76 LEONELL C. STRONG 



a. Races used. The common wild house mouse fulfills the 

 requirement of a homogeneous race^ to a marked degree. If 

 collected in the same locality (an isolated group of buildings, 

 a small island, etc.), one may be fairly sure that such stock is 

 homogeneous for several reasons. Strange mice very seldom 

 invade any particular location already occupied by a well- 

 established colony of mice. This is evidenced by the fact that 

 slight variations in color and form tend to be restricted to the 

 same corner of a building, etc. 



Wild adult female mice will not breed readily in captivity, 

 nor will wild pregnant females (caught wild) usually care for 

 their young when born. It was necessary to find the breeding 

 places (nests) of the wild mice. The young found were reared 

 by foster-mothers from an albino stock. Care must be employed 

 in placing the new young in the nest. The foster-mother must 

 first be removed from the box. After the young have been in 

 the nest long enough for them to acquire the odor of their new 

 surroundings, the foster-mother may be replaced. If this pre- 

 caution is not taken the females may eat the wild young. 



The mice were kept in a closed room in which the temperature 

 did not vary more than a few degrees throughout the day and 

 night. The wild mice are numbered consecutively, their serial 

 number being preceded in each case by the letter W, so that no 

 confusion would arise as to their origin. 



The dilute hrown (silver fawn) stock. This is a special strain 

 of mice produced by inbreeding during the last eleven years. 

 With the exception of the Japanese waltzing-mouse strain of 

 Mr, Lambert (Boston, Massachusetts), this strain represents, 

 no doubt, the nearest approach to a homogeneous strain of mice 

 employed in cancer research. It has a distinct advantage over 

 the Japanese waltzers in the matter of breeding and of rearing 

 the young. This strain consists of animals containing the three 

 recessive characters for coat color: 1) dilution of pigmentation, 



^ Theoretically, there is no question but that homozygosity would be produced 

 by continued inbreeding. In the absence of controlled experimental evidence, 

 it seems better to employ the term 'genetic homogeneity' or merely 'homogeneity' 

 to express this result. 



