356 HELEN DEAN KING 



Out of 124 stock Albino females reared in our own colony during 

 the past three years and intended for breeding purposes thirty- 

 two, or 28.8 per cent, were completely sterile, while about 10 per 

 cent of those that did breed cast only one or two litters. Un- 

 fortunately, no records have been kept that give information 

 regarding the exact proportion of sterile females in the first six 

 generations of the inbred series. The number was relatively very 

 large, and must have included at least one-half of the total number 

 of females that lived to be six months old. Sterility in these 

 females was, for the most part, the result of poor nutrition, 

 and it disappeared as soon as the nutritive conditions were im- 

 proved. Loeb ('17) has shown that in the guinea-pig ''under- 

 feeding prevents maturation of the follicles and thus causes 

 sterility which lasts as long as the effect of the underfeeding is 

 present in the ovary." In the guinea-pig, as in the rat, adequate 

 nutrition reestablishes normal conditions in the ovary and 

 sterility almost entirely disappears. 



In Drosophila, according to Castle et al. ('06), low productive- 

 ness (sterility) is directly transmitted by inheritance and is 

 amenable in selection. In the rat, sterility seems to depend not 

 entirely on genetic factors, but to a marked extent upon condi- 

 tions, such as malnutrition and disease, that act unfavorably upon 

 reproduction. In the present experiments, by selecting for breed- 

 ing only the most vigorous individuals (which it seems were also 

 the most fertile), sterility in as far as it may depend on genetic 

 factors would seem to have been practically eliminated from the 

 strain, and it has not reappeared even after twenty-eight genera- 

 tions of brother and sister matings. 



Of the 954 inbred females that w^ere used for breeding during the 

 course of these experiments, 653, or 68.5 per cent, cast four litters 

 each, and many of them, kept for body-weight records, produced 

 several other litters which were not recorded. Of the females 

 that did not cast the required four litters, the great majority died 

 from pneumonia, or were killed because they showed unmis- 

 takable evidence of illness. A few of the females stopped breed- 

 ing after producing one or two litters, although they were appar- 

 ently in good phj^sical condition and were paired for several 



