CELL DIVISION, MORPHOLOGY, VIABILITY 801 



(Darlington and La Cour, 1945). The in vitro nucleic acid studies of 

 Sparrow and Rosenfeld (1946) and Taylor, Greenstein, and Hollaender 

 (1947, 1948) showed a viscosity fall that indicated at least a partial 

 depolymerization of the nucleic acid, and the viscosity continued to fall 

 for several hours after the cessation of X irradiation. This parallels the 

 results obtained by Harrington and Koza (1951), who found that the 

 methyl green staining reaction of X-rayed grasshopper chromosomes 

 reached a minimum as late as 10 hours after X-raying. It is probable 

 that many of the abnormal anaphases studied by Marshak (1938) in 

 Vicia and Allium root tips 3 hours after X irradiation were the result of 

 nonlocalized rather than localized effects. Though depolymerization of 

 nucleic acids is interfered with at high pH, it seems doubtful whether the 

 capacity he found for increasing concentrations of ammonium hydroxide 

 to decrease the percentage of X-ray-induced abnormal anaphases could 

 be due to a raising of the intracellular pH to the level necessary for such 

 an effect. 



Changes may also be induced by X rays in the intranuclear chromo- 

 somes. The studies of Duryee (1939, 1947, 1949, 1950) were made on the 

 later stages of amphibian oocytes, when the "lampbrush" chromosomes 

 of the large germinal vesicle consist of chromonemata with chromomeres 

 spaced at intervals along them and numerous lateral loops attached at 

 both ends to the central chromonemata. Doses of 5000-10,000 r and 

 more produced breakage in the lateral loops. Chromonemata were occa- 

 sionally broken by 10,000 r; multiple fragmentation was produced by 

 30,000 r and more of X rays (Fig. 11-14). Such changes, which were 

 visible 15 minutes after treatment, appear to be prophase manifestations 

 of chromosome breakage that are not discernible in most cells until the 

 succeeding metaphase and anaphase, when they would be classed as 

 chromosome aberrations. 



The chromosomes of the Chortophaga neuroblast, after 250 r of X rays, 

 undergo changes suggestive of reversion to an earlier stage of mitosis 

 (Carlson, 1940). The diameter of the chromosome thread resembles, 

 successively, that of the middle prophase and then of early prophase 

 cells. Instead of being uniform in diameter from end to end, however, it 

 has a beaded appearance, consisting of granules separated by narrower 

 regions. It finally acquires the coarsely granular character of the inter- 

 phase nucleus. This is a reversible, and not a degenerative, change, 

 however, for at the time of recovery these cells pass through mitosis in an 

 apparently normal fashion. 



Marquardt (1938) described excessive relational coiling of chromatids 

 in Bellevalia microspores 2-3 hours and abnormally short metaphase 

 chromosomes 3-4 hours after X-raying. Defective internal and rela- 

 tional coiling of the chromatids, according to Darlington and La Cour 

 (1945), results from X-ray-induced depolymerization of the chromosome 



