720 RADIATION BIOLOGY 



asymmetrical types in these two categories — dicentrics (Plate I-H, I, K, 

 L, M, N) and centric rings (Plate I-I, 0), respectively — can be regularly 

 scored, although symmetrical interchanges can be detected occasionally 

 when they are very unequal (Plate I-J). 



RATIO OF SYMMETRICAL AND ASYMMETRICAL EXCHANGES 



Symmetrical and asymmetrical chromatid interchanges appear to be 

 about equally frequent according to Catcheside, Lea, and Thoday (1946a), 

 although the data of Sax (1940) suggest that asymmetrical interchanges 

 are more frequent. These latter results are probably biased somewhat, 

 since cells at anaphase as well as at metaphase were scored, and sym- 

 metrical exchanges cannot be detected at anaphase. The observations of 

 Catcheside et al. (1946a) also indicate that symmetrical and asymmetrical 

 chromatid intrachanges are equally frequent and these authors state that 

 such results argue against the existence of any polarization in chromo- 

 somes which would prevent the random joining of breakage ends. 



DISTRIBUTION OF BREAKS IN CHROMOSOMES 



It is not possible to determine directly in Tradescantia the distribution 

 of initial breakage, since the processes of restitution and reunion, to be 

 considered later, intervene. There is, in fact, evidence that more aber- 

 rations tend to arise from breaks produced in the proximal rather than in 

 the distal regions of a chromosome (Sax, 1940). Although such evidence 

 may indicate a nonrandom distribution of initial breaks, it appears more 

 likely that it indicates a nonrandom reunion of broken ends, resulting 

 from the operation of secondary factors influencing reunion and restitu- 

 tion. The more proximal distribution of aberrations has been attributed 

 by Sax (1940) to stresses imposed in the region of the centromere by the 

 various coiling mechanisms. 



DISTRIBUTION OF ABERRATION TYPES AMONG NUCLEI 



The distribution of various chromatid and chromosome aberration 

 types among nuclei following X irradiation and neutron irradiation has 

 been studied by Catcheside, Lea, and Thoday (1946a) and Rick (1940). 

 The distributions observed are in accordance with the Poisson formula, 

 and this evidence, especially that for chromatid breaks, is taken as com- 

 patible with the view that single breaks in a given chromosome are pro- 

 duced by the action of single ionizing particles and are unaffected by the 

 presence or absence of other breaks in the cell. 



SPONTANEOUS CHROMOSOME ABERRATIONS 



Of interest in connection with radiation experiments are observations 

 on the types and frequencies of spontaneous aberration types in micro- 

 spores. Such data are in fact necessary as controls for comparison with 



