1178 BIOLOGICAL EFFECTS OF RADIATION 



scheme and that of Belhng consists merely in that according to BelUng 

 only reciprocal translocations and inversions, but no simple translocations, 

 can be produced. A third possibility is that chromosomes are frag- 

 mented by X-rays, and the fragments possessing no spindle fiber are 

 either lost or somehow attached to other chromosomes. Fragmentation 

 of chromosomes due to X-rays was, indeed, observed even before the 

 discovery of inheritable chromosome rearrangements (Hertwig, 53; 

 Mohr, 67; Alberti and Politzer, 1). Direct cytological observations on 

 chromosomes in treated cells would seem the logical way toward a 

 solution of these problems. 



Lewitsky and Araratian (59) studied the effects of irradiation on 

 chromosomes in root tips of Crepis and other plants, employing various 

 intervals of time after treatment. They observed secondary, tertiary, 

 and even quaternary reconstructions of the chromosome apparatus. 

 This suggests that an X-ray treatment may have an aftereffect on the 

 production of chromosome rearrangements. Since no aftereffect is pro- 

 duced on gene mutations (Timofeeff-Ressovsky, 135), further observa- 

 tions are much needed. Lewitsky and Araratian describe no pictures 

 suggesting a rupture of chromosomes due to their sticking together, but 

 neither have they seen a union 'between fragments once separated. 



The problem of the existence of simple translocations is likewise not 

 settled. In plants, especially in maize and Oenothera, all known trans- 

 locations are reciprocal, though the exchanged sections may be extremely 

 unequal in length. Many reciprocal translocations are known in 

 Drosophila (Sturtevant and Dobzhansky, 134; Dobzhansky and Sturte- 

 vant, 36; Bolen, 11; Oliver and Van Atta, 91; and others), but other 

 translocations seem to be simple (Muller and Painter, 80; Muller, 76; 

 Dobzhansky, 25, 29, 32; and others). Both reciprocal and simple 

 translocations occur in Orthoptera (Nabours and Robertson, 84; Helwig, 

 52). It should, however, be kept in mind that what appears to be a 

 simple translocation may actually be a reciprocal one, involving an 

 exchange of extremely unequal sections. The fact that the very small 

 fourth chromosome may participate in reciprocal translocations (Bolen, 

 11) is very suggestive in this respect. The introduction of the method 

 of studying chromosomes in the salivary glands of Drosophila (see below) 

 may be expected to bring a final solution of this problem. 



Although the possibility that all translocations are reciprocal is not 

 excluded, it has, certainly, not been proved conclusively. Some data 

 argue rather against it. Dobzhansky (32) studied nine translocations 

 involving transfers of sections of the second chromosome to the F-chro- 

 mosome of Drosophila. If these translocations were reciprocal, a section 

 of the Y should be found in every case attached to the remnant of the 

 second chromosome. Since the F-chromosome is a long one, these 

 sections might be expected to be cytologically visible in at least some 



