206 CHARLES ZELENY 



October, 1920, or approximately nine months. Dming the last 

 six months observations were made of all individuals in each gen- 

 eration. Facet counts were not made, but the examinations were 

 sufficiently careful to insure the location of all mutations with 

 the exception of heterozygotes between bar and ultra-bar. 



5. Method 



Efforts were concentrated upon the avoidance of contamina- 

 tion and the immediate recognition of mutants. 



1. Contamination. May ('17) has discussed the reasons why 

 contaminations may be excluded as an explanation of the reverse 

 mutations of bar to full. The same argument applies to the 

 different mutations described in the present paper Special care 

 was taken to avoid contamination, but the reader will be more 

 interested in the evidence against such an explanation of the 

 facts. As shown in table 2, a great variety of stocks was used, 

 including a number with various mutant characters in addition 

 to those involving eye facet number. In every case where an 

 eye mutation occurred these other characters were those proper 

 to the particular stock. Also, as in May's observations, the 

 female mutants were all heterozygotes, while on the theory of 

 contamination homozygotes should occasionally appear. 



2. Evidence for immediate recognition of mutations. With the 

 single exception already noted, the heterozygotes between the 

 different members of the bar series are intermediate in character 

 between the parents and sufficiently different from either of them, 

 so that mutations in daughters as well as in sons may be rec- 

 ognized at once. In case the primary change occurs in one of 

 the sex chromosomes of a female it will make itself apparent in 

 the character of a son or heterozygous daughter. In case it 

 occurs in a male a heterozygous daughter will exhibit the new 

 character. It is therefore not possible for a mutation to be trans- 

 mitted unrecognized from one generation to the next. 



3. Procedure in those stocks in which facet counts were not made. 

 These stocks were kept with uniform food conditions at about 

 21°C. The bottles of each stock were arranged in a row in 

 chronological order with a week's difference in tune between two 



