such 

 a barrier be reconciled with the suggestion made above that 

 injected antigenic material acts as a tolerance-conferring stimulus 

 by ehminating in some way certain cell types originating in the 

 thymus ? Experimental results, however, do not conflict with this 

 theory as the existence of a thymus barrier has only been demon- 

 strated in the adult animal, and experiments by BiUingham and 

 Brent (1959) have shown that the thymuses of mice rendered 

 tolerant by neonatal injection of foreign cells all contained a 

 demonstrable population of these cells. 



The next point one would like to investigate is whether different 

 immunological faculties are maturing at different times or at 

 different rates. Recent data obtained in our laboratory suggest 

 that the earher in hfe a thymectomy is performed the less com- 

 petent the mouse becomes to reject skin grafts of increasingly 

 greater "foreignness". Furthermore, when the capacity to reject 

 foreign skin grafts is restored to neonatally thymectomized mice, 

 possibly as a result of thymus grafting, skin from donors that are 

 genetically least related to the host is rejected first while skin from 

 donors that are genetically most closely related to the host comes 

 off last. 



There is much to suggest, but no real direct evidence to prove, 

 that lymphocytes do actually leave the thymus. For instance, 

 mitotic counts are 4 to 6 times higher in the thymus than in other 

 lymphoid organs and DNA turnover is 2 to 5 times as active in 

 the thymus as it is in lymph nodes (Bierring, i960). Under normal 

 conditions, cell death in the thymus is slight, amounting to only 

 about 15 per cent of total cell production (Kindred, 1942). 

 Thymectomy in the adult animal is generally associated with 

 some diminution in the lymphocyte population of the thoracic 

 duct lymph, peripheral blood, lymph nodes and spleen (Bierring, 

 i960; Metcalf, i960). Thymectomy at a very early age is accom- 

 panied by more marked decrease in the lymphocyte content of the 



