THE MODIFICATIONS OF THE CHROMOSOME CYCLE 



69 



Firstly, if two chromosomes are paired at one point, they tend to be 

 paired at nearby points, as might be expected if the chromosome has a 

 certain rigidity, Darlington has described this as a tendency of the 

 chromosomes to behave as if they were made of a small number of 

 pairing blocks, but this phrase must not be taken to imply that the 

 regions paired are the same in different nuclei, or that there are any 

 non-pairing segments; it is only intended to describe the fact that the 



Fig. 28. Zygotene Pairing in Autopolyploids.— A in triploid Tulipa, B in 

 tetraploid Primula sinensis. The figures show parts of the chromosomes, which 

 are too long and coiled to be drawn completely. Notice the changes of partner, 

 and that there are never more than two threads paired at any point. 



(After Darlington.) 



chromosomes do not zig-zag backwards and forwards from one asso- 

 ciation to another very often, but only rather seldom. Secondly, the 

 zygotene pairing may be interfered with by the large number of chromo- 

 somes which are squeezed inside the nucleus. These may produce such 

 a tangle of threads that some possible pairing fails to occur for purely 

 mechanical reasons. 



Both these factors tend to reduce the total amount of zygotene 

 pairing, and since chromosomes not paired at zygotene caimot be held 

 together at metaphase, unassociated univalent chromosomes appear. 

 Even chromosomes which were paired at zygotene will only remain 

 associated if chiasmata are formed between them. If the chiasma fre- 

 quency is low, there may not be enough chiasmata to hold together all 



