S. H. REVELL 



at metaphase would themselves be the result of exchange : hence the ob- 

 served ' breaks ' would not represent the residue of primary aberrations, the 

 rest of which had either reunited in pairs as exchanges or else restituted. Inter- 

 preted in this way, the visible changes thus provide no evidence of ' breakage 

 first ', or indeed of breakage at all. Of course, it might still be postulated 

 that both intra- and /«^«7-changes arise by breakage and reunion, but this would 

 now be an entirely separate question connected with the general problem 

 of whether resting-stage ' chromosomes ' are actually permanent structures. 



If the new hypothesis were correct, then, for the reason already given, it 

 could hardly be expected at present that much could be deduced about the 

 exchange process, except in the most general terms. But it is clear that, for 

 exchange to occur, the loci involved must come close together at some 

 stage ; also, it seems difficult to conceive of such an exchange as other than 

 an organized process. At least for the author, it is difficult to see how ex- 

 change could be the result of any random damage and repair, even if such 

 events were guided by a proximity of 1 [ji. Nor does it seem to the author 

 that the incomplete exchanges themselves constitute evidence that a breakage 

 process is involved : these should be regarded without prejudice simply as 

 imperfect exchanges. The case for considering the exchange as a develop- 

 mental entity, until more is known about it, is greatly strengthened by 

 the fact that exchange does actually occur as a physiologically determined 

 process at meiosis, where it seems to be connected essentially with chromo- 

 some doubling. It would obviously be tempting to regard the aberrations 

 as induced heterologous exchanges of the same developmental type : at 

 present, however, such a comparison must only be considered with caution. 



Thus the question which was posed earlier in this paper in connection 

 with the homologous exchanges alone now arises again with a much wider 

 application. Are there inter- and /«/rfl-chromosomal associations in somatic 

 cells which may be defined by their capacity to exchange in response to 

 stimuli such as ionizing radiations ? 



It seems to the author that, whatever may be the general view of this 

 hypothesis, there need be no essential conflict between it and the observed 

 dose-/n/frchange relation for X-rays, although at first sight this might appear 

 to be so. It is agreed that all types of ' chromatid ' aberration increase 

 linearly with doses of the densely ionizing radiations (Giles^^, Thoday^*' ^^ ; 

 KoTVAL and Gray^^), and because of this and for other reasons it has been 

 deduced that chromosome loci have to be very close together [ca. 1 [x, Lea ^) 

 before exchange is possible between them, and that this is so even with X-rays 

 where two electrons are necessary. This last fact, however, does not by 

 itself appear at all to exclude the possibility that the chromosomes may be 

 in ' contact ' — if this means their being functionally associated so that they 

 can exchange — unless ' contact ' is also specifically defined as capacity to respond to 

 a single X-ray hit. Now this is, of course, exactly the definition which has 

 been adopted, and it is possible that it might be a misleading one since it 

 has always been so closely bound up with the assumption that it is two breaks 

 which are the two primary events associated with the two ionization paths. 

 It is just this assumption which is now in question. 



So far as the author can judge, it may equally well be proposed that an 

 exchange process is initiated at the chromosome association, but that there 



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