98 R. E. BILLINGHAM AND WILLYS K. SILVERS 



a failure to sensitize their hosts adequately. One obvious hypo- 

 thesis, v^hich might account for this, is that the connective tissue 

 of pouch skin in some wslj or another impedes or prevents the 

 escape of foreign transplantation antigens from the grafts into the 

 regional lymphatic drainage system, so that an effective antigenic 

 stimulus fails to impinge upon the host's regional nodes. 



This hypothesis has been tested experimentally in tv^o different 

 ways: (i) Very shallov^ circular depressions, about o* 5-0*7 cm. in 

 diameter, w^ere cut in the centres of relatively large, healthy-look- 

 ing, long-established pouch skin homo- or isografts, leaving 



host skin 



sKin homoaraft 



lonq establislied 

 cheek pouch skin qraft 



Fig. 3. Illustrating the method of transplanting an ear skin homograft 

 within the substance of a long-estabHshed cheek pouch skin graft. 



intact a continuous layer of connective tissue of cheek pouch 

 origin. Small, thin homografts of ear or body skin w^ere then 

 fitted as accurately as possible into the prepared beds (see Fig. 3). 

 Controls were provided by determining the survival times of 

 normal skin homografts of similar size fitted to shallow beds 

 prepared in the skin of the chests of normal hamsters. 



The majority of the skin homografts " inlay ed" into established 

 grafts of pouch skin healed-in very satisfactorily, acquired a good 

 blood supply and regenerated normal hair crops. The survival of 

 most of them was significantly prolonged (Table V; see also 

 Fig. 4). Established pouch skin homografts were just as effective in 

 providing "privileged" sites for the inlayed skin homografts as 



