THE MULLER FRUIT FLY 



More is known about the heredity of these tiny flies than of any other form of life. The 

 ease and great rapidity with which many generations may be raised in the laboratory, and 

 the small number of chromosomes found in the dividing cells of these flies makes them ideal 

 subjects for studying the relation between the cell mechanism, as determined by cytologists, 

 and the facts of heredity brought to light by genetic experiments. In this genus twelve dif- 

 ferent chromosome "patterns" are found (see Figure 2), and the attempt is now being made 

 to discover by genetic experiments how some of these twelve types of chromosomes are 

 related to each other. The individuality of the chromosomes within the species is well proven, 

 and if homologies can be established between the genetic structure of these carriers of 

 heredity in several distinct species, light will be thrown on the evolution of the chromosomes. 

 The photographs shown above are of Drosophila niullcri, greatly enlarged. This species is 

 nearly identical in external appearance with two others, rcplcta and hydci. In fact, no dis- 

 tinction was made between the three until it was found that they would not cross. Their 

 chromosomes proved to be unlike, and minor external differences were later discovered. 

 Photographs by Robert C. Cook. ( Frontispiece. ) 



