INCIDENCE OF BREAKAGE 



429 



movement of the broken parts in the nucleus (Ch. XII). Their 

 high frequency indicates that a proportion of breakages are reunited, 

 as they were before, in ordinary mitotic irradiation. Further 

 quantitative study would be useful to find out what this proportion 

 may be. Another and indirect effect is produced on meiosis after 

 an interval, namely, an increase in chiasma-frequency (Mather, 

 1934 ; cf. Levan, 1935 h, on Allium). All the special properties of 

 meiosis in relation to X-ray treatment depend, therefore, on the 

 circumstances of pachytene pairing. 



When mitosis is studied with a longer interval after irradiation, 

 a considerable reduction in the frequency of changes is found. 

 This is due to two kinds of selection. First, the acentric chromo- 

 somes are eliminated and the dicentric and ring chromosomes 

 reduced in frequency by breakage and non-disjunction {cf, 

 McCUntock, 1933 b). Secondly, cells with severe abnormalities are 

 eliminated in competition with less severely altered cells. Such 

 competition is, of course, particularly severe in pollen. But where 

 all the cells are damaged it becomes possible to effect fertilisation 

 with such pollen grains as could never do so normally, and mono- 

 somic Zea and polysomic Oryza have been raised this way (Stadler, 

 1930 ; Ichijima, 1934). Damaged pollen has similarly been used 

 to stimulate parthenogenesis in Triticum (Ch. XI). 



The conditions of survival of gene or structural changes are 

 very different in diploids and polyploids. Both are less severely 

 selected in polyploids. Most induced gene changes, however, are 

 invisible in the polyploids, e.g., Avena (Stadler, 1930) ; while 

 structural changes are visible, and, being less dangerous, are found 

 as deficiencies such as would be eliminated in the diploid {e.g., 

 Nicotiana, Goodspeed, 1930). 



Organisms are necessarily heterozygous in regard to the structural 

 changes induced in them. Those such as interchanges, which 

 survive to later generations can often but not always be obtained in 

 the homozygous condition (McClintock, 1934 ; Catcheside, 1935 ; 

 E. G. Anderson, 1935 a ; Lewitsky et alii, 1934 ; Muller, 1932). 

 In the heterozygous condition their behaviour resembles that of 

 natural structural hybrids and has been considered in conjunction 

 with them. After an interval of many mitotic cycles there is, as a 



