378 RADIATION BIOLOGY 



length of the chromosome, as seen in the extended interphase stage repre- 

 sented by the chromosomes of the saUvary gland, whereas something like 

 a third of the translocations undergone by this chromosome have had 

 their break in its centromeric heterochromatin. 



It is true that in the condensed X chromosome, as seen during mitosis, 

 the heterochromatin in the neighborhood of centromeres does occupy 

 about a third of its length (Muller and Painter, 1932), and that a similar 

 relation holds in the case of the other chromosomes, but this is due to the 

 large size of certain chromatin accretions called blocks (Muller and 

 Gershenson, 1935) which are embedded in the heterochromatin in the 

 neighborhood of the centromere at this stage. These blocks are of the 

 nature of adventitious nongenetic material (though produced under the 

 influence of certain genes in their vicinity), and they do not indicate a 

 corresponding length of coiled "gene-string" within. Moreover, breaks 

 are seldom if ever produced within these blocks themselves. Now it has 

 been found that when the genes for the blocks are removed, by means of 

 a structural change, to some other chromosome region, the heterochro- 

 matin which has been separated from them is still as susceptible to having 

 structural changes induced in it as before, although it now occupies about 

 as small a fraction of the mitotic chromosome as it does of the salivary 

 gland (interphase) chromosome (Muller, 1944). Similar observations 

 have shown that this susceptibility of the heterochromatin does not 

 depend upon the presence of either the centromere or the nucleolus in its 

 vicinity. It is therefore a property of the heterochromatic region itself. 



This susceptibility of heterochromatin to structural change does not 

 necessarily mean that it is more easily broken by radiation. It is quite 

 possible that it is broken no more readily than euchromatin is, per vmit 

 of length of its chromosome thread, but that it is much less likely than 

 euchromatin to undergo restitutional union of its broken ends as compared 

 with union between parts that were not together before. This might be 

 either because its broken ends became adhesive later than those of 

 euchromatin, during the period when the condensed chromosomes 

 undergo extension for interphase, or, more likely, because they were more 

 subject to movement. The centromeric heterochromatin is often under 

 special tension, which would tend to move its pieces apart if broken. 

 Moreover (as mentioned in the last part of Sect. 3) any heterochromatic 

 region tends to undergo conjugation with any other heterochromatic 

 region, even if the other does not lie in a homologous chromosome posi- 

 tion, and so these regions, in Drosophila at any rate, are probably more 

 subject than euchromatic regions are to forces of attraction which tend 

 to move them about. Further evidence pointing in this direction 

 appears to be provided by the finding of Prokofyeva-Belgovskaya (1939; 

 Prokofyeva-Belgovskaya and Khvostova, 1939), confirmed by Kaufmann 

 (1939), that there are even within the euchromatic portions of Drosoyhila 



