MOVEMENT OF CHIASMATA 505 



proportion of ends of chromosomes is found associated at metaphase 

 by terminal chiasmata in any organism as there were arms earher 

 associated by interstitial chiasmata. One change only may take 

 place, and this in special circumstances, the breakage of the 

 attachment between one pair of chromatids. Where four 

 chromosomes are associated by a quadruple chiasma the fusion of 

 chiasmata necessary for its formation may break one of the four 

 chromatid associations, so that the cross-quadrivalent becomes a 

 special kind of chain-quadrivalent (Fig. 42, IV, c). This gives an 

 imperfect chiasma (D., 1929 c). 



Chiasmata are first seen through the opening of loops separating 

 pairs of chromatids at diplotene. Their position cannot be accu- 

 rately recorded at the very moment of origin, but nevertheless when 

 the records of the earliest stages are compared with those of diaki- 

 nesis and metaphase it is usually found that they have changed in 

 position. The change has not been observed directly in living 

 nuclei, but it is inferred from a comparison of records at the two 

 stages in fixed preparations which show great constancy in its 

 character. This comparison reveals the conditions summarised 

 earlier (Ch. IV) for the purpose of understanding the form of the 

 metaphase bivalents, namely, the occurrence of a range of types of 

 behaviour from those in which the change is scarcely detectable to 

 those in which the form of the bivalent is entirely altered owing to 

 complete terminalisation of chiasmata. The proportion of the total 

 chiasmata that are terminal in any given type of chromosome or 

 at any given stage is conveniently used as a terminalisation coefficient 

 (D., 1931 d). These different kinds of behaviour now call for exact 

 analysis to show the changes in number and position of chiasmata, 

 i.e., in the association of the chromatids, for several reasons. 

 Observations of meiosis are usually confined to metaphase. An 

 exact understanding of the kinds of change in bivalent structure 

 which lead up to metaphase is necessary in interpreting these 

 observations. Such an analysis is also necessary in deciding what 

 forces hold the paired chromosomes together during and after the 

 observed changes in their relationship, and what forces determine 

 these changes. Further, the changes in number and position of 

 chiasmata during prophase had to be followed in order to know 



