816 RADIATION BIOLOGY 



shift occurs and two cells approximately equal in size result. That this is 

 caused by the reduced spindle size is evidenced by a similar phenomenon 

 in colchicine-treated neuroblasts, in which the spindle is also reduced in 

 size (Gaulden and Carlson, 1951). If the dose of radiation is large enough 

 to damage the spindle to such an extent that the chromosome halves do 

 not separate, even at the time when pseudopodia-like outpushings of the 

 cell indicate the onset of telophase, the chromosomes may all be incorpo- 

 rated in a single nucleus. One of the pseudopodia is eventually cut off by 

 a furrow from the nucleated portion of the cell. 



Nucleolus. The form of the neuroblast can be altered drastically by 

 all ultraviolet wave lengths from 2250 to 3130 A (Carlson and McMaster, 

 1951). This effect is studied most readily in hanging-drop preparations 

 of the living cell. Normally the nucleolus of the neuroblast appears as an 

 irregular mass of low refractility during interphase and most of the pro- 

 phase. Within 25-35 minutes after irradiation the nucleolar mass 

 becomes transformed into about ten nucleolar fragments, which gradually 

 separate to form a cluster of highly refractile spherules. If the dose is 

 large, these subsequently fuse to form fewer and larger spheres, until, 

 ultimately, a single, clearly defined, spherical body results. If the dose is 

 less than that sufficient to produce this series of changes, recovery by a 

 return to the original condition may take place at any stage. The 

 2399-2804 A region of the spectrum is most effective in producing this 

 change, which suggests that absorption by both protein and nucleic acid 

 may be involved in its production. 



VIABILITY EFFECTS 



Few studies have been made on the capacity of ultraviolet radiations to 

 kill animal cells soon after treatment; these are all based on mixed wave 

 lengths and contain no exact information on the doses used. Politzer and 

 Alberti (1924) described necrosis of the cells of the upper two layers of the 

 cornea of salamander larvae 1 hour after ultraviolet irradiation. Mollen- 

 dorff and Laqueur (1938) found that, after moderate doses, fibroblasts in 

 culture recover and progress through mitosis normally, but after large 

 doses they break down on attempting division at the end of the radiation- 

 induced, mitosis-free period. They suggest that ultraviolet irradiation 

 produces a toxic substance, which acts at the time of division to prevent 

 the successful completion of mitosis. Buschke' et al. (1945) recorded 

 aggregation of nuclear chromatin in clumps, followed by fragmentation 

 of nuclei within 6 hours of treatment, and finally complete destruction of 

 the cells of the two uppermost layers of the rat cornea. Both low tem- 

 perature and anaerobiosis, produced either by immersion of the enucle- 

 ated eye in buffer solution without stirring or by maintaining it in a 

 vacuum, caused a lag in the appearance of nuclear fragmentation. From 

 this they concluded that cell disintegration resulting from ultraviolet 



