1248 



BIOLOGICAL EFFECTS OF RADIATION 



(178) has tested the mutation rate in young and old males, treated with 

 identical doses of X-rays, and found no difference in mutation tests 

 (Table 27). This he takes to indicate that mutation is not tied up to a 

 particular stage of cell division. If, however, nothing really happens 

 until the sperm gets into the egg, Timofeeff-Ressovsky's experiment has 

 only negative value. 



Table 27. — Age of Mature Sperm at Time op Treatment, and the Frequency 

 OF Sex-linked Mutations in Drosophila melanogaster 

 cf cf treated with 3500 + r-units; Timofeeff-Ressovsky (178) 



Experiment 



Young cf (^ irradiated and mated 

 immediately 



cf cf kept without females for 15 to 

 20 days, irradiated and then mated 

 immediately 



Number 



of 



gametes 



tested 



718 



539 



Number 



of 



lethal 



mutations 



82 



57 



Number 



of 

 "visible" 

 mutations 



Percent- 

 age of 

 sex-linked 

 mutations 



11.9 



11.1 



From all these considerations it follows that the conclusion either 

 of direct or of indirect effect cannot as yet be reached. The data are not 

 critical in this regard. It may also be remarked, that it is difficult to 

 conceive, when the mechanism of comparatively simple radiochemical 

 reactions is considered, that the process of gene mutation should be 

 affected by the release of electrons directly in the chemical sense, without 

 the intervention of successive reactions. 



THE CAUSES OF "SPONTANEOUS" MUTATION 



The "spontaneous" mutations constitute a single point on the curve 

 relating intensity to mutation rate. But, just as photosynthesis is 

 dependent on other factors than light, other agents than short-wave 

 radiation may be effective in changing the rate of mutation. The results 

 from other treatments — references need not be given — have been negative 

 in the main, with one exception. Yet it is not unlikely that the reason 

 for the failure is the high degree to which the nucleus is protected from 

 changes in the external environment. The one exception — changes of 

 temperature — agrees with the rule, since the extent of penetration does 

 not enter into such experiments. 



The temperature experiments constitute only an indirect attack 

 on the role of such other factors. Muller and Altenburg (114) and Muller 

 (107) found evidence of a slight increase in mutation rate which indicated 

 a temperature coefficient for the mutation process of the order of magni- 

 tude for chemical reactions. Later Goldschmidt (49), and following 



