814 RADIATION BIOLOGY 



cleavage delay in Arbacia: streptomycin, adenosine, folic acid, 2,4- 

 diamino-5-p-chlorphenoxypyrimidine, or riboflavin. 



CELL MORPHOLOGY EFFECTS 



Chromosomes. After generative nuclei of Tradescantia pollen tubes 

 growing on artificial sugar-agar-gelatin medium had been treated in 

 prophase with a sublethal dose (3000 ergs/mm 2 ) of 2537 A radiation, 

 the metaphase chromosomes were much shorter and thicker than normal 

 (Swanson, 1942). This was apparently due to partial despiralization of 

 the chromonemata, for the number of coils in these chromosomes was 

 reduced from the normal twenty to about seven to ten per chromosome 

 and the gyres were increased in width. The chromosome matrix, which 

 was rarely seen under normal conditions, appeared as a transparent 

 hyaline mass surrounding and holding together the two chromatids. 

 Frequently the matrixes of two or more chromosomes were fused, 

 a change suggestive of the nonlocalized or "stickiness" effect found 

 immediately after large doses of ionizing radiations. None of these 

 changes were seen after treatment with a mixture of 2967 and 3022 A 

 radiation. 



If neuroblasts of the grasshopper are exposed to large doses of 2537 or 

 2650 A radiation and examined subsequently in the living, unstained 

 state, the chromosomes of cells in prometaphase, metaphase, and ana- 

 phase appear blurred and indistinct. Fusions between chromosomes may 

 lead to defective anaphase separation and the hourglass-shaped telophase 

 chromatin masses seen soon after large doses of ionizing radiations 

 (Carlson, 1941). A change in the prophase chromosomes from threads of 

 uniform diameter to a moniliform shape accompanies ultraviolet-induced 

 mitotic reversion at this stage. 



Achromatic Figure and Cleavage. One of the striking effects of certain 

 wave lengths of ultraviolet is their inhibitory action on spindle develop- 

 ment (Nebel et al., 1937; Carlson and Hollaender, 1948) and their destruc- 

 tive action on the fully formed spindle (Carlson and Hollaender, 1948). 

 Ultraviolet of 2250 A acts on the spindle and its precursor, the karyo- 

 lymph, in the grasshopper neuroblast very much as colchicine does 

 (Gaulden and Carlson, 1951). If the spindle is not well formed, the 

 karyolymph of the nucleus accumulates in one or more hyaline globules 

 and the spindle remains small, the centromeres of the chromosomes being 

 held in a compact group by the small spindle. Later, certain of these 

 centromeres may move poleward along the spindle, but anaphase is not 

 initiated until all have returned to the equatorial plate. Wave length 

 2804 A is equally effective in altering the spindle; wave lengths 2399, 

 2537, 2650, 2967, and 3022 A are less effective (Carlson and McMaster, 

 unpublished). The action spectrum resembles, therefore, a protein 

 absorption curve. Destruction of the spindle probably accounts for the 



