THE UNIQUENESS OF THE INDIVIDUAL 



condition was clear enough, for most litter-mates in cattle are 

 of the kind that share a common blood circulation from a very 

 early stage of embryonic life until birth. All blood cells, there- 

 fore, and all cells which may circulate in the blood stream on 

 their way elsewhere, can be exchanged between the twins 

 before they are born. But what is specially interesting is that 

 the state of chimerism, of red-cell intermixture, may last for 

 years or perhaps for Life — certainly for much longer than the 

 lifetime of a red blood corpuscle, which is not likely to exceed 

 a hundred days. It follows, then, that not merely red cells but 

 the cells which make red cells must have been exchanged in the 

 foetal cross-transfusion; and that, in defiance of the principles 

 formulated in this article, they survived when the animals grew 

 up and continued to manufacture the red corpuscles charac- 

 teristic of their original owners. 



It is for exactly the same reason that twan chickens are red- 

 cell chimeras, for the blood systems of the two separate 

 embryos within the single shell communicate directly with each 

 other. Moreover, my colleagues and I have shown, that the 

 state of chimerism can be brought about artificially by joining 

 two eggs together across a vascular bridge, using the method 

 first devised by the Czechoslovak scientist Milan Hasek. 

 Natural chimerism has been described in a small minority of 

 twin sheep, and in one human being, a Mrs McK, who was not 

 known to be twin when, at the age of twenty-five, she was found 

 to contain a mixture of two genetically different types of red 

 blood corpuscle.^ Her oa\ti Avere of blood O, but they were 

 mixed with corpuscles of group A, the cellular relicts of a 

 male twin who had died when three months old. There is 

 no knowing how long Mrs McK will remain a chimera, 

 but she has now been so for twenty-eight years; probably, in 

 the long run, her twin brother'^s red blood cells will slowly dis- 



1 I. Dunsford, C. C. Bowley, A. M. Hutchison, J. S. Thompson, R. 

 Sanger and R. R. Race, British Medical Journal j 2, p. 81, 1953. 



151 



