CO-ORIENTATION OF CENTROMERES 131 



somes is shorter, and the cross-arms of an interstitial chiasma 

 strengthen the connection while a terminal chiasma opposes the 

 minimum resistance to any kind of torsion. The rigidity of a 

 configuration with interstitial chiasmata is necessarily three- 

 dimensional, that in one with only terminal chiasmata is two- 

 dimensional (v. Ch. XII). 



The second condition determines whether or not the multivalent 

 is physically capable of K'ing in one axis, as does a bivalent. The 

 third determines whether there is room for the chromosomes to lie 

 transversely across the plate, as is necessary if each is to separate or 



^ ) u V 



r 



L C/ C2 P Df Dz I 



Fig. 44. — Different methods of co-orientation in trivalents (linear, 

 L ; convergent, C ; and independent, I) and quadrivalents 

 (parallel, P, and discordant, D). In Dg the two loops are at right 

 angles. 



" disjoin " (to use a common phrase) from the one with which it is 

 associated. 



From the operation of these different factors the following 

 variations may be observed : — 



(i) The several centromeres may have one of four different 

 relationships with one another (c/. D., 1936). 



{a) Linear : all the centromeres lie in one spindle-axis. If this 

 axis is near the side of the spindle it is arc-shaped, so that 

 the centromeres do not lie in a straight line. 

 (6) Parallel : pairs of centromeres, where there are four or more, 



lie on parallel and independent axes, 

 (c) Convergent : two centromeres are lying axially with respect 

 to a third and lie therefore on axes which converge. 



5—2 



