252 CHARLES W. METZ 



definitely paired. A late prophase group from one of these 

 nuclei is shown in figure 47 (note the association of X and Y). 

 Subsequently to this stage pairing remains constant throughout 

 the development of the fly.^^ 



As to the causes of chromosome pairing in the Diptera very 

 little may positively be said, but there are certain facts about 

 the phenomena which should be considered in this connection. 

 The facts indicate for instance, that pairing is not due to purely 

 mechanical causes, but is dependent in some way upon the 

 qualitative nature of the chromosomes. This conclusion seems 

 evident from the fact that paired chromosomes are corresponding 

 or similar chromosomes. It is difficult to conceive how purely 

 mechanical forces can cause anything more than random pair- 

 ing, while as a matter of fact the actual pairing is selective to 

 the highest degree. That this association is not merely an assort- 

 ment according to size is shown by the pairing of unequal sex- 

 chromosomes in the males (figs. 41, 42, 44, 45, 86, 88, etc.), 

 where X is often several times as large as Y. 



A suggestion as to the significance of pairing may be obtained 

 from tetraploid groups such as are found occasionally in embryon- 

 ic somatic tissues. ^^ Qne such is shown in figure 96. In this 

 case there is twice the normal number of chromosomes (24 

 instead of 12), which means that in place of two chromosomes 

 of each kind, there are four. On the assumption that homol- 

 ogous chromosomes associate together, these 24 chromosomes 

 ought to associate in groups of four ; and this is actually their ar- 

 rangement. In figure 97 is shown a multiple group containing 

 four times the normal chromosome number, or 48. In this case 

 the chromosomes are so crowded together that their grouping 

 is confused, and it is impossible to tell how they are associated. 

 In prophase nuclei of a similar kind, however, the association 

 of homologous chromosomes is clearly evident. Figures 109 



" In respect to the intermingling of the two parental groups the flies thus 

 agree with other insects such as Hemiptera, Orthoptera, Coleoptera, in which 

 an intermingling occurs at least before the adult stage is reached, and presumably- 

 much earlier. 



^^ These are only of sporadic occurrence, and in my material have been found 

 only in a few cells or in small bits of tissue, never throughout the body. 



