THE GENERAL OUTLINE OF MEIOSIS 69 



satisfactory answers to all these questions cannot be 

 given as yet, but a series of preliminary hypotheses 

 have been put forward ^^ ; it seems certain that the 

 initial breakage relieves a loca^lized strain as the 

 spirally twisted chromosomes separate. It is clear 

 that the moment of breakage is after the chromosomes 

 have split and before they have separated, i.e. in the 

 very short four -strand pachytene stage. 



Once the loops between the chiasmata have opened 

 out (as a result of the repulsion mentioned above) and 

 the diplotene bivalents have acquired their character- 

 istic appearance (Fig. 9(i) three kinds of changes 

 begin to take place in the bivalents : the first two of 

 these are universally found, while the third occurs 

 in many organisms but not in all. 



The first change is a shortening and thickening of 

 the chromatid threads ; this can easily be seen by 

 comparing Fig. 9^ and e. It probably takes place 

 in exactly the same way as the contraction of mitotic 

 chromosomes between mid- and late prophase. (See 

 Chap. II, p. 12.) 



The second change is most marked in bivalents 

 with a single chiasma and consists of a relative rota- 

 tion of two arms of the cross through about 180° 

 (relative to the other two arms). The result is that 

 a bivalent with a single chiasma which looks 

 like Fig. 136 at early diplotene comes to look 

 like Fig. \M at late diplotene. In the case of 

 bivalents with several chiasmata the rotation is 

 usually through only about 90° — thus the successive 

 loops between the chiasmata come to lie in planes at 

 right-angles to one another, the alternate ones lying 

 in the same plane. The appearance of the bivalent 

 is similar to tliat of a chain stretched out tight. 



The third change consists of an actual moving of 

 the chiasmata towards the ends of the chromosomes. 

 This is shown in Fig. 14a, b and c, which represent 

 successive stages in the process. Of course the point 

 where the paternal and maternal portions of the 



