PROTECTION AGAINST RUNTING 295 



Cumulative mortality curves were also established. No 

 systematic pathological study was attempted. 



F. Test for specific acquired tolerance 



Occurrence of specific acquired tolerance was tested by skin 

 grafts performed at the age of 6-8 weeks on the A mice injected 

 at birth with CBA spleen cells (plus enhancing or normal CBA 

 serum). The grafts were full-thickness CBA female skin; they 

 were circular, shaped with scissors, with a diameter of approxi- 

 mately 1 6 mm. and were fixed on a previously prepared bed over 

 the stratum carnosum with interrupted stitches using " Sertix" oo 

 silk suture thread. A piece of saline-moistened hydrophilic cotton 

 was applied and an adequate pressure assured by means of an 

 elastic "Sealtex Latex Bandage". 



Six days later the bandage was removed and by this time the 

 grafts were usually well established and vascularized, the edges had 

 joined together and the stitches were easily removed with the 

 cotton to which they adhered. Grafted animals were placed in 

 separate cages and observed for several months, often throughout 

 the normal life span of the animal. No attempt was made to 

 determine the histological picture of the epidermis at any time 

 since the expected increase in survival time was more than the 

 normal survival time (lo days) observed in unprepared animals 

 of the same strain combination. Such an increase in survival 

 time (21 days) was required before one could speak of tolerance. 

 Partial tolerance was considered as concerning survival time of 

 from 21 days to 100 days. When a graft started to undergo 

 changes before 100 days and was rejected after 100 days, it was 

 considered as a case of partial tolerance. The only animals 

 considered as permanently tolerant were those which, after 100 days, 

 bore healthy grafts with normal hair growth of the same agouti 

 color as the original donors and growing in a direction which 

 differed by 180° from that of the surrounding host fur. It has been 

 our constant experience that grafts which were in that healthy 



