APPENDIX 191 



E. Methods of Culture for Drosophila (Exercise 2, Heredity and 

 Variation) 



Flies can be reared in half-pint milk bottles plugged with 

 cotton wads wrapped in cheese cloth. Ordinary milk-bottle 

 caps which contain fine perforations can be used instead of 

 cotton plugs but beginners may have more trouble with 

 them. The culture medium is prepared by dissolving 15 gm. 

 of agar-agar in 750 c.c. of hot water. When this is com- 

 pletely dissolved slowly sprinkle in 100 gm. of cornmeal, 

 stirring continually in order to prevent lumping. Allow 

 this mixture to come to a boil and then add 135 gm. of 

 corn syrup (Karo red label is satisfactory). Boil slowly for 

 five to ten minutes. Pour the medium into the milk bottles 

 to a depth of one-half to three-fourths of an inch and im- 

 mediately drop one end of a strip of paper toweling about 

 an inch and a half wide and three inches long into the food 

 pad ; as the mixture solidifies the paper will be held in place 

 and provide a suitable surface for egg-laying. Plug the 

 bottles and sterilize for twenty minutes at fifteen pounds 

 pressure. About twenty-five to thirty culture bottles can 

 be prepared with the amounts stated. So long as the plugs 

 are not removed and the food pad remains moist the bottles 

 can be used successfully. 



Selection of mutant stocks for crossing with wild-type flies 

 should be done with the student's experience and equipment 

 in mind. Character differences recognizable with a handlens 

 and not too much conditioned by the age of the fly are not 

 easy to find. Vestigial wing is one of the easiest to identify; 

 ebony body color is another. Both of these morphological 

 effects are associated with somewhat lowered viability. 

 Sepia and cinnabar eye-color are both readily distinguished 

 from wild type if the flies are not freshly emerged, and 

 viability is relatively good. Among sex-linked characters, 

 which are probably rather confusing to use for a beginning 

 experiment, white eye-color and miniature wing are very 

 easy to classify. 



Because most mutant stocks are less viable than wild-type 

 flies perfect ratios are not to be expected. Better results 

 will be obtained if culture conditions are good. Do not use 

 crowded cultures or too scant or dry food; take every pre- 

 caution against contamination by molds. The times stated 



