800 



RADIATION BIOLOGY 



Darlington and La Cour, 1945; Rugh, 1950; Deufel, 1951). For the most 

 part these effects have been seen in fixed and stained material soon after 

 treatment, and the mitotic stage at which they were induced is not known 

 with certainty. Direct observations of the postirradiation history of 

 selected living grasshopper neuroblasts in stages of mitosis identified 

 before treatment has provided us with some exact information regarding 

 the stages at which these nonlocalized effects are produced and the rela- 

 tion of this to dosage (Carlson and Harrington, unpublished). With a 

 series of doses ranging from 64 to 4096 r it is found that successively 

 increasing doses give rise to successively increasing degrees of response as 

 follows: (1) delayed anaphase separation of a few of the sister chromatids 



Fig. 11-13. Chromosomes of Chortophaga neuroblast, showing X-ray-induced sticki- 

 ness at anaphase. 512 r; 30 minutes after treatment: A, early anaphase; B, late 

 anaphase. 



in cells that were in very late prophase at the time of treatment and, 

 therefore, the last cells to pass through mitosis before the interval of 

 minimal mitotic activity, (2) a similar effect but with many sister 

 chromatids involved (Fig. 11-13), (3) stickiness involving different 

 chromosomes, induced at very late prophase and evident in prometa- 

 phase, and anaphase, (4) fusion of different chromosomes by treatment at 

 the same stage in which it is observed, viz., prometaphase, metaphase, or 

 anaphase, and (5) fusion of all the chromosomes into a single, irregular 

 mass at prometaphase, metaphase, or anaphase as a result of treatment at 

 the same stage. Subsequently, this chromosomal mass elongates in the 

 direction of the poles and appears to be divided by a pressing inward of 

 the cleavage furrow (Carlson, 1941). 



It has been postulated that the stickiness induced in chromosomes by 

 irradiation is due to depolymerization of the thymonucleic acid of the 

 chromosomes (Darlington, 1942) and to an excess of nucleic acid charge 



