1308 BIOLOGICAL EFFECTS OF RADIATION 



normal insulating properties of the chromosome sheaths and permits 

 intimate contacts between the chromosomes." He also suggests more 

 specifically that irradiation, by decreasing protoplasmic viscosity, 

 "serves to solate the gelatinous "sheath," permitting chromosomes 

 to come into contact with one another and to interact in such a way as to 

 cause segmental rearrangements and gene mutations." 



The present information from induced chromosomal alterations in 

 maize is in accord with the view that irradiation acts in a general way 

 on the chromosomes or their surrounding medium, and that interchange 

 between strands may take place wherever chromosomes happen to lie 

 in contact. On this view the increased frequency of interchange with 

 increase of dosage should be due to intensification of the change in the 

 physical condition of the chromosomes or their surrounding medium; 

 whereas the frequency distribution of interchanges in the chromosome 

 should be merely a probability distribution of contacts between chromo- 

 somes at the proper stage of mitosis. 



REFERENCES 



This bibliography includes the more important papers dealing with tetraploids, 

 triploids, trisomies and chromosome eliminations as well as chromosome alterations 



1. Anderson, E. G. A chromosomal interchange in maize involving the attach- 

 ment to the nucleolus. Amer. Nat. 68: 345-350. 1934. 



2. Anderson, E. G. Chromosomal interchanges in maize. Genetics 20: 70-83. 

 1935. 



3. Anderson, E. G. and I. W. Clokey. Chromosomes involved in a series of 

 interchanges in maize. Amer. Nat. 68: 440-445. 1934. 



4. Beadle, G. W. The relation of crossing-over to chromosome association in 

 Zea-Euchlaena hybrids. Genetics 17 : 481-501. 1932. 



5. Beadle, G. W. A gene for sticky chromosomes in Zea mays. Zeitsch. Indukt. 

 Abstamm. und Vererbungsl. 63: 195-217. 1932. 



6. Brink, R. A. The occurrence of semi-sterility in maize. Jour. Hered. 18 : 266- 

 270. 1927. 



7. Brink, R. A. Are the chromosomes aggregates of groups of physiologically 

 interdependent genes? Amer. Nat. 66: 444-451. 1932. 



8. Brink, R. A. and C. R. Burnham. Inheritance of semisterility in maize. 

 Amer. Nat. 63: 301-316. 1929. 



9. Brink, R. A. and D. C. Cooper. The association of semisterile —1 in maize 

 with two linkage groups. Genetics 16 : 595-628. 1931. 



10. Brink, R. A., and D. C. Cooper. A strain of maize homozygous for segmental 

 interchanges involving both ends of the P-br chromosome. Proc. Nation. Acad. 

 Sci. 18: 441-447. 1932. 



11. Brink, R. A., and D. C. Cooper. Chromosome rings in maize and Oenothera. 

 Proc. Nation. Acad. Sci. 18: 447-455. 1932. 



12. Brink, R. A., and D. C. Cooper. A structural change in the chromosomes of 

 maize leading to chain formation. Amer. Nat. 66: 310-322. 1932. 



13. Burnham, C. R. Gcnetical and cytological studies of semisterility and related 

 phenomena in maize. Proc. Nation. Acad. Sci. 16: 269-277. 1930. 



14. Burnham, C. A. .\n interchange in maize giving low sterility and chain con- 

 figurations. Proc. Nation. Acad. Sci. 18: 434-440. 1932. 



