SEX-LINKED INHEEITANCE IN DROSOPHILA 101 



selves are expected, since one of the exchanges always goes into 

 the polar body; but on an average the loss to the polar body 

 should be as often of one kind as of the other. In order to test 

 this more fully the 'exchanges' should be equally viable, which is 

 not the case in the present experiments, where a correction for 

 viability has to be supplied. 



It will be noted that the few cases, in which impossible female 

 classes appear, have been 'explained away' as due to mutation 

 or to error. We believe that we are justified in making this 

 assumption on the basis of our general knowledge of the behavior 

 of these factors in heredity and of the possible experimental errors. 

 But a critic will not be slow in pointing out that these cases can 

 be explained if crossing-over is admitted for the male. The fair- 

 ness of this criticism must be admitted. Whether it is justified, 

 further work must show, and this work is under way. 



