308 



T{ADI\TT<)N HIOLOCY 



delay by the nidiation than are whole amehae. This cannot be inter- 

 preted as the result of shieldinp; of the nucleus by the larger amount of 

 cytoplasm in whole amebae, for both whole and half amel)ae si)read over 

 the substrate so that they are of approximately the same thickness. 

 Mazia and Ilirshfield (H)51) sug{i;est that the increased sensitivity 

 reflects an effect of the cytoplasm on recovery processes. They also 

 find evidence for a cytoplasmic effect of the radiation in the fact that 

 irradiated enucleate halves die more rapidly than the unirradiated halves. 



lOOn 



90- 



80- 



70 



o 60- 



50 



40- 



30- 



20- 



10 



2400 



— I 



3000 



2600 2800 



WAVE LENGTH, A 



Fig. 8-4. Action spectra for retardation of cell division in Paramecium, modified from 

 Giese (1945bj. Solid circle = time to third division — starved. Open circle = time 

 to recovery — well-fed. 



For Paramecium, the evidence as to localization of the effect is not ade- 

 quate. Giese (1945b) has found an ultraviolet action spectrum for 

 retardation of the third division in starved P. caudatum similar to the 

 absorption spectrum for nucleoproteins (Fig. 8-4). A similar action 

 spectrum was found for recovery of the normal rate in \vell-fed animals. 

 However, the action spectrum for time to the third division in well- 

 fed animals is rather nondescript, having a very slight maximum at 2804 A 

 (Fig. 8-5). Kimball et al. (1952) have been able to confirm, although 

 with differences in detail, the nondescript action spectrum for well-fed 

 P. aurelia (Fig. 8-5) but have been unable to demonstrate a nucleoprotein- 

 like spectrum for recovery of the normal rate. Giese (1947a) concludes 

 that "the immediate effect is upon the cytoplasm but the more lasting 

 effect is upon the nucleus." It does not seem to the reviewer that this 



