510 



CELL MECHANICS 



longest chromosomes it is never complete. In Fritillaria (D., 

 1930 c) terminalisation is complete in the small fragments at late 

 diplotene although it never has any effect in the long chromosomes, 

 beyond leading to the movement of the distal interstitial chiasmata 

 to the ends. This difference is due in part to the chiasmata being 

 formed nearer the ends in short chromosomes and in part to their 

 being nearer the centromere. 



(h) Number of Chiasmata. Where the degree of terminalisation 

 is low, as in Lilium (Belling, 1931) and Tulipa (D. and Janaki- 

 Ammal, 1932), the number of chiasmata terminal, and even the 



I J V 



Number of Chmmafa 



[only Late 



Diplotene Oiplotene 



forly 

 Diakinesis 



Lafe 

 Diakinesis 



Fig, 143. — (Left) Chiasma frequency polygon of Zea Mays where 

 no fusion of chiasmata occurs. (Right) Terminalisation in 

 bivalents with different total numbers of chiasmata, cf. Fig. 93. 

 (D., 1934) 



proportion terminal, is higher the greater the number of chiasmata 

 in the bivalent (Fig. 142). The Tulipa observations show that 

 this is due to failure of chiasmata lying in one arm only to be ter- 

 minalised. It appears therefore that, other things being equal, 

 movement is increased by the centromere lying in a closed 

 loop. 



(c) Chromosome Homology. In certain organisms with more or 

 less complete terminalisation interstitial chiasmata are occasionally 

 found, sometimes very near the centromere. When these were 

 observed in a known structural hybrid (Tradescantia, D., 1929 c) 

 the explanation suggested was that terminalisation was arrested by 

 a change in the homology of the chromosomes. Thus, if two dis- 

 similar chromosomes with a linear sequence abcdef and abcxyz with 



