DIRECTION AND FREQUENCY OF MUTATION 217 



3. Ultra-bar eye 



The mutations from ultra-bar are given in table 4. The first 

 two items, consisting of 3184 individuals, or over one-third of the 

 whole, are made up of flies in which eye facet counts were recorded 

 and in these sets the chance of overlooking mutations to bar is 

 less than in the others. Nevertheless, there is no essential 

 difference in the rates of mutation in the two groups, though the 

 numbers are too small for any valid comparison. 



Among the 8681 ultra-bar individuals examined there were 

 five mutations to full, or one in 1726, which gives a coefficient of 

 0.00058. There were also three mutations to bar, or one in 

 2894, a coeflftcient of 0.00035. The reverse mutations to full 

 are nearly as frequent as those of bar to full which have a coeffi- 

 cient of 0.00061. Reverse mutations to bar are more frequent 

 than direct mutations from bar to ultra-bar which have a coeffi- 

 cient of only 0.00003. Notwithstanding the small number of 

 mutations involved, the difference, though not its degree, may 

 be a significant one. 



In view of its origin from bar, it is a matter of special interest 

 to note that ultra-bar may revert either to bar or directly to full. 



The comparative frequencies of the various changes between 

 full, bar, and ultra-bar are given in figure 3 and table 5. In figure 

 3 the observed jumps are indicated by arrows which are marked 

 with the respective frequencies. The much greater frequency 

 of the reverse mutations is apparent. 



III. DISCUSSION AND CONCLUSIONS 



1. Are there definite and recurring stopping-places in mutation? 

 An examination of the data shows clearly that the germinal 

 changes have definite and recurring stopping-places (fig. 3) . In 

 the present observations there are no demonstrated cases of inter- 

 mediate conditions between the three components of the series, 

 A possible exception was described by the author (Zeleny, '20, 

 p. 308) as mutant D and has a dominance intermediate between 

 that of bar and ultra-bar. It may be a case in which ultra-bar 

 had mutated in the direction of bar, but not to the same point as 



