234 



CALVIN B. BRIDGES. 



the right of hairless). The ebony rough distance was so great 

 that in constructing a map a correction was required on account 

 of double crossing over. The presence of white-ocelli between 

 ebony and rough gave an opportunity to make a direct test 

 of the amount of double crossing over and consequently of the 

 amount of correction required. The results of the spineless 

 white-ocelli X hairless rough back-cross are given below: 



There was 6.6 per cent, of crossing over between hairless and 

 white-ocelli, which agrees with the value 6.8 found in the previous 

 experiment. Likewise, the white-ocelli rough value of 15.8 is in 

 agreement with the expectation from the usual value of 22 for 

 hairless rough. There was only one double crossover in the 

 hairless white-ocelli rough section a percentage relatively very 

 low. Comparisons show that in the third chromosome (as in 

 the second) the region near the end of the chromosome has a far 

 lower 'coincidence' than has the mid-region. The amount of 

 correction of the observed crossover value for the hairless ebony 

 interval is thus .063 per cent., or somewhat less than one tenth 

 of one unit. Other back-cross tests involving this region have 

 produced a total of 45,971 flies, of which 19.6 per cent, were 

 crossovers. This value is to be corrected to 19.7, which is the 

 map-distance between ebony and rough. 



THE VIABILITY OF THE WHITE-OCELLI MUTANT. 



As just seen, the linkage of white-ocelli was worked out 

 through use of the more convenient spineless white-ocelli stock, 

 while the black white-ocelli stock was discarded. The white- 

 ocelli character persisted in the original black stock. No effort 

 was made to eliminate it, nor, on the other hand, to aid in its 

 survival. In May, 1919, a census of the flies of this black stock 

 showed that approximately half were white-ocellars. That is, 



