386 RADIATION BIOLOGY 



also showed that centrifuging, which of course causes the chromosome 

 parts to be moved about, increases the frequency of structurally changed 

 chromosomes produced by irradiation. So too does sonic vibration 

 (Conger, 1948). 



Another limiting factor in the production of structural changes is their 

 time restriction. According to Sax's (1939, 1940) interpretation of his 

 results with Tradescantia microspores — an interpretation accepted by 

 most other workers but now disputed by Lane (1951), the broken end of 

 an interphase chromosome has little chance of undergoing union with 

 that derived from another break unless the two breaks have been pro- 

 duced within a rather short time interval of each other, usually of the 

 order of some tens of minutes ; otherwise the broken end will in the great 

 majority of cases have come into contact with the complementary end 

 derived from the same break, so as to undergo restitution. This interpre- 

 tation is based on Sax's finding that a given dose of radiation applied to 

 interphase cells of microspores, if fractionated in time or delivered at a 

 low intensity over a long period, produces fewer structural changes, pre- 

 sumably because of more restitutions, than if delivered in concentrated 

 manner in a few minutes or seconds. A higher temperature during treat- 

 ment has been shown by Sax and Enzmann (1939) to have an effect on 

 this result similar to that of lengthening the time; this would be because 

 it causes a speeding up of the movements whereby the ends meet each 

 other.2 Lane (1951) finds, however, in his experiments on Tradescantia 

 microspores, that irradiation causes chromosomes to acquire a resistance 

 (which for a considerable period continues to increase) to breakage by 

 later irradiation, and he believes the lesser efficiency of prolonged and of 

 fractionated radiation (and presumably of that applied at a higher 

 temperature) to be entirely explained in this way, without assuming that 

 union can occur during interphase, in this material. Nevertheless, the 

 fact that in many experiments the structural changes produced by 

 irradiating long before mitosis involve whole chromosomes rather than 

 chromatids shows that in these cases union occurred before effective 

 chromosome reproduction into chromatids occurred, i.e., at some time 

 during interphase, when the chromosomes were in an extended condition. 



In recapitulation, it may be observed that the far lower frequency of 

 both viable (eucentric) and inviable (aneucentric) structural changes 

 induced by irradiation of ordinary interphase stages than by that of 

 spermatozoan and mitotic stages must in considerable measure be the 

 result of the greater spatial limitation on union of broken ends when the 



2 It will be noted in Chap. 8, however, that there are one or more other ways in 

 which temperature can influence the frequency of production of structural changes, 

 quite apart from that discussed above. These other types of temperature influence 

 may operate even when condensed chromosomes are irradiated. 



