been established at that time. )ll 4) Double 

 centromeres, which result from the fusion of 

 two whole chromosomes, or of two proximal 

 fragments, or of one of each. With every divi- 

 sion of the cell some of these are lost. 5) Ring 

 chromosomes, which are formed by the fusion 

 of two ends of a single normal chromosome or 

 of a proximal fragment. (Recently the appear- 

 ance of ring chromosomes has been explained 

 differently. )11 From this we can see that the 

 only chromosomal abnormalities capable of 

 maintaining themselves through a series of cell 

 divisions are translocations and proximal frag- 

 ments. Having made these observations con- 

 cerning the origin and survival of definite ab- 

 normalities, Mather and Stone set out to solve 

 one of the important cytological problems, 

 namely: at what point in mitosis do the chromo- 

 somes split? On the basis of Stone's premise 

 that irradiation does not affect chromosomes 

 that are in division, they concluded that the 

 abnormalities must arise during the resting 

 stage, which has been retarded by X rays. If 

 the chromosomes split at metaphase or ana- 

 phase of the preceding division, unequal abnor- 

 malities should then arise as a consequence of 

 the existence of two elements — chromatids. 

 The appearance of two unequal chromatids, how- 

 ever, has not been observed in any species. If, 

 on the other hand, chromosomes arise after the 

 appearance of abnormalities, the chromatids 

 which appear will be equally abnormal. Since 

 Mather and Stone observed only equally abnor- 

 mal chromatids, then concluded that the chromo- 

 somes must split during early prophase. In 

 other words, in the first case the X rays affect 

 the chromatids; in the second case they affect 

 the entire split chromosome. 



Mather (1934) compared the behavior of 

 irradiated meiotic chromosomes in Vicia faba 

 and Tradescantia bracteata , which differ from 

 each other in that Vicia faba has a higher 

 chiasma -frequency and a lower terminal ization 

 than Tradescantia . The haploid chromosome 

 set of Vicia faba consists of five short sub- 

 terminally attached chromosomes (the m chro- 

 mosomes) and one chromosome twice as long 

 as the others with a median attachment (the M 

 chromosome). The mean chiasma -frequency 

 of the M bivalent is nearly twice that of the 

 m bivalents. X radiation causes the appearance 

 of fragmentation and translocations at first 

 metaphase. The translocations give rise to 

 various configurations. One of these configura- 

 tions consists of four chromosomes and appar- 

 ently results from segmental interchange, 

 reciprocal translocation, or an inversion of a 

 segment of one chromosome. Another config- 

 uration involves one m and the M chromosome. 

 One form of break is more frequent than others 



1 1 These 2 parenthetical comments appeared as foot- 

 notes in original text. 



and that is a break in the "loop" or arm which 

 carries the centromere. The latter is not 

 clearly drawn out at early metaphase and here 

 [at this stage] the break occurs more rarely. 

 At late metaphase, when the centromeres move 

 toward the poles and the chromosomes are 

 thinnest at this point, the break occurs more 

 frequently. This indicates that the chromo- 

 somes are weakened by the action of X rays 

 and break easily [when the strain of separation 

 is put on them]. At early metaphase breaks 

 occur along the whole length of the chromosome, 

 and at anaphase and telophase several large 

 chromosomal fragments are frequently encount- 

 ered. These fragments do not contain the 

 centromeres and, consequently, are not able 

 to migrate to the poles. They lag at the equator 

 and form micronuclei at interphase, and during 

 the second division they lie apart from other 

 chromosomes. The two centromeres of a single 

 chromosome can go either to the same pole or 

 to opposite ones. In the first case the double 

 centromere can be retained, and in the second 

 division it can give rise to a new chromosomal 

 type of pollen grain. In the second case two 

 daughter nuclei will be connected by a stretched 

 chromosome [bridge] until it breaks. In 

 Tradescantia from 4 to 7 days after irradiation 

 almost all the chromosomes in meiosis are 

 fragmented. Many translocations are also ob- 

 served. Cells at telophase and interphase are 

 frequently abnormal in the number and size of 

 the chromosomes. This is due to the presence 

 of lagging chromosome fragments, which 

 eventually give rise to micronuclei. The ab- 

 normalities of the first division manifest them- 

 selves at metaphase of the second division by 

 formation of more than two [nuclear] plates, or 

 by abnormal structure of the chromosomes, 

 either forming rings or having from 2 to 4 

 centromeres. Sometimes chromosomes com- 

 posed of identical chromatids are observed at 

 second metaphase. The tetrad stage is irregu- 

 lar. At microspore metaphase fragments are 

 observed in various numbers. The chief differ- 

 ence in the behavior of Vicia and Tradescantia 

 chromosomes is fragmentation before meta- 

 phase in the former and up to metaphase in the 

 latter. Apparently, the various stages at which 

 fragmentation occurs are related to a certain 

 difference in the behavior of normal chromo- 

 somes, namely, terminalization of chiasmata 

 is low in Vicia whereas in Tradescantia it is 

 high. (The term terminalization was used by 

 Darlington to refer to the movement of chias- 

 mata away from the centromere due to the 

 repulsion of the centromeres during prophase. ) 



Marshak (1935) studied the effects of X rays 

 on various stages of meiosis in Gasteria . This 

 plant was selected because each of its flower 

 buds contains six anthers with the pollen mother 

 cells in the same stage of development. By 

 this means Marshak avoided the usual diffi- 

 culties encountered in work with the biological 

 effects of X rays that arise because of lack of 



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