406 AN INTRODUCTION TO MODERN GENETICS 



bottle with mouth wide enough to fit against the mouth of a milk 

 bottle. The flies can be shaken straight into the etherizer, which is 

 closed with a cork, to which is attached a pad of cotton wool on which 

 the ether is placed. The same bottle can be used for flies from vials, 

 though a better, but more elaborate arrangement is to have a funnel 

 fitted into the cork, so that both milk bottles and vials fit when placed 

 against it. The flies should, of course, be quite still after etherization, 

 but they should not be allowed to get to the point when their wings 

 are folded backwards above the body, as they then rarely recover. 



When opening a bottle of flies, for transferring or etherizing, it is 

 necessary first of all to shake them ofi:"the cotton wool plug by bumping 

 the bottle sharply downwards on to the palm of the hand. Then remove 

 the plug and immediately place the mouths of the two bottles in contact. 

 Hold them together and vertical with the flies in the upper bottle, 

 then by jarring the side of the upper bottle, shake the flies out of it 

 into the lower bottle. 



b. Sorting 



The flies are tipped from the etherizing bottle on to a plate for 

 sorting. Note that all flies are removed from the bottle; if too much 

 ether has been used the sides of the bottle may get damp and some 

 flies stuck, remaining in the bottle to come out later and upset some 

 other result. The counting plate may be white cardboard, white tile, 

 or unused photographic plates. The flies are handled with a "pusher"; 

 either a small paint brush or a pointed piece of stiff card or sheet 

 metal, cut so as to be convenient to hold. It is easiest to arrange the 

 flies in a line along the centre of the plate, and, working from one end 

 of the line, sort the flies into two or more parallel rows according to 

 how many sorts are present. Some hereditary characters can be sorted 

 by the naked eye, but many require a hand lens. 



c. Discarded Flies 



Discarded flies should be killed; it is dangerous to have mutant flies 

 loose in a laboratory where they may contaminate experimental 

 cultures. They can be poured, still etherized, into a "fly-morgue," 

 i.e. a wide-mouthed jar containing 70 per cent alcohol, or, better, car 

 engine oil. 



d. Making Crosses 



In crosses for experiments on heredity, the female must be virgin. 



