318 RADIATION BIOLOGY 



lo heat. Gipso (194r)a) has shown that viRil)l(' lijj;ht hy itself can do so 

 Tor the ciliatc liU phitrismn, wliicli contnins u natural pliolodynjimic 

 pifiiiKMit . 



Ciicsc ami Heath ( M> IS) dcnioiistrated that paramocia can be sensitized 

 to heat by sublethal doses of X rays. They emphasize that dosage calcu- 

 lations with the thill window 1ul)e employed are subject to much ((uestion 

 l)ut give 50,000 r/min as an approximate rate. On this basis, it can be 

 estimated from their data that do.ses between lOO.OOO and 300,000 r are 

 necessary, depending on the nutiitive state of the animals, to reduce the 

 time for death at 42°C" to half. Well-fed, rai)idly dividing animals are 

 more sensitive than starved ones. This finding contrasts with Giese and 

 Reed's finding (1940) that well-fed paramecia are more resistant to the 

 division-retarding eifects of ultraviolet than are starved ones. Thus one 

 cannot speak of a general resistance to radiation but only resistance to 

 specific effects of specific radiations. Recovery from heat sensitization 

 was shown to occur when the paramecia were fed after irradiation but 

 not when they were starved. Giese and Heath (1948) conclude that 

 recovery must involve the synthesis of new materials bj'' the cell. 



Giese and Grossman (1945b) .suggest that radiation partially denatures 

 proteins. Exposure to heat is then supposed to complete the denatura- 

 tion and so lead to death more rapidly than in animals not exposed to 

 radiation. Giese (1947b) found evidence for sensitization of nucleo- 

 proteins to heat by ultraviolet. Partially purified nucleoprotein from 

 Strongylocentrotus sperm, dissolved in 2 M sodium chloride, forms threads 

 when poured into dilute sodium chloride. Brief exposures to ultraviolet 

 (mainly 2537 A) followed by a 10-min exposure to 80°G changed the 

 luicleoprotein so that the threads did form. The reverse procedure, 

 exposure to 80°C and then to ultraviolet, had no effect. 



Miscellaneous Microscopicalli/ Visible Changes. ^Microscopically visible 

 changes in the nucleus of the cell are treated by Garlson (Ghap. 1 1 , volume 

 1 of this series). Some of the more recent observations of various changes J 

 in both the nucleus and cytoplasm in protozoa and invertebrate eggs are | 

 summarized in Table 8-3. No attempt has been made to include papers 

 in which microscopic observations were merely incidental to other work. 

 As has been pointed out in a previous section, many of these changes may 

 be those that occur in dying cells. 



Various Physiological, Biophysical, and Biochemical Effects. For the 

 most part, the work on protozoa and invertebrate gametes has not been 

 directed to a study of the enzyme systems affected by the radiation, the 

 colloidal changes in protoplasm, the changes in permeability of mem- 

 branes, and alterations in the chemical composition of the cell. Infor- 

 mation of an indirect sort on some of these matters has been mentioned 

 in preceding sections of this chapter but direct investigations are scarce 

 and are listed briefly in Table 8-4 without discussion in the text. 



