DIFFERENTIALS OF CLOSELY INBRED ANIMALS 85 



came from generations 91 and 92, and a larger number from generations 102 

 to 106. Somewhere between generations 92 and 102, family B died out and 

 from then on only strain A and hybrids between strains A and B were still 

 available. There was therefore a wide range of inbred generations, extending 

 from the 36th to the 106th, and a time span of about 15 years in the progressive 

 inbreeding of the rats, which were used in these experiments. 



Series I (Table III). The grades in transplantations in different litters in 

 inbred strains are slightly better than the average grades in homoiogenous 

 transplantations in non-inbred rats and the grades are somewhat higher in 

 strain B than in strain A. The combined grade of the transplants between 

 brothers and sisters in strains A and B is higher than the grade of transplants 

 between different litters. This is quite definite in strain B, while in strain A 

 there is no marked difference between the two grades ; the reaction happens to 

 be even slightly less severe in transplants between rats belonging to different 

 litters. Transplants from family A to family B (grade 1.67), and from family 

 B to family A (grade 1.57) may serve as controls. Both these average grades 

 correspond about to the grades of ordinary homoiotransplants ; they are lower 

 than the average grades of transplanted tissues exchanged within family A 

 or family B. From these data we may conclude that as a result of close 

 inbreeding in rats for 37 to 47 generations in families A and B, only a very 

 slight progress towards a homozygous condition has been accomplished. 



Series II (Table IV). A comparison of the grades in series I and II shows 

 that there is an improvement in the grades in transplantations between differ- 

 ent litters neither in family A nor in family B in series II over the corre- 

 sponding grades in series I. However, in the second series, in both families the 

 grades obtained in transplantations between litter mates are not only better 

 than the grades obtained in transplantations between different litters, but they 

 are also better than in the transplantations between litter mates in series I. 

 Also, the grades in transplantations between hybrids (AXB)F 4 or F 5 are 

 improved to a certain degree if litter mates are used. But in these transplan- 

 tations Ihe results are not so good as in transplantations between litter mates 

 in families A or B. Such a difference might be expected, because in hybrids 

 there is a greater chance for unlike genes to accumulate in brothers when 

 both A and B contribute to the genes of the fertilized germ cells. Trans- 

 plantations from parent to hybrid give somewhat better results than the 

 reciprocal transplantations, but both elicit severe homoiogenous reactions as 

 an indication that a homozygous genetic constitution has not yet been reached 

 in either family A or B. Exchange of tissues between families A and B like- 

 wise corresponds to a severe homoio-reaction, in accordance with expectations. 



It is especially the results of transplantations between litter mates in inbred 

 strains A and B which suggest that some progress towards a greater homo- 

 geneity in the constitution of the individuality differentials has been made 

 through continued close inbreeding. In the exchange of tissues between broth- 

 ers and sisters of the same inbred family, the factor of a difference in the dis- 

 tance of relationship between different litters is eliminated. It is not probable 

 that the lack of a diminution in the severity of the reaction in the exchange of 



