350 INTRODUCTION TO CYTOLOGY 



principal types as they appear with their characteristic arrangements at 

 the time of cell-division. Type A is that found in Drosophila melano- 

 gaster (formerly known as D. ampelophila) , upon which the greater part 

 of the genetic work in these flies has been done; it is also characteristic 

 of several other species. Here there are two pairs of large bent "euchro- 

 mosomes," one pair of sex chromosomes, and one pair of very small 

 "m-chromosomes." In some of the other species it is seen that the 

 position of one or both of the large pairs is occupied by two pairs but 



M II.. lie U 

 tr \\// ///, 



^ & : ^ *' 



o 



M 



L 



FIG. 137. The 12 principal types of chromosome groups found in the cells of the Droso- 

 philidae. (After Melz, 1916.) 



half as large, and there is much evidence to show that these have arisen 

 by a segmentation of the large chromosomes. Furthermore, the m- 

 chromosomes do not appear in some species, but it is not yet certain 

 whether they are actually lost through irregular mitoses or are fused 

 with some of the larger chromosomes of the group. Since irregular 

 mitoses resulting in abnormal distributions of chromosomes are actually 

 observed in Drosophila, and are known to be accompanied by changes 

 in the hereditary constitution, there can be little doubt that by this and 

 other means all the types of chromosome groups have been derived from 



