DIRECTION AND FREQUENCY OF MUTATION 205 



3. The origin of the series 



The original change, that from full to bar, was obtained in 

 February, 1913, by Tice ('14) in a single male in an experiment 

 involving rudunentary and long-winged flies with normal full 

 eyes. No recurrence of the mutation has been mentioned by 

 later observers. 



The second change, that of bar to ultra-bar, also occurred in 

 a single male (Zeleny, '20, p. 300). It was observed on October 

 20, 1917, in the second generation of selection for low facet 

 number in a white bar stock obtained by crossing red bar with 

 white full. 



That the change from full to bar is reversible was demonstrated 

 in this laboratory by May ('17), who obtained eleven mutations 

 from bar to full during the period November 15, 1915, to June 7, 

 1916. 



4. Material for the present report 



The present report is based upon four sets of observations i^ 



1. General observations of full-eyed stocks were made over the 

 period 1914 to 1920. 



2. The selection series in white bar number 127 at 27°C. con- 

 sists of forty-two generations of selection for low and for high 

 facet nmnber extending over the period September 26, 1917, to 

 July 15, 1919, or approxunately tw^enty-two months. During 

 this time facet counts were made of all the individuals, and muta- 

 tions were therefore located at once. 



3. With the establishment of the ultra-bar stock, careful ob- 

 servations of all individuals, with facet counts at 27°C., were 

 made over the period October 20, 1917, to July 23, 1918, or 

 approximately nine months. As in the previous case, mutations 

 could be accurately located. 



4. The whole group of full, bar, and ultra-bar stocks was kept 

 under observation for mutations during the period January to 



2 For excellent care of the living; material and for continued help during the 

 course of the investigation I am indebted to Dr. Joseph Krafka, Jr., who was my 

 assistant from 1916 to 1919, and to Mr. D. H. Thompson, who has occupied the 

 same position since that time. 



