66 HEREDITY AND VARIATION 



add 1-2 drops of a concentrated solution of compressed yeast to the 

 surface of the food pad in each bottle; replace the plug. Now obtain 

 pairs of flies that differ with respect to the character being studied in 

 order to make the original or P^ cross. Both parent flies are from 

 pure-breeding stocks. Half of the class should use wild-type males, 

 and half, wild-type females. Understand that virgin females must be 

 used and learn how they are obtained. Since the flies will move up 

 and toward the light they can be transferred from the vials to the 

 culture bottle. Remove the plug from the culture bottle and hold 

 between two fingers or place on a clean sheet of paper. Unless you 

 use care, mold spores will be introduced into your culture bottle from 

 the plug and hinder or stop the production of flies. Hold the open 

 end of the culture bottle away from the light and somewhat lower 

 than the base. Tap the base of the vial containing the fly on the 

 table to throw the fly away from the cork, quickly uncork the vial, and 

 hold the open end of the vial within the open end of the culture 

 bottle. The fly will crawl to the open end of the vial and can then 

 be shaken off into the culture bottle in which the plug is immediately 

 replaced. Repeat the process for the other fly; do not allow the first 

 one to escape. Since virgin females are more trouble to obtain than 

 males it is better to introduce the male into the culture bottle first. 

 Because there are numerous reasons why cultures fail, each student 

 is to start duplicate cultures. Make a label containing information 

 about parents and date of beginning experiment and attach to each 

 bottle. 



(d) Keep the culture bottles at room temperature and examine 

 each day to make sure that the parent flies are still alive and have 

 not escaped. On the fourth day you should be able to see the small, 

 cream-colored larvae moving about in the food pad. AVhen do you 

 first find the pale pupal cases on the side of the jar or on the paper? 

 Remove the parent flies on the seventh or eighth day. As soon as the 

 pupal cases become dark — probably on the ninth day — carefully re- 

 move six to eight of them with a needle without puncturing and place 

 in separate vials each of which contains a moistened piece of filter 

 paper. When do you observe the first adults in the culture bottle? 

 Eight or nine days after the first adults are seen transfer the flics from 

 the culture bottle to an etherizing bottle. The mouth of this bottle 

 should fit the mouth of the culture bottle, and the flies may be allowed 

 to crawl up from one to the other or the transfer can be hastened by 

 tapping on the culture bottle while it is held above, but in contact 

 with, the etherizing bottle. Several drops of ether should be placed 

 on the plug of the etherizing bottle. As soon as all the flies are in it 



