812 RADIATION BIOLOGY 



also studied. The latter gave a slight increase in the number of dividing 

 cells during the first 3 hours after treatment, while the former reduced the 

 number of mitoses to slightly below normal during 24 hours after treat- 

 ment. Since the numbers of cells on which these percentages are based 

 are not given, it is not apparent how significant they are. It is note- 

 worthy, however, that exposures of 0.5, 1.0, 1.5, and 3.0 hours all showed 

 a consistently similar effect on mitosis. It is a question also whether 

 the ultraviolet — especially the shorter wave lengths — would penetrate 

 the root tip to the periblem, let alone to its deeper layers. A differential 

 penetration by the wave lengths used is indicated by the different effects 

 of 2537 and 3650 A. Seide (1925) treated Ascaris eggs in various stages 

 of the first cleavage division with small doses of ultraviolet radiation of 

 2804 A and of mixed wave lengths from the mercury quartz lamp, but was 

 unable to demonstrate a positive shortening of the time required by the 

 eggs to reach the two- and four-celled stages. In 1945 Buschke, Frieden- 

 wald, and Moses described what appeared to be increased mitotic activity 

 following weak doses of nonmonochromatic ultraviolet radiation of the 

 rat cornea, but it was demonstrated in a later study (Friedenwald et al, 

 1948) that the effective agent was a gas — probably ozone — generated by 

 the ultraviolet lamp used in the treatments. 



Of the tissues examined for mitotic activity after exposure to ultra- 

 violet, only the corneal epithelium of the rat exhibits a compensatory 

 effect at recovery, i.e., a temporary increase in the number of mitoses in 

 excess of normal immediately after recovery (Friedenwald et al, 1948). 

 This peak is reached approximately 12 hours after treatment. 



It appears from present evidence that interphase and early prophase 

 are the stages most sensitive to ultraviolet radiation. Investigating the 

 radiosensitivity of different parts of the interval between the first and 

 second cleavages of the Arbacia egg, Blum and Price (1950) found that a 

 dose of radiation applied in the early part of this interval caused the 

 greatest mitotic delay, the radiosensitivity then diminished to a minimum 

 at 12 minutes; and from this time to the completion of the second cleavage 

 division, irradiation did not induce delay. In studying the effects on the 

 grasshopper neuroblast of 2250 and 2537 A radiation, Carlson and Hol- 

 laender (1944, 1948) found that, if the radiosensitivity was measured by 

 the time required for a cell treated in one stage to reach the next stage, 

 early prophase was the most sensitive; for cells treated in either inter- 

 phase or early prophase remained in early prophase an excessively long 

 time. If, on the other hand, the increase in the time required by the cell 

 to progress from the stage in which it was treated to anaphase was the 

 criterion of sensitivity used, at both wave lengths interphase was found 

 to be more sensitive than early prophase (Carlson and Hollaender, 1948). 

 A detailed analysis of the radiosensitivities of different stages of mitosis to 

 2250 A, as indicated by their delay in reaching anaphase, showed middle 



