2 NUCLEUS 231 



in the Drosophila chromosomes is so large that for reasons of space 

 each gene must be bound to relatively small molecules of about the 

 same order of magnitude as found in the reserve proteins (compare 

 Fig. 90, p. 136) investigated in Svedberg's ultracentrifuge. It is 

 difficult to see, however, how such freely moving particles are able 

 to intervene decisively in the processes of development. To be able 

 to do this their carriers must have fixed mutual positions and it is 

 best to imagine that they are fixed on beaded protein fibrils. In this 

 way we comply with the requirement of Hnear arrangement in a 

 manner which could hardly be improved. 



In spite of its great probability, however, irrefutable proof of the 

 existence of the submicroscopic fibrillar structure has not yet been 

 produced. As has been shown, the quantitative evaluation of the 

 optical results suggests that the protein is not in a pronounced fibrillar 

 state possessing the characteristics of a chain lattice ; and thus far the 

 electron microscope has failed, because even the pachytene and diplo- 

 tene chromosomes yield only compact black shadows (Elvers, 1943) 

 showing fewer particulars than a good light optical image. It is of so 

 much the greater value that the experimental investigation of mutation 

 or ray genetics (Zimmer and Timofeeff-Ressovsky, 1942) opens new 

 perspectives. 



Target theory. Artificial mutations are induced by ionizing rays (UV- 

 rays, X-rays, y-rays). The dose of rays is measured by the X-ray unit 

 r (roentgen), which is defined as that amount of rays which will bring 

 about enough conductivity under prescribed conditions in a chamber 

 of I cm^ of air to permit a charge of one electrostatic unit to be 

 measured at saturation current. It is now established that the mutation 

 rate induced artificially by radiation is proportional to the dose of 

 rays brought to bear (Timofeeff-Ressovsky, 1940). The effect is 

 independent of the wavelength and the dose (intensity X time) can 

 be given all at once, concentrated or diluted, or else given at intervals. 

 There appears, therefore, to be no recovery. For instance, whereas the 

 sex-linked mutations in the x-chromosome of Drosophila have a 

 natural mutation rate of approximately 0.2%, the irradiation of 2500 r' 

 produces a rate of 7% and 5000 r produces 14^%. If the mutation rates 

 are plotted as a function of the dose, a straight line resuhs which 

 intersects the zero point; thus there is no threshold value and any 

 small dose will give an effect. 



