revealed in meiosis also the reciprocal charac- 

 ter of the translocation, which rarely appears in 

 the form of a ring, apparently due to the insig- 

 nificant size of one of the chromosome parts 

 participating in the exchange. Several modified 

 individuals were obtained from each of the irradi- 

 ated parent plants; these modifications were of 

 various types. In some cases several individuals 

 with identical chromosomal changes arose from 

 the same parent plant, indicating that they de- 

 rived from a single modified cell of the parent 

 plant, which gave rise to a whole aberrant 

 "sector" or part in the mother plant. 



All of these changes were first obtained in a 

 heterozygous condition, and later in a homozy- 

 gous one. In addition to balanced [i.e., recipro- 

 cal] changes, duplications were obtained from 

 whole chromosomes (all three types) and from 

 the distal and proximal parts of the D-chromo- 

 some. The last of these changes was obtained 

 in tetrasomic condition. Some of the aberrants 

 that were examined showed a depressed fertility. 

 This decrease of fertility usually was found in a 

 heterozygous state; in some cases, however, it 

 was found in a homozygous one. In the former, 

 according to LevitskiT, this phenomenon is due 

 to cytological causes. Among karyotypes of 

 pollen grains of structurally heterozygous aber- 

 rants, the normal karyotypes are more numer- 

 ous than balanced aberrant karyotypes. Two 

 heterozygous translocants had many pollen 

 grains with karyotypes possessing an additional 

 chromosome or part of a chromosome. There 

 are a total of nine various types of gametophytes. 



From Gerasimova's two experiments (1940a 

 and 1940b*) which have exceptional genetic signif- 

 icance, we shall take only the purely cytological 

 data for this part of our exposition of the effect of 

 X rays. Ripe ova of Crepis tectorum were ex- 

 posed to X rays (1940a) and then fertilized by nor- 

 mal pollen. From 20 ovules of a single plant 8 

 very infertile plants came up; seven of them had 

 normal chromosomes, one was a translocant. 

 The translocation occurred between B and D, the 

 break in them took place along the satellite of the 

 D-chromosome not far from the proximal arm of 

 the B-chromosome. In the translocation almost 

 the entire distal arm of the B-chromosome 

 transferred itself to the remnant of the satellite 

 of the D-chromosome, becoming a giant satel- 

 lite suspended by a long thread. Apparently, 

 the D-chromosome gave up part of its satellite 

 to the B-chromosome, but this part was so 

 minute that it was not noticeable. Cytologists 

 examined 189 descendants of these plants; 84 

 were normal, 59 turned out to be heterozygous 

 translocants, and one was a homozygous trans- 

 locant. In 34 of the plants only one chromosome 

 was changed. In a paper published in the same 

 yearly the sizes of the chromosome satellites 

 are examined. As has already been pointed out. 



"Only 1 1940 reference given. 



one plant arose whose D-chromosome had a 

 giant satellite due to X radiation of the ova of 

 Crepis tectorum . This satellite consisted of 

 the entire distal arm of the B-chromosome (of 

 which only the short proximal arm was left). 

 The descendants of this plant included homozy- 

 gous specimens of this translocation. In some 

 cells the giant satellite would lose a small or 

 large fragment of itself, sometimes disappear- 

 ing entirely. Together with metaphase cells, 

 which were characterized by a partial or com- 

 plete loss of the giant satellite, cells were 

 encountered with additional nuclei, chromatin 

 spheres, etc. , which were indicative of loss of 

 fragments or whole chromosomes during cell 

 division. It was natural to associate this with 

 changes in the giant satellite. An analysis of 

 the figures indicated to Gerasimova that when a 

 satellite exceeds a certain size, it runs a great 

 risk of being lost or diminished. 



In order to conclude the literature survey of 

 the effect of X rays on the nucleus and the 

 chromosomes, it is essential to examine the 

 changes arising in the two plants that have been 

 most thoroughly studied in this respect, i. e. , 

 Nicotiana and Zea mays. The irradiation of 

 Nicotiana tabacum by high doses of X rays 

 produced in the germ cells, seeds, and seed- 

 lings an extreme degree of fragmentation and 

 fusion of the chromosomes, accompanied by 

 changes in the spindle mechanism. Cytological 

 investigations revealed such an extreme and 

 obvious destruction of the chromosome mecha- 

 nism in mitosis and meiosis that the resulting 

 lethal effect brought about complete sterility in 

 gametes and zygotes. Nevertheless, in some 

 instances a small number of seeds did develop, 

 and they gave rise to plants that showed extreme 

 morphological modifications. In one case 48 

 descendants were obtained from a plant and 

 only 2 of them were anywhere near normal; the 

 rest exhibited a most extreme variety of external 

 characteristics. However, the offspring of one 

 of the two outwardly normal plants showed a 

 very great range of variations. 



The scope of chromosomal aberrations dis- 

 played by Nicotiana tabacum is partially due to 

 the fact that this plant is of polyploid origin. 

 Goodspeed [1936] observed two haploids in the 

 genus Nicotiana : one of them from the variety 

 purpurea which arose from irradiated seedlings, 

 and the other from irradiated seeds which arose 

 in the X 1 . In addition, two haploids appeared in 

 N. glutinosa in the X3 generation. A total of 11 

 haploids were observed by Goodspeed, the oc- 

 currence of which the author ascribes not to the 

 direct action of irradiation but rather to the sec- 

 ondary effects of irradiation. Goodspeed as- 

 sumes that a parthenogenetic development of the 

 ova is possible. He also notes the appearance of 

 numerous triploid plants in the variety purpurea , 

 but only three of them were checked cytologi- 

 cally. They probably arose from parents with 

 an unbalanced chromosome condition, which is 



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