646 



RADIATION BIOLOGY 



tion to the general rule, since they are transported to the spindle poles 

 together with the main body of the chromosomes from which they have 

 been detached (White, 1936; Bauer and LeCalvez, 1944; Hughes- 

 Schrader and Ris, 1941; Ris, 1942). Exclusion of detached fragments 

 from the daughter nuclei upsets genie balance ; it has been suggested that 

 the reduction in fertility that follows irradiation of gametes in various 

 species of animals is due in large measure to fragment production. This 

 topic has been reviewed by Lea (1947b). 



0.8 



1.5 1.7 3.0 



Fig. 9-10. Salivary-gland chromosome map of the prune-echinus region of the X 

 chromosome of Drosophila melanogaster, indicating the extent of 14 deficiencies that 

 have been studied. (From Demerec, in Demerec and Kaufmann, 1937.) 



Terminal deficiencies are probably produced by irradiation of sperma- 

 tozoa of Drosophila, but they presumably have dominant lethal effects 

 (Pontecorvo, 1941, 1942; Pontecorvo and Muller, 1941; Muller, 1941; 

 Fano, 1941; Demerec and Fano, 1944; Lea and Catcheside, 1945; Catche- 

 side and Lea, 1945a). Loss of a fragment from one of the longer auto- 

 somes in cleavage mitosis will upset genie balance and cause abortion of 

 the embryo. The mechanism probably involves establishment of a 

 breakage-fusion-bridge cycle in the centric portion of the chromosome 

 (similar to that described by McClintock, 1939, for maize) as a conse- 



