Cellular Growth 



113 



or a c-nieiosis begins. Ojlchicine reduces the pairing as shown by the 

 reduction in diiasniala and increased Irecjuency ol univalents.- The 

 calculations ironi several independent studies (onfirni the action on 

 pairing. Allium ceriniinji rarely showed luiivalents in controls, but 

 among treated cases, 8 cells out of 31 had no bivalents. Moreover, no 

 cell among 31 jjollen mother cells studied had more than 5 bivalents 

 ^\hen the total with Itdl pairing could have been 7.''* Among Trades- 

 ((Dilld. I-! univalents (Fig. 4.3C') were produced by a lull c-meiosis. 

 .Similar cases are reported with other species. 



The terminali/ation of chiasmata is dirterent when colchicine is 

 piesent; therefore, there is reduction in chiasmata as well as change 

 m the kind of chiasmata (Table 4.3).- Whether crossing-over is 

 changed has not Ijecn tested geneticalh. but the cytological picture 

 seems to warrant a conclusion that cross-overs would occur in places 

 they are not generally expected. 



If recovery sets in while the univalents are distributed through 

 the cell, there is no congregation into the equatorial plate. But the 



TABLE 4.1 



Relation Between Treatment and Stage 



(After Levan) 



univalents collect at jjoles where the jjarticidar chromosomes happen 

 to lie. On the other hand, bivalents, if they have persisted, upon re- 

 coverv orient in the ecjuator. 



Unlike the tendency toward supercontraction at the metaphase of 

 a c-mitosis, the c-meiotic chromosomes do not show the usual contrac- 

 tion. ''•' In fact, they are less contracted; this is a very striking action 

 induced by colchicine. Such lack of contraction is correlated with a 

 decrease in the frequency of chiasmata. These are the major effects 

 iKjted ^vhen colchicine acts during piemeiotic stages. Full action up- 



