THE UNIQUENESS OF THE INDIVIDUAL 



and their immediate or later issue. It is clear, then, that genetic 

 uniformity of donors and hosts is not necessary for the success- 

 ful use of homografts; what is necessary is that the donors 

 should not possess any substances making for incompatibility 

 which are not also present in the hosts. Mice of the first 

 generation of a cross between the members of two highly inbred 

 strains contain representatives of all the hereditary factors 

 possessed by either parental strain; skin grafted from their 

 parents or their progeny cannot therefore come to them, 

 genetically speaking, as a surprise. All this is of great import- 

 ance experimentally, but it has no bearing at all on practical 

 everyday affairs, for human beings are, genetically, a most 

 diverse assembly and even the most strenuous effort of abstrac- 

 tion cannot liken them to inbred mice. 



The second exception to the rule that skin grafts are sure to 

 perish after transplantation from one individual to another is 

 this: it does not apply to embryos. Embryos will accept not 

 merely homografts but grafts from members of quite different 

 species as well, "heterografts". The age at which an animal 

 becomes competent to recognize foreign tissue as foreign varies 

 from one species to another. In sheep certainly,^ and almost 

 certainly in cattle, the power to react upon and reject homo- 

 grafts has already developed two-thirds of the way through 

 pregnancy; in mice and chickens, the transition from the 

 immature or embryonic to the adult mode of response is 

 marked, to a good enough approximation, by birth itself. 

 These variations are perfectly understandable: if we compare 

 one species with another, it soon becomes clear that birth is a 

 movable feast in the calendar of development, for mice are 

 born at a stage of development not much different from that of 

 a human foetus only three months old, and sheep and cattle 

 are much more mature at birth than man. 



1 P. G. Schinkel and K. A. Ferguson, Australian J. Biol. Sci., 6, p. 533, 

 1953. 



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