224 CHARLES W. METZ 



in those cases in which the two members of a pair have become 

 separated and do not He side by side in metaphase. A few cases 

 have been found in which the two members of a pair lie on op- 

 posite sides of the spindle. In anaphase, each of these is seen 

 to have divided and sent a daughter half to either pole. Figure 

 29 (same species as 27 and 28), for instance, shows a metaphase 

 in which the two large members lie on opposite sides of the 

 groups. In figure 30 a similarly arranged group is seen in ana- 

 phase. It is perfectly clear from the position of the large chromo- 

 somes in figure 30 that the two large elements going to one pole 

 are not sister halves of one chromosome, but are daughter halves 

 of two separate chromosomes, else they could not lie on opposite 

 sides of the spindle at this stage. A comparison with figures 

 27 and 31 shows how this differs from the normal condition in 

 which the large as well as the small chromosomes are paired. 

 The duality of the chromosomes in figure 31, if this figure were 

 taken by itself, might be interpreted as indicating a precocious 

 division of single chromosomes, rather than as indicating pairs 

 of chromosomes, but other facts, as just described, preclude such 

 an explanation. It is doubtless such appearances as those given 

 by figure 31 that have led some authors to misinterpret entirely 

 the nature of Diptera chromosomes. 



Fully as convincing evidence is furnished by other cases in 

 which the two members of a pair have become only slightly 

 displaced, instead of lying on opposite sides of the spindle. 

 Such cases are shown in figures 7, 9, 12, 16, 28 and others. Fig- 

 ures 7, 9, 12 and 16 are different stages in nuclei containing the 

 same type of chromosome group. It is obvious that here one of 

 the large pairs has been disturbed in such a manner that its two 

 members resemble two horse-shoes placed side by side. Ac- 

 cording to the ideas of Taylor and of Lomen these two members 

 should go to opposite poles, but it is clear that they do not. 

 On the contrary each divides and sends a daughter half to either 

 pole. Figure 12 represents a particularly interesting case, for 

 here the chromosomes have all divided and the daughter halves 

 have separated. The figure on the left represents the upper 

 group, chat on the right the lower group (displaced in order to 



