TRANSLOCATION 



149 



united at identical ends, i.e., if the parent chromosome was ahcdef 

 then the new chromosome was abccba. It gave a series of con- 

 figurations in accordance with this assumption. The most interest- 

 ing of these are the ones in which the two ends of the same 

 chromosome are associated, by a terminal chiasma, for they show 

 most clearly that the chromosome does not behave as a unit in 

 pairing and that pairing is not determined by a difference in the 

 origin of the mates. Similar configurations have been found in 

 trisomic Matthiola (Philp and Huskins, 1931). This condition has 



Pachyf-ene. 



Diphtene, 



Diskinests. 



Me tap h 



ase. 



Fig, 50. — Diagram showing the normal course of chromosome asso- 

 ciation in a simple interchange-heterozygote of the constitution 

 AB-BC-CD-DA (type, Campanula). The A segments 

 form two chiasmata at diplotene, the others one chiasma. 

 Terminalisation is complete at diakinesis. The arrangement is 

 " disjunctional " at metaphase ; circles denote the centro- 

 meres. (From D., 1931 c.) 



also been shown in regard to a small segment of the chromosome in 

 tetraploid Aucuba japonica (Meurman, 1929 b) and Tradescantia 

 virginiana. 



(v) Interchange Heterozygotes. In many diploid plants, both 

 natural races and artificial hybrids [v. Table 19) four, six or 

 more chromosomes instead of forming two, three or more pairs at 

 meiosis form a single association. This association, like those in 

 polyploids, is not constant, but is liable to be replaced by smaller 

 ones. Unlike those in polyploids, however, it is limited in its 

 variation, as all associations must be in diploids, by the fact that 

 each part of each chromosome has only one possible partner. Such 



