fusion of the chromosomes occur at every point 

 in the chromosome from the "hit" received by 

 the X rays, then the break can occur at random 

 at any point along the whole length of the chro- 

 mosome. But if secondary factors are involved 

 (such as torsion of chromosomes, or the rela- 

 tive positions, or differential contraction), then 

 the breaks must be localized. In order to solve 

 this question Sax analyzed the position of the 

 breaks in cells that had been irradiated when 

 the chromosomes were in the resting stage. 

 His analysis showed that 50% of the chromosome 

 breaks occur in the proximal third of the chro- 

 mosome arm. They are less frequent in the 

 central part of the arm and more^^ frequent in 

 the distal parts of the chromosomes. Then 

 plants which had been irradiated with 150 r were 

 analyzed in order to determine the types and 

 frequencies of chromosomal aberrations in 

 various stages of meiosis and mitosis. Up to 

 24 to 30 hours after irradiation the number of 

 chromosomal aberrations was 11% at 48 to 55 

 hours the number diminished, and between the 

 third and the eleventh day it went down to 3%. 

 At the same time the fertility of the pollen 

 attained 80%. On the twelfth day after irradia- 

 tion a small increase in the number of chromo- 

 somal aberrations was observed; on the four- 

 teenth day their number went up to 40%, with a 

 sharp rise in pollen sterility. Between the 

 fifteenth and the nineteenth days the sterility of 

 the pollen is so great that it is difficult to inves- 

 tigate the chromosomes. On the ninth dayl^the 

 fertility of the pollen increases up to 50% and 

 remains such for 10 days. By the end of the 

 fourth week the cycles of mitosis and meiosis 

 approach completion and the fertility of the 

 pollen becomes normal despite irradiation of 

 the premeiotic cells. 



It turns out that chromosomes in meiosis are 

 ten times as sensitive as chromosomes in the 

 resting stage of the microspore. Prophase of 

 mitosis is twice as sensitive to X rays as the 

 resting stage, but [after irradiation] at prophase 

 half of the breaks occur in the chromatids, 

 whereas [after irradiation] resting nuclei show 

 only chromosome breaks. 



In addition, by his experiments Sax [1938] 

 confirmed the observation of many other inves- 

 tigators that X-ray mutations are independent 

 of the temperature at time of irradiation. 



Savchenko (1940*) irradiated young shoots of 

 Crepis capillaris and fixed the rootlets immedi- 

 ately after irradiation. A cytological examina- 

 tion did not disclose any mitoses. On the basis 

 of this observation Savchenko comes to the 



*' Editor's note: instead of "more" the words "still 

 less" should have been used. 



'^Editor's note: text should read "19th day." 



conclusion that X rays do not impede the proc- 

 ess of mitosis if it is already under way, but 

 they do prevent the commencement of new 

 mitoses. In order to be able to form an accu- 

 rate concept of the nature of the X-ray effect on 

 cell division, the cells were chilled in order to 

 slow down their reaction rate. It turned out 

 that the chromosomes simply become swollen; 

 there were no other changes. Consequently, 

 X rays do not exert any direct effect on the 

 forming of chromosomes. Under irradiation, 

 cells simply cease to divide, and after irradia- 

 tion is concluded they remain in the resting 

 stage for some time. The duration of this con- 

 dition differs: 12 hours after irradiation the 

 author observed prophases and occasionally 

 metaphases; 18 hours after irradiation he ob- 

 served anaphases and telophases. In the first 

 cellular division after irradiation, fragments, 

 chromosomal breaks, and fusion of chromo- 

 somes were observed. 



Immediately after Muller's communication 

 was published in 1929 [1927?], LevitskiT also set 

 up experiments with irradiation of Crepis capil - 

 laris, whose chromosome set consists of three 

 pairs of distinctly differentiated chromosomes 

 (A, C, and D). The transformations of the 

 chromosomes were described in 1931 (Levit- 

 skiT, Araratyan, [Mardzhanishvil and Shepe- 

 leva]). In the spring of 1932 shoots of this plant 

 were irradiated once more and 20 specimens 

 that showed marked deviations were selected 

 from the plants which developed. From the 

 seeds produced by these freely self-pollinated 

 plants, Xi was obtained. This generation was 

 subjected to the cytological examination that 

 established the karyotype characteristics of 

 numerous plants. These data were published 

 in 1934 (Levitskii, Shepeleva, and Titova). Of 

 the 491 Xi plants examined, 84, or 17%, showed 

 modifications. The majority of them (74) were 

 "structural" modifications of the chromosomes, 

 either in pure form or in combination with 

 triploid (4) or trisomic (1) sports. There were 

 only 10 pure numerical changes: 6 triploids, 

 1 tetraploid, and 3 trisomies (of all the 3 possi- 

 ble [chromosomal] types). The most frequent 

 of the structural changes were translocations, 

 which previous to 1934 had been explained as a 

 simple transfer of part of one chromosome to 

 another. The same reasoning was applied to 

 explain the mechanism of transformations in- 

 duced by X rays (Levitskii and Sizova, 1934). 

 This reasoning, it appears, was confirmed by 

 an examination of reduction-division in karyo- 

 typic aber rants performed by Petrov (1935) on 

 Levitskii' s materials. Petrov described a 

 chain-like linkage in diakinesis as typical of 

 simple translocations. A more detailed exami- 

 nation of bulk material showed that "X-ray 

 aberrations in Crepis in the overwhelming 

 majority of cases (and perhaps in all cases) 

 are reciprocal" (LevitskiT and Sizova, 1934). 

 On the other hand, a reexamination of Petrov' s 

 material, performed on earlier stages, 



74 



