Genic and Non-genic Parts of the Chromosome 83 



heterochromatin distal to the peach locus, or proximal to this locus; 

 or by placing euchromatin either distal or proximal to the locus. This 

 shows clearly that the cause of the variegated position effect is 

 simply the disruption of heterochromatin by a break. This means that 

 it is a break, a change of pattern, which produces the position effect: 

 Lewis' ( 1950 ) artificial distinction in principle between variegated 

 and other position effect is not necessary, and is actually mistaken. 

 The variegation in the presence of the heterochromatic break is, I 

 think, to be explained here in the same way as above: the hetero- 

 chromatin affects the penetrance and expressivity of the mutant-Hke 

 action of the break (i.e., position effect) during development by 

 lifting the developmental processes above a threshold or not (this 

 includes chemical syntheses in the case of eye colors). Baker him- 

 self gives an interpretation in the currently popular terms of inter- 

 action between necessary substrates of gene and heterochromatin, 

 but he adds that he is aware that such a model is only one of many 

 possible schemes. I prefer the one presented in this chapter. (We 

 shall not go into further details at this point, since only hetero- 

 chromatic function is under discussion, not the theory of position 

 effect, which will be discussed below.) 



As far as our present discussion of the function of heterochro- 

 matin is concerned, all the different facts show in the end that 

 chromocentral heterochromatin has a generalized function connected 

 with rates of cell division and quantitative features of intracellular 

 metabolism. 



cc. Intercalary heterochromatin 



The other type of heterochromatin, the intercalary one, was also 

 discovered by Heitz, who found that mitotic chromosomes of Dro- 

 sophila had at certain stages a beaded appearance, showing a number 

 of more darkly staining blocks. Kodani (1942) proved later for the 

 X-chromosome that Heitz's heterochromatic segments correspond 

 closely to those he could delineate by specific reaction to urea- 

 alkali. Similar observations were made in other animals and plants. 

 Darlington and La Cour (1938) showed the alternation of eu- and 

 heterochromatic segments in the chromosomes of Fritillaria. Later 

 they found that cold treatment reduced the stainability of these 

 chromosomes in segments, which closely corresponded to those seen in 

 pachytene chromosomes and therefore are heterochromatic (their 

 so-called chromatin starvation). In other chromosomes (see fig. 10) 

 intercalary heterochromatin appears in the form of larger and con- 



