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THE AMERICAN NATURALIST [Vol. XLVIII 



at which time the eggs have hatched) can stand more 

 ether. Twice daily throughout their larval and pupal 

 lives (approximately 11 days) 6, or 7, or 8, or 9 or even 10 

 drops of ether were added to the quart bottles. The ether 

 excited the larvae at first, then quieted them ; later they re- 

 covered. In such tests the larvae were kept almost con- 

 tinuously in an atmosphere of ether from birth to emer- 

 gence of the fly and in a few cases the etherization was con- 

 tinued with the flies also. By covering a wide range of 

 stages and conditions I hoped to find the critical point, if 

 any such existed, when ether would act. Since, as the 

 sequel will show, no specific results were obtained it seems 

 unnecessary to give the details of all these trials. 



Double, and in one case triple recessive, stocks were 

 used for the work, because experience had already shown 

 that even with great care contamination may occur. One 

 or two flies that came from escaped mutants would ruin 

 the value of the data, but the operator can protect himself by 

 using stocks that have already two or more recessive char- 

 acters. If such flies mutate in one of the characters in- 

 volved the presence of the other one will make it certain 

 that the mutant belonged to this culture, and had not come 

 in from outside ; if a change appeared in some other part, 

 the double recessive character would still identify the 

 stock. Two of the stocks used had sex-linked characters, 

 i. e., eosin miniature and cherry club vermilion. If a 

 mutation should appear that involved these characters it 

 would become evident at once in the male offspring; for, 

 the male gets his single sex chromosome from his mother 

 and exhibits her sex-linked factors. Of course this 

 would be equally true for any other sex-linked char- 

 acter that appeared, but in practise it is impossible to 

 thoroughly examine each fly in every possible part, so that 

 I had to confine my attention to certain organs, and in 

 these cases I concentrated on the mutant characters. Con- 

 spicuous mutations in other parts would, I think, have 

 been picked up, but minor ones would probably have been 

 missed. On the other hand, if changes taking place in 



