526 



CELL MECHANICS 



hindered by the body repulsions of the other chromosomes on a 

 crowded plate, and in this case one bivalent may form an accessory 

 plate half-way between the primary plates and a pole (D., 1936 c). 

 The sex chromosomes are particularly liable to behave in this way 

 (Roller and D. , 1934) . Orientation, as well as even distribution, may 

 fail with interlocking (Dark, 1936, Pceonia). 



Orientation is also liable to fail where the centromeres are excep- 

 tionally far apart . For example, in a hybrid where two long chromo- 

 somes have formed a single chiasma, instead of three or four, the 



Fig. 151. — Non-congression and non-orientation of bivalents in 



Podophyllum versipelle [n = 6). x 1,800 (D., 19366). 



centromeres may be five times their usual distance apart, further 

 apart indeed than the centromeres of different bivalents. They 

 then fail to orientate and also fail to show any effective repulsion 

 even such as bivalents show at diakinesis (Richardson, 1936, 

 Lilium ; D., 1936 h, Podophyllum). This type of observation shows 

 that repulsion is the agent of orientation. The converse is also 

 true ; bivalents that have not become orientated and continue to 

 lie crosswise in the spindle fail to show tension between their 

 centromeres. 



Such repulsion as occurs crosswise in the spindle is less than 

 that shown at diakinesis, This is true equally of the centromeres 



