290 BIOLOGY OF THE LABORATORY MOUSE 



Strong (75), working with an adenocarcinoma dBrC which originated in 

 the dilute brown (dba) strain, found that it grew in all animals of that strain 

 which were inoculated. It also grew in 180 Fi mice produced by a cross 

 between strain dba and A albinos. The F2 generation gave a ratio (line i, 

 Table 9) which indicates that probably six genes were involved. 



During routine inoculations of this tumor a very rapidly growing sub- 

 strain of it was observed. This was designated as tumor dBrCX (line 12 in 

 Table 9) . From this tumor two further sub-strains seeming to show a dififer- 

 ence in growth rate and specificity were isolated. These were called dBrCm 

 and dBrCsp. They were carefully tested with Fo animals and gave results 

 shown in lines 5 and 9 of Table 9. There had evidently been a genetic 

 change from a probable six factor tumor to two factors in the case of dBrCm 

 and to one factor in the case of dBrCsp. 



Similar results with other tumors have later been described by Bittner 

 (7) and by Cloudman (22). In every case the change has been in the direc- 

 tion of decreased specificity and there have been ratios indicative of fewer 

 factors after the change than before it. 



Since the changes appear to be sudden and since they are perpetuated 

 from one cell generation to another, they are properly definable as mutations. 

 It will, of course, be necessary' to discover a method of identifying the genes 

 borne within the tumor cells before the mutations can be considered as estab- 

 lished "gene" mutations. They are, however, abrupt genetic modifications 

 which are self-perpetuating. 



Tr.\nspl.\ntation of Leukemia 



One of the most extensively studied types of neoplasm in mice is the series 

 of leukemias reported by MacDowell and his associates. 



An excellent discussion and review of this field has been given by 

 MacDowell (57). In all of his work he has employed significant numbers of 

 inbred genetic strains which provide authenticity and a sound foundation for 

 future investigation. 



Having demonstrated that the estabhshment of a true leukemic condition 

 depends upon the multiplication of an invasion by inoculated leukemic cells, 

 the parallel between that situation and the growth of transplanted tumors is 

 established. 



The elimination of extra-cellular agents including bacteria further 

 strengthens the similarity of the two processes. 



There also exists a high degree of specificity within the inbred genetic 

 strain so that transplantation of leukemic cells within the strain is uniformly 



