130 MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



The first condition determines the rigidity of the combination, 

 for multivalents with interstitial chiasmata seem to be less pliable 



I 



Fig. 43. — Diagram showing the division of trivalents in Hyacinihus. 

 Ai and B^, metaphase ; Ag, Bg and B3 the anaphase forms to 

 which they give rise. A divides into 2 -|- i, B divides into 

 i^ -f i^. Arrows mark the positions of centromeres which are 

 median. Note : (i) The chromatids are pulled apart distal to the 

 chiasmata. (ii) Where, as in Bg, the chromatids of one chromo- 

 some are pulled in opposite directions, they are held together at 

 the centromere, (iii) The centromere of a single chromatid is 

 pulled back in B3, where it is associated with a chromatid of a 

 whole chromosome. (After D., 1929 b. Aj, Fig. 34 f ; Aj, 

 Fig. 36 b and 38 ; B^, Fig. 34 r ; Bg, Fig. 36 n ; B3, Fig. 39). 



N.B. Fig. Bg shows that the holding together of the 

 chromatids at the centromere is something essentially different 

 from their attraction to one another in pairs elsewhere. This 

 difference is probably described by saying that the centromere 

 has not yet divided and the separation of the chromatids waits 

 upon its division. 



than those with only terminal chiasmata, and for two reasons : 

 the distance separating the centromeres of the associated chromo- 



