64 THE CHROMOSOMES 



comparison with the prophase of an ordinary mitosis, 

 but it is probably better to regard the splitting of 

 the chromosomes as being delayed. 



Pachytene 



As a result of pairing the apparent number of 

 chromosome threads (in a diploid organism) has been 

 reduced to half ; if there were 2n chromosomes in 

 leptotene there will be n associations of two chromo- 

 somes at the beginning of pachytene. These associa- 

 tions of pairs of chromosomes are called bivalents. 

 Each bivalent has a split down the middle and thus 

 closely simulates an ordinary mitotic chromosome at 



Attract/on 



■ Pater nat <C^ ^^^^^^^^^Attracth^ 



J. . Repulsion 



^^^ll^liiin in i/Timii 



Fig. 12. — Diagrams to show how the attraction between 

 homologous chromosomes before they have spHt becomes 

 converted into a repulsion as soon as splitting has 

 occurred. 



mid-prophase, although it has arisen in a totally 

 different way, by pairing of two entirely distinct 

 chromosomes 'instead of by splitting of a single one. 

 Another important point of difference is that the 

 pachytene bivalent has two distinct spindle attach- 

 ment points, whereas the mitotic prophase chromo- 

 some has only one which does not divide until 

 prometaphase. 



Naturally the double pachytene threads are twice 

 as thick as the single leptotene threads ; they are 

 also a good deal shorter since contraction takes place 

 during zygotene. 



Half-way through pachytene the splitting which at 

 mitosis has occurred before the beginning of prophase 

 takes place. Thus the division process is not sup- 



