there are apparent exchanges of segments between homologous half- 

 chromosomes as indicated by X arrangements known as chiasmata. 



It has generally been assumed that chiasmata represent the physical 

 basis of crossing over since it has been demonstrated that genetic ex- 

 change also involves chromosome exchange (Stern, 1931; Creighton and 

 McClintock, 1931), and it has also been shown that both chiasmata 

 and crossing over involve only two out of four strands at any one locus. 

 Various theories concerning the origin of chiasmata and their assumed 

 relationship with crossing over will be discussed in Chapter 6. 



Diplotene. Contraction continues as well as the opening out between 

 homologues, and the chromosomes tend to clump in the center of the 

 cell. Formation of the major coil usually begins at this stage, though in 

 some cases its inception may be slightly earlier. Because of the charac- 

 teristic clumping, this is a difficult stage to study. 



Diakinesis. As a rule, this is the best stage for studying chromosome 

 associations. Contraction is near a maximum and the chromosome pairs 

 are well spread throughout the cell as though by mutual repulsion. Pairs 

 of homologous chromosomes are still held together at chiasmata but are 

 elsewhere separated (Figures 5-6 and 5-7). This has given rise to the 

 idea that the chiasmata hold pairs together, but there are a number of 

 cases of continued association in which no chiasmata can be demon- 

 strated. The nucleolus generally disappears during this stage but may 

 persist, usually in reduced size, until anaphase. The major coil is usually 

 completely formed by the end of this stage. 



SAeiaphase I 



Pairs (bivalents, tetrahomads, tetrads) line up on the equatorial plate 

 with homologous kinetochores oriented toward opposite poles. 



Anaphase I 



Homologous kinetochores move toward opposite poles. Each kineto- 

 chore is attached to two chromatids (half chromosomes) which are 

 sisters in some regions and homologues in others, depending on where 

 crossing over has occurred. The chiasmata are fully resolved, apparently 

 by being pulled off the ends. 



Telophase I 



This is a regrouping of the chromosomes at the poles similar to the 

 equivalent stage of somatic mitosis. In some species both this stage and 

 the following one may be virtually eliminated. 



MITOSIS AND MEIOSIS / 129 



