86 THE CHROMOSOMES 



tinuous ring of chromosomes is formed instead of a 

 number of bivalents. Chiasmata arise in the paired 

 terminal segments so that the ring is maintained up 

 to the metaphase of the first meiotic division. Occa- 

 sionally a chiasma fails to develop in one of the 

 paired terminal regions, so that the ring breaks at 

 one point to give an open chain. It is obvious from 

 the homologies of the pairing regions that the relative 

 positions of the 14 chromosomes in the ring are 

 constant. 



At the prometaphase of the first division the spindle 

 attachments arrange themselves in a zig-zag round 

 the equator of the spindle so that alternate ones go 

 to the same pole at anaphase (Fig. 19d). This is a 

 natural consequence of the repulsion between the 

 spindle attachments, and is a phenomenon we have 

 already met in the case of trivalents. It results in 

 all the C median segments going to one pole and 

 the R ones to the other. Ci . . . C, and Rj . . . R7 

 are thus inherited as units and are referred to in the 

 genetical terminology as the cur vans and rig ens com- 

 plexes Oenothera muricata thus consists of a rigens 

 complex, a curvans complex and seven pairing 

 segments each of which is represented twice in the 

 somatic chromosome set. 



The process of meiosis is essentially the same in 

 macro- and micro -sporogenesis so that two kinds of 

 megaspores and two kinds of pollen grains are 

 formed. One of these contains the curvans segments 

 C1-C7 and the other the rigens segments Rj-R,. 

 Fertilization of a curvans ovule by a curvans pollen 

 tube or a rigens ovule by a rigens pollen tube leads, 

 however, to an in viable type of zygote. Thus only 

 the curvans x rigens and rigens x curvans zygotes 

 survive and the species is kept in a state of permanent 

 heterozygosity as far as the middle parts of its 

 chromosomes are concerned. 



The condition described above, in which all the 

 chromosomes form a ring is found in a number of 



