S. H. REVELL 



Other than at the associations, although of course this is by itself quite incon- 

 clusive. If the exchange interpretation were correct then the nature of the 

 primary events would have to be known, and a method would have to be 

 devised which could detect them wherever they occurred, before the ' con- 

 tact ' question could be decided conclusively. 



It will be clear that the present exchange hypothesis is much less unlike 

 the breakage-and-reunion hypothesis of Sax, Catcheside and Lea than the 

 somewhat different version of this hypothesis proposed by Darlington and 

 others. It may be pointed out that the exchange hypothesis disposes of 

 those arguments for delaved reunion which are based on the observation 

 of chromatid and chromosome ' breaks ' in the same cell, and on the obser- 

 vation of certain rearrangements such as triradials. 



In conclusion, the reservation made in the introduction to this paper must 

 be repeated : the present hypothesis is only suggested as an explanation 

 of the aberrations which characteristically occur in cells about to undergo 

 mitosis, namely ' chromatid ' changes. Such a restriction may seem arbi- 

 trary, for it has been customary to consider observations on all types of 

 chromosome change together, and in general to seek a common explana- 

 tion for them all. If all this information were to be considered now, much 

 of it could certainly not be reconciled with the present scheme. In spite 

 of these discrepancies, the author considers that there is enough evidence in 

 favour of an exchange hypothesis for ' chromatid ' changes to justify its 

 being regarded as a possible alternative to the orthodox hypothesis. 



ACKNOWLEDGEMENTS 



The author is greatly indebted to Dr. L. H. Gray, to Dr. L. F. Lamerton, and to 

 Dr. A. Loveless for their advice and for criticism of the ideas presented in this paper. 

 This investigation has been supported by grants to the Royal Cancer Hospital and 

 Chester Beatty Research Institute from the British Empire Cancer Campaign, the 

 Jane Coffin Childs Memorial Fund for Medical Research, the Anna Fuller Fund, 

 and the Xational Cancer Institute of the National Institutes of Health, U.S. Public 

 Health Services. 



REFERENCES 



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2 Sax, K. Genetics, 1938, 23 494. 



3 Sax, K. ibid. 1940,25 41. 



* Catcheside. D. C, Lea, D. E. and Thoday, J. M. J. Genet. 1946, 47 137. 



5 Ford, C. E. Proc. 8th Intern. Cong. Genet. Suppl. to Hereditas, 1948, 570. 



« MuLLER, H. J. J. Genet. 1940, 40 1. 



■^ Loveless, A. Suppl. to Heredity, 1953. 6 293. 



8 Catcheside, D. G. Advanc. Genet. 1948, 2 271. 



^ Lea, D. E. Actions of radiations on living cells. Cambridge University Press, 1946. 

 10 Morrison, J. W. Suppl. to Heredity, 1953. 6 83. 

 " Darlington, C. D. and La Cour, L. F. J. Genet. 1945. 46 180. 

 1^ Lewitsky, G. a. and Araratian, A. G. Bull. appl. Bot. Genet. PL-Breed. 1931, 



27 part 1 (U.S.S.R.), 265. 

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 i» Thod.\y, J. M. Brit. J. Radiol. 1951, 24 572. 622. 

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253 



