CELL DIVISION, MORPHOLOGY, VIABILITY 799 



immediately after irradiation during the period of diminishing mitotic 

 activity, while localized effects occur most frequently in those cells that 

 enter metaphase and anaphase after the period of irradiation-induced 

 minimum mitotic activity. That localized effects are not restricted to 

 the postinhibition period is evidenced by the recording of acentric frag- 

 ments 7^2 hours after treatment in Scilla root tips (Marquardt, 1938), 

 shortly after irradiation in Tradescantia microspores (Sax and Swanson, 

 1941), at 4 hours in Trillium, Allium, and Vicia root tips (Darlington and 

 La Cour. 1945), in anaphases of the first meiotic division in Habrobracon 

 eggs treated in late prophase or metaphase (Whiting, 1945), at 1 hour in 

 root tips of Vicia (Deufel, 1951), and 22 minutes after treatment in 

 neuroblasts of the grasshopper (Gaulden, unpublished). The explana- 

 tion offered by Sax and Swanson that such fragments may be due to 

 breaks resulting from stresses imposed by stickiness of sister chromatids 

 at anaphase may also apply to the Habrobracon egg treated at prophase I, 

 for anaphases of the latter showed fusions between separating chromo- 

 somes. It is unlikely that this would explain their presence at anaphase 

 I in the Habrobracon egg treated in metaphase I or at anaphase in the 

 grasshopper neuroblast treated in very late prophase, for in these no 

 fusions between separating chromosomes were evident. It will certainly 

 not account for their presence in metaphases of Scilla, Vicia, Allium, and 

 Trillium root tips. The frequency of fragments in irradiated cells before 

 the period of minimum mitotic activity is very low in most cells compared 

 to the number found in cells after the resumption of mitotic activity. 

 The high percentage of cells with fragments found by Deufel (1951) in 

 root tips of Vicia as early as 1 hour after 150 r may not be inconsistent 

 with other results because he includes in his tabulation "fragments" 

 connected by constrictions with the remainder of the chromosome as well 

 as those completely separated from it, and no breakdown of the two types 

 is given. It may be that the ion densities of X rays of the wave lengths 

 usually used is not high enough to produce many breaks of the heavily 

 condensed late prophase-through-anaphase chromosomes; perhaps neu- 

 trons or a particles would produce a greater frequency of breaks during 

 this period. It is of interest to note that, though localized and nonlocal- 

 ized effects in the form of breaks and fusions, respectively, may occur 

 during the period of decreasing mitotic activity, fusions apparently do 

 not occur at the breakage loci to give translocations, as Marquardt 

 (1938) has pointed out. 



Nonlocalized effects in the form of clumping of metaphase chromo- 

 somes or the sticking together of sister chromatids to give anaphase 

 bridges have been described by numerous investigators (Alberti and 

 Politzer, 1923, 1924; Strangeways and Oakley, 1923; Kemp and Juul, 

 1930; Lewitsky and Araratian, 1931; Crow, 1933; White, 1937; Mar- 

 quardt, 1938, 1950; Sax and Swanson, 1941; Carlson, 1941; Roller, 1943; 



