56 Reproduction and Meiosis 



mosome are still twisted about the two chromatids of the homolo- 

 gous chromosome in relational coiling, but this is further com- 

 plicated by a coiling of the two chromatids of each chromosome 

 around each other. Thus two threads (chromatids) which are 

 coiled about each other are coiled relationally about two other 



Fig. 18. One bivalent in stages of the first meiotic division. Top, 

 zygotene or early pachytene before the chromosomes have become double. 

 Center, pachytene with each chromosome consisting of two chromatids. 

 Bottom, diplotene showing chiasmata. 



threads (chromatids) which are coiled about each other. It can 

 be seen from this that the chromosomes are under considerable 

 stram. Before the chromosomes become double, there is an 

 attraction of an unknown nature which causes them to remain 

 paired. Once the chromosomes are doubled this attraction ceases 

 and is translated into an attraction between the chromatids of 

 each pair. AVhen the attraction between chromosomes lapses, 

 one pair of chromatids begins to repel the other pair, increasing 

 the strain. The result will be that at one or more places one of 

 the four threads will break, and it will not necessarily be the 

 same thread that breaks at any two places. 



