Pairing without Chiasmata 215 



places. The short arm of each homologue is associated at one 

 interstitial region, and the longer arm at one interstitial region 

 and again at the end. At other regions on these chromosomes, 

 the two arms either fail to attract or actually repel one another. 

 As prophase advances, the paired regions separate from one 

 another, and during metaphase they are not paired but tend to 

 lie next to each other on the equatorial plate. The sex chromo- 

 somes do not pair during midprophase but they do lie next to 

 one another at metaphase. 



During the first meiotic division in the males of Olfersia, at a 

 stage comparable to late diakinesis, the autosomes are associated 

 as bivalents at apparently the same regions where they were 

 joined during the somatic divisions. The rod-shaped chromo- 

 somes are together at their distal region, whereas the V-shaped 

 ones are associated in three places and open out into two loops, 

 which gives this bivalent the appearance of one with three 

 chiasmata. The two dot-like autosomes lie very close to each 

 other, but the sex chromosomes are not paired and may fre- 

 quently be widely separated in the nuclei. By late diakinesis, 

 however, the sex chromosomes are found associated together in 

 regions fairly close to the centromere. 



At first metaphase, the two dot-like autosomes are usually 

 separated from one another, but the other three pairs of chromo- 

 somes form bivalents. They are still paired, but not as in most 

 organisms, for they are held together not by chiasmata but by 

 small chromosomal segments, conjunctive segments, each of 

 which appears to have the power of adhering to the similar 

 segment of the homologous chromosome. Both the autosomes 

 and sex chromosomes behave as though they possessed one or 

 more of these relatively short conjunctive segments. They may 

 initiate the approach of the homologues and they are responsible 

 for their cohesion in bivalents. Without them, chromosomes in 

 the male fly would not pair and therefore would not disjoin so 

 regularly at meiosis. They may also be the regions from which 

 emanate the hypothetical forces believed to be responsible for 

 somatic pairing. It has been further suggested that similar 

 conjunctive segments rather than reciprocal chiasmata are the 

 basis of the association of the X and Y chromosomes of Dro- 

 sophila and that the autosomes associate as the result of essen- 

 tially the same forces, but that these forces are distributed 



