THE UNIQUENESS OF THE INDIVIDUAL 



they were identical twins. (The reason for this pawky reserva- 

 tion will be explained later.) As to Pierre and Victor, the skin 

 grafting test proved nothing except that they were not 

 identical twins; but as blood grouping had shown that Pierre 

 had certainly been allotted to the wrong mother, Eric'*s mother 

 at last accepted him as her own. 



Just as skin grafts can be exchanged between identical 

 twins, so also can they be exchanged between mice and guinea- 

 pigs which, by having been inbred brother to sister for upwards 

 of twenty successive generations, have come to resemble each 

 other in hereditary make-up almost as closely as if they were 

 identical twins. (This is not necessarily true for all animals, for 

 some have mechanisms which prevent their achieving a very 

 high degree of genetic uniformity.) But even here there is a 

 curious exception of some theoretical importance^: in some 

 inbred strains of mice, females will not permanently accept 

 skin homografts from males. It is now pretty well certain that 

 this is because agents of the type that cause the breakdown 

 of homografts are present on, or caused to be formed by, the 

 Y-chromosome, that which is peculiar to males. There is 

 accordingly no reason why females should not accept grafts 

 from other females, nor males from females or other males, 

 as indeed they do. 



When members of two different inbred lines of mice are 

 crossed (supposing them already to have achieved and retained 

 a sufficiently high degree of genetic uniformity), their hybrid 

 progeny, forming the so-called F^ generation, are also uniform, 

 and will accept grafts from one another. They will also accept 

 grafts from their parents and, mutations apart, from their own 

 progeny of the first or any subsequent generation. Excepting 

 only the special case of grafts transplanted from males to 

 females, F^ hybrids are therefore the ""universal recipients'* of a 

 little microcosm of mice comprising the inbred parental strains 



1 E. J.Eichwald and C.R.Silmser, Transplantation Bull. ,2, p. 148, 1955. 



148 



