CHROMOSOME BREAKAGE BY DIEPOXIDE AND BY X-RAYS 



between homologous chromosomes at apparently homologous points, as 

 found by Revell^- "^ in Vicia. As regards localization of breaks in general, 

 this is difficult to estimate in Tradescantia owing to the close similarity in 

 form between the different chromosomes. Further analysis should enable 

 me to make a decision on this important point. 



A few minute chromosome fragments (m) are found after all treatments, 

 often associated with chromatids of unequal length or chromatid breaks. 

 These are probably mainly minute interchanges of the type figured by 



Table II. — Frequency of mitoses and of micronuclei after treatment with M 12,000 di-{2 : 3-epoxypropyl) 

 ether. Roots grown in 0-05 per cent colchicine for 5 hours before fixation. Two slides for each 

 treatment 



RevelP in relation to his explanation of the mode of origin of the chromosome 

 breaks with sister reunion. 



An occasional dicentric chromosome appears in the 48-hr sample from 

 10-min treatment. At this time-interval after the smaller dose no primary 

 breakage is seen. Most of the mitoses are evidently of the second cell 

 generation after treatment (Tg). These dicentrics are accompanied by a 

 micronucleus in the same or neighbouring mitosis, and clearly have their 

 origin in asymmetric interchanges, the micronucleus representing the acentric 

 fragment. Similarly, neighbouring cells in this sample are found with short 

 chromosomes of equal size, the products of separation of centric fragments 

 which have not undergone sister reunion. Sister reunion is frequently 

 absent in the centric fragment when the break is close to the centromere. 



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