388 J. F. A. p. MILLER 



loss of weight, lethargy, ruffled fur, hunched posture, diarrhoea 

 and death within two to three weeks. 



Peripheral blood 



Absolute and differential leucocyte counts were performed on 

 tail blood at 6 weeks of age in healthy mice that had been thymec- 

 tomized or sham-thymectomized at birth. The total white 

 cell count in the thymectomized group was half that in the sham- 

 thymectomized controls, the decrease being entirely due to a 

 lymphopenia (Table I). There was no significant difference 



Table I 

 Peripheral blood leucocyte levels of six-week-old (Ak x T6)Fi mice 



Total leucocytes per mm.^ 

 Lymphocytes per mm.^ 

 Neutrophils per mm.^ 

 Eosinophils and monocytes 

 per mm.^ 



in the levels of the polymorphonuclear neutrophils, eosinophils 

 and monocytes. 



Histology of lymphoid tissue 



Neonatally thymectomized, healthy-looking, 6-week-old mice 

 were killed and their tissues examined histologically. All these 

 mice showed one striking anatomical abnormality : involution of 

 the lymphoid tissues. The spleen was greatly reduced in size and 

 the average spleen weights were fifty per cent below those of 

 sham-thymectomized control mice of the same age. Histological 

 examination revealed ill-defmed, inactive follicles (Figs. 2 and 3) 

 showing little basophilia and few mitoses. The lymph nodes were 

 also considerably diminished in size and showed inactive follicles 

 with poor cellularity (Figs. 4-7). There were no germinal centres 



