5i8 



CELL MECHANICS 



the displacing association is the same as the terminal one owing to 

 the chiasmata being comparate) or the lateral association at the 

 end of the displaced thread will be lost (as in cases (c) and (d) where 



D/fi/o/'ene. 



^^•- Z-'^'s^ 



Diakinesis. 



•' — -^ (P) 





Fig. 147. — Diagram to show terminalisation in a quadrivalent giving 

 a quadruple chiasma and the possibility of " double reduction " 

 (or "equational exceptions ") following crossing-over between 

 chromatids (on the chiasmatype theory), a, b, c, d and e, diplo- 

 tene chiasmata and points of crossing-over, x, y and z, terminal 

 chiasmata. P and p, alternative factors in respect of which the 

 organism is triplex (PPPp). Segregation will give double 

 reduction in a proportion of cases yielding reduced nuclei of the 

 constitution pp. Arrows show the positions of the centromere. 



the three associations are different owing to the chiasmata being 

 disparate). The terminal association, therefore, cannot be 

 eliminated by cancellation (as suggested earlier by the present 

 writer, 1929 c), it can only be changed (as in {b) ) to become the 

 lateral one. 



