84 Nature of the Genetic Material 



densed chromomeres, a kind of heteropyknosis. White (1945) actually 

 defines (all) heterochromatin as those chromosomal regions which 

 become heteropyknotic at some time during the chromosomal cycle. 



The best information on intercalary heterochromatin again comes 

 from the salivary gland chromosomes in Drosophila. On a purely 

 morphological basis, it is difficult to define the intercalary heterochro- 

 matic segments. In a general way they show less stainability and 

 differentiation of dark bands, features which, however, are not equally 

 clear in all regions, but are just the opposite of the behavior in 

 metaphase and prophase chromosomes where heterochromatin is 

 highly chromatic. However, there are a group of criteria which 

 permit a fair delineation of these segments. The first is the tendency 

 of heterochromatic segments to get attached to other heterochromatin 

 in th6 same or other chromosomes. By checking carefully the bands 

 which show this unspecific attraction (so-called ectopic pairing), 

 as opposed to the specific band-by-band attraction of euchromatin, 

 Prokofyeva (1939) could map the distribution of intercalary hetero- 

 chromatin. A less exact method was found by Kodani (1942), who 

 showed that heterochromatic segments are more quickly dissolved in 

 alkali-urea and can thus be delineated approximately if inversions 

 are used as markers. As far as this goes the results agree with 

 Prokofyeva's and similar studies by Kaufmann (1946, 1948a). A 

 third method is based upon the property of the heterochromatic seg- 

 ments of being more easily broken by X rays or spontaneously. By 

 mapping easily broken sections, Prokofyeva (1939), Slizynski (1945), 

 and Kaufmann (1946) again localized the heterochromatic sectors. 

 The results of these studies are well summarized in Hannah's (1951) 

 scheme (fig. 10) for the first, second, and third chromosome. It does 

 not include Kodani's data, but includes for the X-chromosome so- 

 called repeats, assumed to be heterochromatic, and the location of 

 the Minutes, which are suspected to be heterochromatic. The salivary 

 chromosome is represented with the numbered sections as laid down 

 in Bridges' maps, and the crossover maps are added for comparison. 

 The rather good fit of the different checks is apparent. 



A word should be added about repeats and Minutes, which are 

 represented in figure 9. Bridges described many places in the 

 salivary chromosomes which look as if the same band or bands had 

 been repeated by some process of doubling. Much theoretical inter- 

 est is attached to these. At this point we are concerned only with 

 the idea that repeats are heterochromatic. It cannot be said that this 

 is actually proved, though the locahzation points to it. One idea 



