ROLE OF THE THYMUS 389 



and there was often a deficiency of plasma cells in many of the 

 lymphoid tissues examined. 



Autopsies were perform-ed on neonatally thymectomized mice 

 that died as "runts" between 2 and 4 months of age. The involu- 

 tion of lymphoid tissue in these mice was extreme. The lymph 

 nodes were minute and showed no proper structure and no plasma 

 cells. Sometimes they were reduced to a small piece of adipose 

 tissue containing only a few lymphoblasts. The spleens were 

 markedly shrunken and the follicular structure had disappeared. 

 There were few or no lymphocytes and no plasma cells. The 

 Peyer's patches were poorly developed, inactive and deficient in 

 lymphocytes. 



Survival of allogeneic skin grafts 



The survival of allogeneic skin grafts in mice thymectomized at 

 birth, at 4 to 7 days, at 3 weeks, in mice sham-thymectomized at 

 birth and in entirely normal mice is shown in Tables II and III. 

 It can be seen that thymectomy in the neonatal period was 

 associated with prolonged survival of skin grafts not only from 

 donors which differed at the H- 2 locus (Fig. 8) but also from donors 

 of a different species (Fig. 9). These grafts all grew normal tufts 

 of hair. Rejection of grafts, when it occurred, was characterized 

 by gradual diminution in size of the graft and progressive thinning 

 of the hair until the graft eventually disappeared. There were no 

 signs such as thickening, reddening, oedematous swelling, bleed- 

 ing or scab formation. 



None of the mice that rejected skin grafts by 25 days and 

 only a few of those that rejected grafts of mouse origin by 

 60 days died as runts. All the mice that carried rat skin for longer 

 than 25 days became runted and died with the graft intact. 

 The majority of the deaths from runting occurred in the group 

 of mice that were tolerant of foreign grafts for more than 60 

 days. 



