58 Nature of the Genetic Material 



amount or concentration of DNA in the chromomeres. (Darlington 

 speaks of the charge with DNA; see Darlington and La Cour, 1940.) 

 These differences are specific at different times of the mitotic cycle. 

 In inteiphase, when the chromosomes (i.e., their euchromatic regions) 

 become invisible through disintegration (scattering or swelling by 

 hydration, not chemical disintegration) of the chromomeric material, 

 heterochromatin is highly stainable and is condensed into a block, 

 called a chromocenter if it is a single mass, or chromocentric blocks if 

 there are many. This phenomenon is called heteropyknosis (Gutherz, 

 1907). At other stages of the mitotic cycle when the euchromatin is 

 very dense, the heterochromatin stains differently. This phase dif- 

 ference ( also called positive and negative heteropyknosis ) , based upon 

 the minute arrangement of the chromomeres, is called allocycly ( Dar- 

 lington and La Cour, 1940). It involves dense coiling and chromomeres 

 of rather large size. These are the two most distinctive properties of 

 heterochromatin on the cytological level, but there are innumerable 

 additional details and variants: the structural differences in the salivary 

 gland chromosomes of Diptera, the specific features of the sex chro- 

 mosomes, and of course all the facts relating to genetics. 



Let us first view the purely cytological features and, without 

 going into all the variations, mention the salient facts which we con- 

 sider of importance for the theoretical analysis in special cases. Hetero- 

 chromatin is essentially a cytological concept based upon hetero- 

 pyknosis and allocycly. If these criteria alone are used, it is hardly 

 possible to give a description which fits all cells. White (1951) 

 actually speaks of a continuous spectrum between the extremes, eu- 

 chromatin and heterochromatin. (For details see the reviews by 

 Resende, 1945, and Barigozzi, 1950fl; also Darlington and La Cour, 

 1947. ) Resting nuclei may show chromocentral accumulations or none 

 at all, even within the same organism. In the same way, mitotic 

 chromosomes may have allocyclic segments or not. The observable 

 quantity of heterochromatin may be very different in nearly related 

 species (White, 1945) or even within the same chromosome after 

 treatment at different temperatures. White (1945) speaks of micro- 

 heterochromatic species with allocycly of only the sex chromosomes, 

 and megaheterochromatic species with much allocyclic material in the 

 autosomes. Virkki ( 1951 ) has tabulated these conditions for a large 

 number of scarabaeids (Coleoptera) and discussed other cases; Smith 

 (1952) has done the same for tenebrionid beetles. This is a good 

 illustration of the cytological variability of the phenomenon, which. 



