554 ' RADIATION BIOLOGY 



attachments, and bridges of chromosomes (e.g., Koernicke, 1904a, b, 

 1905; Guilleminot, 1908; Gager, 1908; Amato, 1911; Barratt and Arnold, 

 1911; P. Hertwig, 1911). It was natural therefore to suspect that the 

 observed cell damage was largely caused by the chromatm damage (Bohn, 

 1903; Levy, 1906). This supposition was soon strengthened materially 

 by observations showing that irradiation of germ cells (even of male germ 

 cells alone, a more decisive criterion) leads to abnormal development 

 (Bardeen, 1907, and McGregor, 1908, on anurans; Regaud and Debreuil, 

 1908, on rabbits; the Hertwigs and Oppermann and their co-workers, 

 1911-1913, on sea urchins, fish, amphibia, and angiosperms, reviewed by 

 G. Hertwig, 1920), and that these effects are accompanied by chromosome 

 abnormalities (see G. Hertwig, 1920). 



That cells near the metaphase of mitosis are more readily damaged by 

 radiation than those in which the chromosomes are more extended had 

 already been noted by Koernicke (1904a, b, 1905), in so far as chromo- 

 some changes were concerned, and by Krause and Ziegler (1906), in so far 

 as cell damage in general was concerned; the latter authors used these 

 results to explain the greater damage to tissues in which mitosis was pro- 

 ceeding more actively at the time of irradiation. This interpretation 

 was strengthened by Holthusen's (1921) classical study of the damaging 

 effects of X rays, applied at various stages, on the development of Ascaris 

 eggs. He traced in detail the variation of radiation susceptibility with 

 the mitotic cycle and proved that there is a marked peak in the neighbor- 

 hood of metaphase. The later work of Alberti and Politzer (1923, 1924) 

 on cells in the cornea of salamanders and to some extent that of Strange- 

 ways and Oakley (1923) on cells in tissue cultures broke down the effects 

 on chromatin into those immediately observable (these are usually less 

 abundant and often consist of a mere stickiness) and the more definitive 

 "secondary" chromosome fragmentations and junctions not observable 

 until the mitosis following the irradiation. 



A review of most of the voluminous later literature on the variations 

 in the frequency of induced chromosome changes of different kinds with 

 the stages of the mitotic and meiotic cycles and with accompanying con- 

 ditions may be left to the chapters by other authors, dealing more spe- 

 cifically with these effects. Here it may be noted that (as pointed out in 

 Chap. 7 and in Sects. 2 and 3 of this chapter) a considerable part, at least, 

 of the observed differences in results have long been known to have their 

 basis in the differences in joinability of broken ends, and in the relative 

 opportunities for joining in one way rather than another, e.g., restitu- 

 tionally or by isochromosome formation or by exchange of parts of the 

 same or different chromosomes. These opportunities in turn depend on 

 factors such as chromosome and chromatid arrangement and amount and 

 type of movement of parts; these arrangements and mo^'ements can be 

 influenced by intrachromosome tension, degree of spiralization, properties 



