120 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



Each particle of the chromosome is potentially independent of its 

 neighbours, so that exchanges of partner take place here and there 

 along the paired chromosomes, and the paired threads at pachytene 



^■y 



% 



/ 



\z JJ 





yiyiy!!.! y 



Fig. 38. — Diagram illustrating the structure of the pairing chromo- 

 somes during prophase and metaphase of the first division in 

 triploids and tetraploids, with chromosomes having median 

 centromeres and complete terminalisation. Z., zygotene ; 

 P., pachytene ; Dp., diplotene ; Dk., diakinesis (with partial 

 terminalisation) ; M., metaphase (with completed terminalisa- 

 tion). The lines represent chromosomes in the first two stages, 

 chromatids afterwards. There are three stages of development 

 when behaviour varies : (i) choice of partner at zygotene ; 

 (ii) formation of chiasmata at diplotene ; (iii) orientation in 

 the spindle at metaphase. Variations through the last two 

 chances are shown but the more complicated possibilities are 

 omitted [v. Figs. 40, 42, 43, 45). 



in the tetraploid resemble those at diplotene in the diploid. As 

 many as six exchanges have been observed in one association 

 (Newton and D., 1929 ; D., 1929 b). 



