I06 R. E, BILLINGHAM AND WILLYS K. SILVERS 



cornea is certainly an immunologically privileged site, so long as it 

 remains unvascularized (Maumenee, 195 1; Billingham and 

 Boswell, 1953). 



The fact that healthy-looking pouch skin homografts of well 

 beyond 100 days' standing showed no evidence of loss of suscep- 

 tibility to subsequent sensitization of their hosts with normal 

 homologous skin or leucocytes lends no support to a hypothesis 

 advanced by Woodruff and Woodruff (1950) (see alsoWoodruff, 

 1959), according to which homografts once estabhshed might 

 eventually pass a "critical period", becoming capable of surviving 

 an immunological resistance on the part of their host. 



So far, attempts to discover other connective tissues having 

 biological properties resembling those of the connective tissue of 

 the hamster's cheek pouch have been unsuccessful. Nevertheless, 

 one cannot entirely dismiss the possibility that principles similar 

 to that responsible for the anomalous survival of pouch skin 

 homografts in this species may be important in some naturally 

 occurring situations in which animals are chronically confronted 

 with potential antigens against which they do not normally react. 

 These include the potential auto-antigenicity of certain ingredients 

 of some organs of the body such as the adrenal, thyroid, testis, and 

 the brain, and the potential isoantigenicity of the trophoblast 

 which establishes such intimate contact with the maternal tissue 

 in the placenta. 



Summary 



In the course of a study of the basis of the immunologically 

 privileged environment afforded to tissue grafts of foreign origin 

 by the cheek pouches of Syrian hamsters, an investigation has 

 been made of the fate of homografts of cheek pouch skin trans- 

 planted to recipient areas prepared in normal skin. It has been 

 found that the majority of these grafts long outlive homografts of 

 normal skin, and some survive indefinitely. Nevertheless, well- 

 established, healthy pouch skin homografts have been found to be_ 



