TRIPLOIDS 



135 



an equatorial division of the chromosomes takes place, 

 giving two giant cells with 36 chromosomes each. 



As a rule very few functional pollen grains are formed 

 in the triploid, but apparently the egg-cells are more 

 often functional. For instance, when a triploid is polli- 

 nated by a normal plant, the number of normal offspring 

 (2n) produced is much beyond expectation on the as- 

 sumption that the chromosomes of the egg are freely 

 assorted. 



Fig. 82. 



a, Normal or diploid female, and b, triploid of Drosophila melano- 



gaster. 



Triploid Drosophilas have been found by Bridges 

 (Fig. 82). They are females because they have three X- 

 chromosomes balanced against three of each kind of auto- 

 some. This is the same balance that produces the normal 

 female. Since genetic factors in all the chromosomes are 

 known, it has been possible to study the behavior of the 



