70 Nature of the Genetic Material 



mosome. He also tried to fit heterochromatin into his scheme. He 

 concludes that heterochromatic regions (and chromosomes) differ 

 from euchromatic ones in retaining the peripheric nucleoproteins while 

 those in euchromatic regions are disintegrating, and this appHes 

 especially to the part closely bound to the chromonemata. The reason 

 is the non-functioning of these regions from a genetic point of view. 

 Heterochromasy may be permanent or transitory- and does not play a 

 great role. An excess of heterochromatin represents a charge for the 

 cell, because heterochromatic regions are slower in di\iding and 

 pairing, and many heterochromatic chromosomes have a tendency to 

 be lost. "Heterochromatic regions are maintained by a balance 

 between their usefulness in mechanisms of sex determination, in 

 rendering inert certain duphcated or unbalanced fractions of the geno- 

 type, and in making stable certain special zones of the chromosomes 

 like terminal regions and those adjacent to the centromere in many 

 species, on one side — and on the other, their detrimental properties 

 upon the mechanisms of di\-ision and pairing. Natural selection acts 

 upon the net result of this balance and maintains or rejects hetero- 

 chromatic chromosomes." 



This interpretation fails completely to explain the facts discussed 

 thus far and also those to be discussed below. It practically denies a 

 functional role on the basis of morphological obser\ations in plant 

 cells and makes use of the equivocal concept of inert material. Thus it 

 represents a rather negative point of \-iew, which, I think, the facts do 

 not require. 



It seems that an explanation cannot be given which fits all the 

 remarkable and conspicuous facts. However, in a general way we may 

 accept the \iew that heterochromatin is involved in rapid cell division, 

 either prev'enting it from taking place if retained in the chromosomes 

 or encouraging it if removed into the c\1:oplasm. A further role in the 

 production of cell constancy and endomitosis is probable. It is not 

 known why the distribution of and happenings to the heterochromatin 

 are so conspicuous in some organisms, while in others nothing of the 

 kind is seen but must be supposed to occur on a submicroscopic level, 

 as must also be assumed for the sloughing off of RXA. \-isible only in 

 Lepidoptera. 



d. Cytology and genetics of heterochromatin in Drosophila 



A remarkable body of knowledge on heterochromatin is derived 

 from the study of Drosophila, in which CNiiolog)' and genetic experi- 

 mentation can collaborate. Though much is still obscure even in this 



