No. 633] SHORTER ARTICLES AND DISCUSSION 373 



coccus infection we have therefore limited our counts to animals 

 of the same age-group, that is to mice of at least four weeks old 

 and not yet used for breeding. 



At the general weighing of January 4, 1919, no losses were 

 observed among the Japanese mice. Shortly afterward the 

 Japanese started to die off. And at the general weighing of 

 February 14, the last Japanese mouse was found dead. 



The data given in this paper are taken from the records of 

 this general weighing of February 14, 1919. They include 

 litters of six kinds, pure Japanese, pure Robertson's whites, F, 

 hybrids, F 2 hybrids, mice with one parent F 1 and the other 

 Japanese, and such with one parent F 1 and one large parent. 



As noted above all the Japanese left in the mousery died 

 between January 4 and February 14, 59 in all. Of these 23 

 were in the class of weaned young, not yet breeding. 



TABLE I 



As to the Robertson large strain, no deaths were observed 

 w it bin this period of six weeks among mice of this age class. A 

 very considerable number of these weaned young were growing 

 up in cages together with Japanese of their age and sex. 



Between January 4 and February 14 we lost no F, animals 

 after weaning age. Strictly eomparahle to the other lots were 

 only three litters, which were weaned within the critical six- 

 weeks and not yet put to breeding. These litters contained 

 fourteen young. All were living on February 14. 



