CELL DIVISION, MORPHOLOGY, VIABILITY 



813 



and late prophase to be only slightly less affected than early prophase, 

 and metaphase to be much less sensitive. Cells in middle prophase were 

 caused to undergo temporary regression by a dose of 34,650 ergs/cm 2 

 and late prophase by one-fourth as large a dose. Recovery and progres- 

 sion through anaphase followed. Delay induced in prometaphase and 

 metaphase cells is probably the result of damage to the spindle-forming 

 substance and spindle, respectively (see p. 814). 



Intensity. In time-intensity studies the problems encountered in the 

 choice of materials and methods and in the interpretation of results are 

 essentially the same for ultraviolet as for high-energy radiations (see p. 



t.oo 



0.80 

 0.60 



0.40 

 Q 



I- 

 < 



cr 

 o 



p 0.20 



0.10 

 0.08 



0.06 







20 



24 



4 8 12 16 



xlO 3 ERGS PER CM 2 

 O LOW INTENSITY • HIGH INTENSITY 



Fic;. 11-16. Effects of different intensities of 2537 A ultraviolet radiation on the 

 mitotic ratio of treated to control cells (Chortophaga neuroblasts) (after Carlson and 

 Hollaender, 1945). 



788). In addition, the relatively slight penetration of many ultraviolet 

 wave lengths into organic materials limits drastically the kinds of cells 

 amenable to mitotic studies. Carlson and Hollaender (1945) compared 

 the effectiveness of selected doses of 2537 A radiation delivered in 3.5-13 

 seconds and in 3.75-4 hours. It was found that the number of cells pass- 

 ing through mitosis in a 2-hour period beginning approximately 4 hours 

 after the mid-point of the irradiation period was decreased to approxi- 

 mately the same extent by doses of 5760 or 11,520 ergs/cm 2 , but that at 

 doses of 17,280 or 23,040 ergs/cm 2 the brief, high-intensity exposure was 

 considerably more effective in reducing the mitotic activity than the pro- 

 longed, low-intensity one (Fig. 11-16). 



Chemical Agents. Marshak (1949) found that none of the following 

 chemical agents were effective in altering 2537 A ultraviolet-induced 



