52 HELEN DEAN KING 



and she succeeded in making them quite as gentle as are 

 stock albinos. These tamed rats were then used in experi- 

 ments requiring daily injections of solutions through the ab- 

 dominal wall. They submitted to these injections as readily 

 as do albinos, even though they were held by bare hands. 

 Gray rats, therefore, can now be used for all forms of experi- 

 mental work. Large, vigorous, and seemingly little susceptible 

 to certain diseases prevalent in other strains of rats, they 

 should be valuable for laboratory work in which it seems 

 desirable to use individuals from a pure strain of known 

 ancestry. 



MUTATIONS 



Several mutations affecting the color or the structure of the 

 hair appeared in captive gray rats during the period covered 

 by this report. As only a brief report of these mutations has 

 been published (King, '32), a more detailed account is given 

 here, including genetic studies made to determine the mode 

 of inheritance of certain types. 



One of the six wild gray females (2) used as foundation 

 stock for the colony had a small spot of white hair on the 

 ventral surface of the body between the forelimbs, similar in 

 size and position to the spot shown in figure 14. This female 

 was mated with a wild male (2), presumably a brother since 

 the two rats were trapped at the same time and appeared to 

 be the same age. From the matings of this pair of rats 

 twenty-nine young were obtained, of which seventeen had a 

 patch of white hair on the ventral surface varying in size 

 from a mere dot to a spot no larger than that in figure 14. 

 Experiments were then begun to determine whether the 

 amount of white in coat could be increased by selective breed- 

 ing. For two generations rats having white spots were inbred, 

 brother and sister. Subsequent matings were made between 

 individuals with the greatest amount of white in their coats, 

 regardless of their relationship. 



During early generations there was but little increase in 

 the size of the white area, but in the sixth generation indi- 

 viduals appeared with irregularly shaped patches of white 



