Pathways of Radiobiological Damage 259 



the chromosome actually irradiated but which spread in the 

 course of 30 minutes to three times the original length. This 

 effect was only produced by protons when the dose was in- 

 creased more than 100-fold. Micro-beam irradiation of pro- 

 phase chromosomes with a few dozen protons was found to be 

 very effective in producing chromosome stickiness, and chrom- 

 osomes lacking functional centromeres and chromosome frag- 

 ments. Irradiation of metaphase chromosomes was about as 

 effective in producing chromosome stickiness but very rarely 

 destroyed the function of the centromere or produced chromo- 

 some fragments. In contrast to the marked effects of a few 

 dozen protons delivered to small chromosomal regions, 

 relatively huge numbers were ineffective when delivered to 

 similarly small extrachromosomal regions. Thousands were 

 delivered to ends of spindles, and hundreds of thousands to 

 cytoplasm. No effects could be seen at the irradiation sites 

 or elsewhere. An occasional sticky chromosome which was 

 seen could be ascribed to protons which were scattered into 

 the chromosomes when very large numbers were aimed at 

 extrachromosomal targets. 



In contrast to the protons, heretochromatic u.v. radiation 

 produced very striking effects when extrachromosomal regions 

 were given exposures of the same order as those which, given 

 to chromosomal regions, produced stickiness and loss of 

 centromere function. To destroy the spindle and induce a 

 deranged metaphase it is not necessary to include part of 

 the spindle in the u.v. irradiated region. A slightly greater 

 exposure of an equal volume of cytoplasm produces the same 

 effect, from which it was concluded that effects on the spindle 

 were quite probably due entirely to the absorption of u.v. 

 radiation by some cytoplasmic component. We may recall 

 here the observations of Carlson and McMaster (1951), who 

 used a variety of monochromatic u.v. radiations, that in 

 grasshopper neuroblasts the derangement of spindle mechan- 

 ism exhibited a protein-like action spectrum. 



It had, of course, long been believed that the chromosome 

 fragments seen at metaphase following the irradiation of 



