THE UNIQUENESS OF THE INDIVIDUAL 



about the classification of the twins as dizygotic, for the pairs 

 we chose for our critical tests were of unlike sex and as different 

 as possible in other ways. Dizygotic twin calves were therefore 

 tolerant of homografts of each other's skin — though not, it 

 should be observed, of the skin of any other cattle. Obviously 

 the cross- transfusion in foetal life which led to their becoming 

 red-cell chimeras had destroyed their power to recognize each 

 other*'s skin as foreign. Simonsen later reported the successful 

 exchange of whole kidneys between dizygotic cattle, so it is 

 likely, as more recent experimental work has shown to be true 

 of mice and other laboratory animals, that dizygotic twin cattle 

 are tolerant of homografts of aZZ tissues from their twins, though 

 of no tissue from any other animal. 



Billingham, Brent and I therefore set ourselves to reproduce 

 experimentally, in laboratory animals, the state of affairs that 

 occurs by a felicitous natural accident in twin cattle and in 

 those other natural chimeras mentioned in Section 2; and after 

 a year's labour we succeeded. ^ A typical experiment was con- 

 ducted thus. The seventeen-day old embryos of white mice of 

 strain A were injected, while still i7i utero, with a mixture of 

 cells taken from an adult donor belonging to the quite different, 

 brown, strain of mice, strain CBA. The injected mice were born 

 a few days after their injection, and allowed to grow up. A 

 normal adult mouse of strain A rejects skin homografts from 

 CBA mice within eleven days of their transplantation, but adult 

 mice which had been injected before birth with CBA cells were, 

 in the extreme case, completely tolerant of grafts transplanted 

 from CBA donors. Graft hybrids could therefore be made at 

 will. Since these first experiments were done, the technique has 

 been greatlv simplified — with some strains of mice, for example, 

 the preparatory injection can be delayed until immediately 



1 A full account of this work is contained in R. E. Billingham, L. Brent 

 and P. B. Medawar's monograph in Philos. Trans, Roy. Soc. B, 239, 

 p. 857, 1956. 



M 177 



