716 



THE AMEBIC AN NATURALIST [Vol. XLVIII 



defy Mendelian analysis. Though all the details of the 

 case have not been worked out, enough has been done to 

 show that it is brought about by factors which segregate 

 in the ordinary Mendelian fashion, and that the diffi- 

 culties which it still presents are not opposed to that 

 hypothesis. 



The case under consideration is that of Beaded wings, 

 which, according to Morgan (1911a), first appeared in 

 May, 1910, among flies that had been exposed during part 

 of their early life to radium rays. 



The appearance of these wings can best be understood 

 from the figures (Figs. 1-12), which represent a few of 

 the forms that may appear in a stock culture. All grada- 

 tions may be found between wings perfectly normal and 

 mere strips, such as shown by Figure 11. 



In the early days of its history, according to Morgan, 

 the Beaded-winged flies did not breed true, but for many 

 generations produced many normal-winged offspring. 

 At the time when I took up the experiment, however, the 

 stock bred almost 100 per cent, pure ; that is, almost every 

 fly hatched had wings more or less Beaded. I have at 

 present a strain which breeds true, throwing only Beaded- 

 winged offspring, and most of the offspring have the 

 Beading in an extreme form. Most of my work has been 

 done with this stock. 



II. THE GERMINAL CONSTITUTION OF BEADED FLIES 



A. Crosses between Beaded and "Wild Flies 

 1. Behavior in First Generation 

 When a Beaded fly is mated to a normal fly of a normal 

 Wild stock, a considerable number of flies with Beaded 

 wings usually appears in the first generation (F t ). The 

 percentage is not constant, but varies between zero and 

 about fifty per cent. (See Table I.) From Chart 1, it 

 appears possible that the average percentage of Beaded- 

 winged offspring per pair is near 10-15 per cent, or else 

 near 30-35 per cent, of the total offspring. The exact 



