A. TYLER 351 



1943), even when one of the animals has been previously immu- 

 nized against tissues of the other (Grobstein and Youngner, 

 1949). 



As far as phagocytic ability is concerned, tests have shown that 

 this is present in the cells of all three germ layers of early ( 2-day ) 

 chick embryos and becomes progressively delimited to the famil- 

 iar sites of the mature organism (Heine, 1936; Steinmiiller, 1937). 



Not only do homoplastic and heteroplastic grafts succeed well 

 in embryos but the foreign cells may also persist well into the 

 adult, as illustrated in the experiments of Willier and Rawles 

 (1940, 1944) on neural crest tissue. The same sort of persistence 

 of foreign cells is evident in the obsei-vations of Owen ( 1945 ) on 

 erythrocyte mosaicism of cattle twins, which can be attributed to 

 a transfer of erythropoietic tissue from one twin to the other 

 through the conjoined placentas, a phenomenon also recently re- 

 ported to occur in human twins (Dunsford et al., 1953). Such 

 persistence implies that the adult has become incapable of form- 

 ing antibodies, or a destiiictive amount of antibody, against the 

 foreign antigen introduced in the embryonic stage. 



The recent remarkable experiments of Billingham, Brent, and 

 Medawar (1953, 1955a,b, 1956) have now demonstrated such 

 failure to react immunologically against specific antigens on 

 the part of adult animals that had been injected with the anti- 

 gen during fetal life. In particular they showed that adult mice 

 of CBA strain that had been injected in utero with tissue 

 (chopped-up adult testis, kidney, and spleen) of A strain can 

 tolerate A strain skin grafts, whereas without such injection the 

 typical incompatibility reaction would ensue. In chickens they 

 accomplished the same thing by transfusing blood between em- 

 bryos of different strains. They termed this phenomenon "actively 

 acquired tolerance" to skin grafts, but it is quite clear from the 

 earlier analyses of the incompatibility reaction that this relates to 

 specific antibody-forming ability. They showed also that the adult 

 lacks only ability to respond to the specific antigens that it re- 

 ceived in fetal life, its ability to form antibodies against other 

 antigens being unimpaired. Furthermore, if lymph nodes from a 

 normal animal are transplanted to a tolerant animal, the latter 



