THE VINEGAR FLY 366 



The Vinegar Fly. — Investigations in heredity have 

 probably made greater progress in the last dozen years or 

 so than in all previous time. This has in large measure 

 been made possible through the use of the vinegar fly, 

 Drosophila. This insect (see Fig. 105) is small and breeds 

 readily in captivity. It mutates (see p. 350) frequently 

 and its life is short. In a little over ten years niore genera- 

 tions of this fly have been reared and carefully observed 

 under the microscope than there have been generations 

 of men since the dawn of recorded history. 



The nucleus (Fig. 103) of the cell of the vinegar fly is 

 almost ideally suited for studies in heredity. In the first 



9 d 



Fig. 103. — The Nuclei of the Cells of Vinegar Flies. The figure 

 to the left shows that the female has 4 pairs of chromosomes in each 

 cell nucleus and that the members of each pair are visibly alike. The 

 right-hand figure shows that the male has three pairs like those of 

 the female but that the members of the lower pair are visibly dif- 

 ferent from one another. So far as investigations have gone up to 

 the present the one with the hook (the Y-chromosome so-called) 

 carries no genes or hereditary factors. In certain abnormal flies 

 it is entirely missing and that apparently without making any 

 essential difference in heredit3^ 



Morgan, Sturtevant, MuUer, Bridges, Mechanism of Mendelian 

 Heredity. 



place it has only four pairs of chromosomes and hence 

 can have only four groups of linked characters. In the 

 second place, its chromosome pairs can be distinguished 

 from one another by their form and size. This has been 



