116 O. MUHLBOCK 



their genetic constitution can change only by mutation 

 (Russell, 1941). In mice, it requires at least 20 generations of 

 inbreeding before we can speak of an inbred strain. If a 

 strain is to be maintained with maximum homozygosis, there 

 should be no relaxation of inbreeding. It should be recognized 

 that the strain may break up into many separate lines unless 

 the choice of the breeding pairs is made with reference to a 

 pedigree chart. It is essential that only one central trunk of 

 the inbreeding tree be maintained. Because of mutations, 

 brother-to-sister matings alone, i.e. without limitation to the 

 central trunk, can produce a multitude of sub-lines all of 

 which may differ (Heston, 1945). 



The criteria used in proving that all individuals of a pure 

 strain are genetically identical should first be considered. It 

 need not be said that the general aspect and the morphology 

 of the organs should be identical. Another requirement to be 

 met is that neoplasms developing in an animal of a certain 

 strain should be transplantable to another individual of that 

 strain. The same holds true for tissues and organs which, 

 following transplantation to another individual, continue to 

 grow and to function as they did in the original animal. 



The ovaries are known to be transplantable even in incom- 

 pletely homozygous strains; the skin, however, requires a 

 considerably more marked genetic similarity, as indicated 

 by the experiments of Billingham and Parkes (1955). The 

 most exact criterion used in our laboratory, however, has been 

 found to be a procedure known as parabiosis, i.e. the union of 

 two animals by side-to-side suturing, not only of the skin but 

 also of the thoracic and abdominal muscles and, for fixation, 

 of the shoulders and the iliac bones. Communication of the 

 two circulations is established within a few days. A satis- 

 factory parabiosis with normal lifespan of both partners, 

 however, is established only in completely genetically identical 

 animals. 



Besides the accurate administration of the inbred strains, 

 the care and hygiene of the animals require maximal atten- 

 tion. It is desirable that the environmental factors be kept as 



