446 EADIATION BIOLOGY 



ation. These studies have estabUshed, among other things, the following 

 previously unknown principles (which were in the preceding sections 

 assumed to be true): (1) that the breakage of chromosomes occurs first; 



(2) that the unions of fragments happen as a later, quite separate step; 



(3) that broken chromosome ends have the property of being adhesive to 

 one another and thereby becoming permanently united; (4) that origi- 

 nally free ends do not have this property and are therefore to be distin- 

 guished as "telomeres"; (5) that the adhesiveness does not manifest itself 

 while the chromosomes are in a condensed stage but remains latent, to be 

 expressed on their emergence therefrom; (6) that the reproduction of a 

 chromosome fragment produced by breakage results in that end of the 

 daughter piece which is homologous to the broken end of the mother 

 piece being itself adhesive; (7) that when union of broken ends occurs 

 it is always two-by-two, i.e., 3 or more pieces cannot join at one point so 

 as to produce a branched chromosome ; and (8) broken ends do not adhere 

 to the side of a chromosome. Breakage, however caused, is the event 

 that triggers the process, but what happens later in the production of a 

 structural change follows in consequence of the operation of the other 

 principles. For evidence on most of these points the reader is referred to 

 the present writer's analysis of the processes (MuUer, 1940a). 



The establishment of the chromosomal interpretation of the ropelike 

 bodies in Dipteran salivary glands — a step most serviceable for progress 

 in genetics — involved the use of chromosomes structurally changed by 

 radiation. Only when the very changes which genetic tests had demon- 

 strated to have been produced by radiation in the linkage maps were 

 directly seen in the salivary gland bodies was the chromosomal nature of 

 the latter revealed unmistakably by Painter (1933, 1934). Then, after 

 varied chromosomes that had been structurally changed by radiation had 

 had their morphology determined by the genetic method of "mapping" 

 through linkage tests, the detailed study of the appearance of these chro- 

 mosomes in the salivary glands made it possible to ascertain exactly which 

 spots on the visible salivary chromosomes corresponded with given points 

 in the linkage maps. In this way verification was provided of the physi- 

 cal validity of even the minutiae of the linkage maps, much as such veri- 

 fication of their gross features had earlier been giA-en by comparisons of 

 the linkage maps of chromosomes structurally altered by radiation with 

 the chromosomes as seen in their condensed (mitotic) stages (MuUer, 

 1928b, d; Muller and Altenburg, 1928; MuUer and Painter, 1929; Painter 

 and Muller, 1929; Dobzhansky, 1929, 1930a, b, 1931, 1932). Moreover, 

 after the point-by-point correspondences had been ascertained in this way 

 for salivary-gland material, various further conclusions could be drawn, 

 such as that the frequency of crossing over varies in given ways from 

 region to region of a chromosome, owing to the influence of the centro- 

 mere and other features, and that the condensed mitotic chromosome 



