QUANTUM THEORY OF RADIATION ABSORPTIONS IN TISSUES 1323 

 1.70 X 10-13 X 8.29 X 10-3 



1280 



= 1.1 X 10-18 cc. 



The figure 1.1 X IQ-^^ cc. is, of course, not the gene volume but rather 

 the purely physical space within which the absorption is effective. The 

 gene could not well be larger than this volume but easily could be smaller. 

 A calculation adopting a somewhat different approach and coming to a 

 somewhat larger figure has also been made by Blackwood (3) on MuUer 

 and Altenburg's data. 



The number of different genes which might be discovered through 

 alterations produced by X-ray in a Drosophila sperm would be of the 

 order of 15,000 to 30,000. The number of loci in any one sperm would 

 be 9000 to 15,000. These estimates are larger than those derived from 

 Muller's or Demerec's data. They are also four times larger than the 

 estimates of the number of gene loci based on the supposition that the 

 bands of the salivary gland chromosomes represent loci of particular 

 genes. 



The foregoing analysis suggests the manner in which the total number 

 of gene loci within an organism may be estimated. It suffices to say here 

 that, depending on certain assumptions, the numbers necessary to this 

 small fruitfly seem to be not lower than 2000 or more than 15,000. 

 The magnitude of either of these numbers is entirely sufficient to show the 

 complexity and importance of the sensitive spot in the physical concept. 

 Possibly all these loci, with their contained genes, are essential. Cer- 

 tainly the rarely occurring natural mutations or the more rapidly pro- 

 duced mutations due to X-rays show that but 8 to 17 per cent of these 

 genes may be mutated and support life. 



It may be asked how the change in susceptibility of an object to 

 X-rays may be related to such a theory. In other words, what may be 

 the biological basis for the variation in sensitiveness of different materials? 

 The Drosophila egg is worth discussing from this viewpoint since so much 

 is known about it. The death rates of these eggs and of Drosophila sperm 

 are shown in Fig. 4. 



The Drosophila eggs at 2 hr. after laying are much more sensitive 

 to the X-rays than are the sperm. The two graphs for the sperm show 

 a fairly wide divergence in their results, perhaps nearly the extremes, 

 similar to that seen in the recessive lethals. The egg-survival curves 

 show a distinct curvature, whereas the curves for the sperm are straight 

 lines when plotted on the semilogarithmic grid. From this fact we 

 would infer that the egg requires more than one absorption in the vital 

 areas whereas the sperm needs but one to kill. Drosophila eggs are 

 about 0.5 mm. long and 0.2 mm. in width and depth. The nuclei within 

 them are diploid. In view of the fact that all eggs hatching earlier than 

 18 hr. are discarded, it would seem that the eggs making up these data 

 are at most only at the few-celled stage when they are laid. The time 



