two homologous fragments. All these fragments 

 can be traced through two generations. A sixth 

 plant had 22 bivalents, two more or less com- 

 plex elements and one univalent. A sister plant 

 had 22 bivalents, one trivalent, and one uni- 

 valent. On the basis of these observations the 

 conclusion was reached that certain changes in 

 the external morphology of the chromosome due 

 to the influence of X rays represent a structural 

 reorganization of the chromosomes. In some 

 plants these changes do not bring about a com- 

 plete loss of viability; hence it is possible to 

 observe the chief categories of structural changes 

 in the chromosome in subsequent generations. 

 The rapid increase in the amount of data avail- 

 able on the genetic effects of X rays calls 

 attention to these changes which have great 

 potential significance for the evolutionary 

 processes, especially since, at the present 

 time, it has been established that such modifi- 

 cations often arise under natural conditions. 



In their subsequent work Goodspeed and 

 Avery (1934) describe 14 types of plants which 

 arose from a single, irradiated sex cell of 

 Nicotiana tabacum . A cytological investigation 

 of the parent plant showed a change in the re- 

 organization of its chromosomes under the 

 influence of X rays. Some of the derivatives of 

 this plant had homologous fragments that conju- 

 gated with each other. One plant (27. 154 P004) 

 had a configuration in the first reduction-division 

 which can be characterized by the formula 

 21n -I- 5i -(- I to 2f (f signifies fragment, II signi- 

 fies bivalent, and I signifies univalent), one of 

 the two fragments being joined to one of the 

 bivalents. Configurations 22n -(- 3i -I- 1 to 2f 

 or 23n + li -H I to 2 f were also observed. These 

 configurations were responsible for the varia- 

 tions in the number of units found in the various 

 pollen mother cells. At least one of the unpaired 

 chromosomes contained in this plant represented 

 structural change as can be deduced from its 

 size. In this experiment no less than five chro- 

 mosomes of the haploid set underwent structural 

 changes under the influence of X rays. This 

 was manifested by the appearance of fragments, 

 small univalents (unpaired chromosomes), and 

 abnormal bivalents (pairs). The authors traced 

 these changes down to the sixth generation from 

 the irradiated plant. These three articles are 

 sufficient to give an idea of the kind of chromo- 

 somal changes found by Goodspeed and his co- 

 workers. 



For the purpose of increasing the number of 

 modifications M. Navashin (1931) exposed seeds 

 of Crepis tectorum to X rays intermittently for 

 40 to 2200 seconds. He used 10 batches of 

 seeds. All these plants produced shoots, but 

 the irradiated plants showed various abnormali- 

 ties in the formation of the first leaves. The 

 number of abnormalities increased proportion- 

 ately to the X-ray dosage. Cytological exami- 

 nation of the root tips, taken from the plants 

 three months after sprouting, revealed an 



abnormally high percentage of modifications. 

 All the changes in the chromosomes were trans- 

 locations. Observations revealed that parts of 

 chromosomes, regardless of size, could be 

 translocated to any other chromosome. It is 

 interesting to note from these observations that 

 the centromere (kinetic spindle) displays an 

 amazing stability. All these chromosomal 

 changes did not affect the viability of the cells 

 or of the organs. 



LevitskiT [et al] (1931) also describes frag- 

 mentation, loss of chromosome parts, and 

 translocation of parts of some chromosomes to 

 others. (The reciprocal nature of the trans- 

 locations was eventually established.)^ In 

 Secale cereale two of the 14 chromosomes 

 (which usually possess two well -developed 

 arms) had only oval heads which apparently 

 were parts of the arms. In one nuclear plate 

 of Crepis capillaris chromosome A' developed 

 a secondary constriction, chromosome D lost 

 the greater part of its long arm and chromo- 

 some C, on the other hand, lengthened signifi- 

 cantly, apparently at the expense of the trans- 

 located part of the D chromosome. In another 

 nuclear plate of the same species, fragmenta- 

 tion of the A' chromosome at its constriction 

 was observed. Both arms formed little heads 

 and constrictions. A third nuclear plate showed 

 that chromosome D' enlarged its head but lost 

 its satellite while chromosome C lost part of 

 its head and acquired a satellite. Afterwards, 

 year after year, Levitskii and his co-workers 

 published papers dealing with the effects of 

 X rays, but since the results of their work are 

 summarized by the author in his 1940 article, 

 they will be presented in their proper place. 



Nadson and Rokhlina (1934), who devote 

 their attention chiefly to the effect of X rays on 

 the protoplasm and chondriosomes, also point 

 out changes in the nuclei of living cells. These 

 changes commence with the contours of the 

 nucleolus becoming more distinct while the 

 texture of the chromatin becomes more coarse. 

 Then coagulation and vacuolation appear in the 

 nucleus, and finally karyolysis sets in. The 

 nucleolus disappears. The outline of the 

 nucleus becomes irregular and very variable; 

 sometimes its size diminishes. The rate of 

 change depends, first of all, on the duration of 

 exposure to irradiation (the dosage), and also 

 on the physiological state of the cell. 



Stone's experiments (1933) with nuclei of 

 Tulipa silvestris and Rhoeo discolor in meiosis 

 and with the somatic nuclei of Crocus olivieri 

 are interesting. Parallel observations of the 

 two groups indicate that X rays induce a physio- 

 logical reaction [in root tip cells] which is 



^Editor's note: This parenthetical remark appeared 

 as a footnote in the original Breslavets text. 



69 



